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Adsorption of l -histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy
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VIBRATIONAL SPECTROSCOPY
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ELSEVIER
Vibrational Spectroscopy 10 (1996) 271-280
Adsorption of L-histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy Saulius Martusevi~ius a, Gediminas Niaura b, Zita Talaikyt6 Valdemaras Razumas a
a, *
a Institute of Biochemistry, Mokslininlat 12, 2600 Vilnius, Lithuania b Institute of Chemistry, A. GoJtauto 9, 2600 Vilnius, Lithuania
Received 1 January 1995
Abstract The adsorption of L-histidine on a copper electrode from H20-and D20-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO2- and C1- ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO2 - and C l ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than - 0 . 2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the a - N n 2 group and the neutral imidazole ring. The C1- ions cause the protonation and detachment of the a-NH 2 group from the surface and the formation of the ion pair NH~- • • • C l can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the a - C O O - group at E >_ - 0 . 2 V. However, this adsorption state is transformed into the adsorption state in which the a-NH 2 group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring. Keywords: Adsorption; Raman spectrometry; Raman scattering, surface enhanced; Histidine; Copper electrode
I. Introduction L-Histidine, one of the natural amino acids, plays an important role in various biological processes. There are numerous indications of the significance of
* Corresponding author.
histidine in enzyme action and in the binding of metal ions to proteins. The imidazole group of histidine is ionized at physiological pH values, thus playing a major role in the buffer action of proteins. In the context of investigations of protein adsorption on different surfaces it is of interest to obtain information about the interfacial behavior of L-histidine per se. Surface-enhanced Raman scattering (SERS) spec-
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S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
troscopy appears to be a powerful tool in the investigation of the adsorption behavior of molecules on the surfaces of SERS-active metals because the vibrations of molecules may change significantly at solid-liquid interfaces. It is possible to determine the geometry of the adsorbate and the groups being bonded to the surface. The groups that are directly involved in adsorption in general could be recognized by the sufficiently large A v value (difference in frequencies between SERS and solution Raman bands) and also by enhancement of the intensity AI (the ratio of the intensity of the SERS band to the intensity of the same band in the solution spectrum). The latter parameter increases most significantly if the vibrational mode of the adsorbate has a large polarizability component perpendicular to the surface. There are grounds to believe that the imidazole group of histidine is highly surface active. For instance, imidazole can react with colloidal silver forming polymeric surface complexes [1]. The formation of the polymeric film on a copper electrode immersed in a benzimidazole solution was also reported [2]. The adsorption of imidazole and its 3-alkyl derivatives on copper was studied by Fourier transform infrared (FI'-IR) spectroscopy [3]. The adsorption of imidazole on a roughened polycrystalline silver electrode has been studied by SERS spectroscopy [4]. The SERS results indicated that different forms of imidazole (anionic, neutral and probably ylide form) can be observed at the Ag surface. The relative amounts of these forms depend on the electrode potential and on the pH of the solution. On the other hand, from the data of SERS spectra of aliphatic amino acids and dipeptides it was established that they may adsorb on colloidal silver through the a-carboxylic and possibly a-NH 2 groups [5,6]. SERS spectra of histidine adsorbed on colloidal silver were presented in Refs. [7-9], but the results were not discussed in sufficient detail. In the present work the dependence upon the pH of the solution and electrode potential over the interval from - 1 . 0 to 0.15 V (versus the normal hydrogen electrode) of the adsorption of L-histidine on a polycrystalline copper electrode was studied. As far as we know, the copper electrode and histidine do
not exhibit electrochemical activity over this potential range. Thus, the effect of potential on the SERS spectra of histidine, if any, should be determined by the adsorptive behaviour of the amino acid.
2. Experimental 2.1. Reagents and solutions
Sulfuric acid of chemically pure grade was doubly distilled at 240°C. Sodium sulfate, chemically pure, was recrystallized once from triply distilled water and annealed at 350°C. L-Histidine (Reanal, Hungary) was recrystallized twice from triply distilled water. For deuteration experiments, 99.8% D20 was used as received from Sigma. Raman and SERS spectra of 0.2 and 0.02 M L-histidine, respectively, were measured in the triply distilled water-or D20-based solutions of 0.1 M H2SO 4 (pH 1.2), 0.1 M Na2SO 4 (pH 3.1 and 7.0) or 0.1 M NaOH (pH 11.9). The pH values of the NazSOa-based solutions were attained by using H 2 S O 4 or NaOH. The pH values presented in the text constitute the function of the pH glass electrode. In specific cases 0.1 M NaCI was added to the standard histidine solution. The Raman spectrum of the 0.02 M L-histidine solution does not differ from that of the background. Before use all solutions were deoxygenated by bubbling ultra-pure argon for about lh. 2.2. Equipment and procedures
Raman spectra of solutions were recorded in a rectangular quartz cell. SERS measurements were performed in a cylindrical three-electrode electrochemical cell. The counter electrode was a Pt wire. Its compartment was separated from the working electrode compartment by glass frits. The SERS-active copper electrode (geometric surface area of 0.07 cm 2) was prepared according to the previously described method [10]. The surface of the working electrode was placed at 5 mm distance from the cell window. The potential of the working electrode was measured versus the saturated Ag/AgC1 reference electrode supplied with a Luggin capillary to minimize the influence of the
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
CI- ions on the adsorption of L-histidine at the copper electrode. The potential of the working electrode was controlled using an OH-105 polarograph (Radelkis, Hungary). All potentials in the text are given versus the normal hydrogen electrode (NHE). An argon laser was used for solution Raman excitation at 488 nm (power approx. 40 mW). The 632.8-nm line of the H e - N e laser was used in the SERS experiments as the excitation source with approx. 10 mW power at the sample. The laser beam was incident on the surface at an angle of 61 ° with respect to the surface normal and focused to a line with a cross-area of approx. 1 mm 2. The scattered Raman light was analyzed using a double monochromator with 1200 lines mm-1 gratings and detected by a cooled photomultiplier and a photon-counting system. The instrumental parameters were as follows: spectral slit width S = 5 cm -1, integration time ~-= 0.5 s.
3. Results and discussion
3.1. Raman spectra Of L-histidine solutions To gain a better insight into the SERS spectra of the adsorbed histidine, in the first stage of the investigations the Raman spectra of the amino acid solutions have been studied at three different pH values: 1.2, 3.1 and 11.9. The Raman spectra were recorded in H 2 0 - a n d D20-based solutions. The band frequencies observed and possible assignments of the vibrational modes on the basis of the literature data [11-19] are listed in Table 1. In aqueous solutions, depending on the pH, histidine may exist in five ionic forms: p K a = 1.8
H4His 2+
~
p K a = 6.0
H3His +
~
COOH pK a=9.1
'~" NH~
H2His °
Im pK a= 14
HHis-
~
His 2-
(1)
Im
where COOH, Im and NH~- denote the a-carboxylic, imidazole and a-amino groups, respectively. The structures of the corresponding forms presented in Eq. 1 are shown in Fig. 1. According to the given p K a values of histidine
273
(Eq. 1), the amino acid solutions at pH 1.2, 3.1 and 11.9 comprise approximately 80% of H4His 2÷ and 20% of H3His +, 5% of H4His 2+ and 95% of H3His +, and 99% of H H i s - and 1% of His 2-, respectively. Hence the data of Table 1 reveal that the bands most characteristic for the positively charged imidazolium ring in the forms of HaHis 2+ and H3His + are located, on the average, at 1194, 1490 and 1630 cm -1 (for the N-deuterated imidazolium ring at 1110, 1408 and 1602 cm-1). On the other hand, the neutral imidazole ring in HHis- is characterized by the presence of strong bands at 1158, 1230, 1320 and 1570 cm -1.
3.2. SERS spectra of L-histidine at a copper electrode in acidic solutions (pH 1.2) SERS spectra of histidine obtained at pH 1.2 in the potential (E) range of the copper electrode from 0.15 to - 0 . 4 V (vs. NHE) are shown in Fig. 2. As noted above, the predominant (80%) ionic form of histidine at this pH is HaHis e÷ (Eq. 1, Fig. 1). First, it is worth noting that the pronounced intensive band at 972 cm -1 is assigned to the totally symmetric vibration of the adsorbed SO 2- ions on the copper surface [20], whereas the others should be assigned to the vibrations of the adsorbed histidine. When E becomes more negative the intensity of the SO 2- band increases, as the intensity of the histidine bands decreases, indicating the desorption of histidine from the surface. The change of intensity of the 972 cm -1 mode of adsorbed SO42- on the copper surface was studied earlier and a parabolic curve of intensity versus potential was observed with a maximum at - 0 . 4 V in 0.1 M n 2 s o 4 solution [20]. From the data of Fig. 2 it is evident that at E > - 0.4 V the histidine ions compete in the adsorption with the SO42- ions, whereas the latter anions are the main adsorbable species at E = - 0 . 4 V. To confirm the assumption of competitive adsorption between the anions and histidine the experiments were performed using chloride ions as additional anions in the solution. The addition of 0.1 M NaCI to the standard histidine solution causes complete disappearance of the SO 2- and histidine bands, regardless of the electrode potential. However, the ~,(Cu-C1) band at 290 cm-1 was clearly defined in all spectra.
S. Martusevirius et al. / Vibrational Spectroscopy 10 (1996) 271-280
274
These results prove conclusively the above-mentioned assumption. The comparison of the frequencies of the adsorbed histidine vibrations (Fig. 2) with those observed in the solution Raman spectrum at pH 1.2 (Table 1) shows that A v of the most prominent bands is small. The same is observed when SERS and Raman spectra obtained in D20-based solutions were compared. The frequencies of the intensive imidazolium ring vibrations - v ( N 1 - C 2 - N 3 ) + 8 ( N - D ) at 1410 cm -1 and v(C4-C 5) at 1603 cm -1
- and the width values at half maximum of these bands in SERS and solution Raman spectra were in close agreement. These results indicate that the imidazole ring of the adsorbed histidine remains protonated and the nitrogen atoms of the heterocycle are not involved in the chemical coordination with the copper surface. If the assumption is made that the electromagnetic mechanism is mainly responsible for the intensity enhancement in this system, it is likely that the plane of the imidazolium ring of H4His 2+ is mainly oriented perpendicularly to the surface, since
Table 1 Raman frequencies (cm -1 ) of histidine in the H20-and D20-based solutions of different pH pH 1.2
pH 3.1
pH 11.9
Assignment
Ref.
H20
D20
HzO
D20
H20
D20
517 w
506 w 595 v w 637 m
518 v w
509 m 597 m 628 m
528 m
522 m
8 ( C O O - ), ¢(NH 2)
[151
621 661 717 769 811
618 663 719 773 814 835 866 919
Im %(R) 6 ( C O O - ) , Im x ( R ) l m (ring breathing) Im x ( C - H ) Im (ring breathing) Im / / ( C - H ) Im fl(R), v ( C - C ) Im x(R), v ( C - C O O - ) Im ~(R) v(C-C) 8(C-CH) Im 8(R) v(C-N) Im 8(R) + / 3 ( C 2 - H ) ~(C-H) Im ~5(R) + 13(C5-H) , 8 ( N - D ) + v(N1C2N3) Im 8(R) + f l ( N l - n ) Im 8(R) Im (ring breathing) Im /3(C2-H) Im 8(R) + / 3 ( C 2 - H ) Im (ring breathing) to(Ell 2)
[15] [15,16] [15] [15] [15] [15] [16] [17,18] [12] [15] [19] [12] [15,16] [15,16] [15] [12,13,15,17] [12,15,17]
v(NIC2N3) + tS(ND), v s ( C O O - ) 8(CH2), 8 ( N ~ - H )
[13-16,18] [12,15,17,18]
Im v(R) + / 3 ( C 2 - H ) Im v(g) + f l ( N 1 - H ) v(C 4 =C5), 8(NH2),Vas(COO-) v(C=O)
[12,15,17] [11,15,17] [11,14,151 [15]
633 664 700 770 812
vw sh vw sh m
854 m 917 m
705 w 773 v w
860 sh 911 m
964 sh
627 664 707 775 812
m vw w w w
854 m 918 m
860 w 903 w
963 sh
975 sh 1004 s 1040 w 1074 vw
993 m
999 m
992 m
1063 sh 1090 w
1074 vw
1070 sh 1092 m 1108 vs
1112 vs 1194 vs
704 w 778 sh 812 s
1104 m
m m w vw m
856 m 918 v w 935 w 962 w 989 m 1008 sh 1044 v w 1068 sh 1088 sh 1096 s 1158 s
m m w vw sh s m vw
994 s 1012 s 1064 w
1168 sh
1194 vs
1270 vs
1264 vs
1270 vs
1254 m 1271 w
1363 w
1357 m
1329 sh 1360 m
1321 w 1355 w
1233 1265 1284 1321 1355
s s vs vs vs
1438 w
1410 vs 1443 w
1406 m 1441 m
1405 vs 1441 m
1407 s 1442 vs
1491 vs
1477 w
1489 vs
1631 vs 1734 m
1603 vs 1734 m
1629 vs
1476 v w 1562 w 1600 vs
1491 s 1571 vs 1637 s,br
1230 1258 1274 1317 1349 1373 1407 1439
vs sh s vs sh s m m
1482 m 1566 vs
[17] [12,15,17] [12,15] [12,15,17] [15]
Intensities: w = weak, m = medium, s = strong, v = very, sh = shoulder, br = broad. Assignments: v = stretch, /~ = deformation, /3 = inplane bend, X = out-of-plane bend, to = wag, ¢ = torsion, s = symmetric, as = antisymmetric.
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
the enhanced bands at 864 and 1270 cm-1 (Fig. 2) are determined by in-plane vibrations. This adsorption state will further be defined as "adsorption state A". It should be mentioned that the potential-dependent spectral changes, described in this and following Sections 3.3-3.5, are totally reversible under changes of potential. 3.3. SERS spectra of L-histidine at a copper electrode in acidic solutions (pH 3.1) SERS spectra of histidine obtained at pH 3.1 in the E range of the copper electrode from 0.15 to - 0 . 8 V are shown in Fig. 3. The predominant ionic form of histidine in a solution of pH 3.1 is H3His ÷ (95%). As can be seen from the data in Fig. 3, the SERS spectrum of histidine at E = 0.15 V and pH 3.1 is very much like the spectrum obtained at pH 1.2 (Fig. 2). Hence there are grounds to believe that the H3His ÷ form adsorbs on the copper surface through the protonated imidazole ring as does the H4His 2+ form (adsorption state A). The above conclusions are supported by the SERS spectrum of histidine in the D20-based solution of pH 3.1, i.e., the bands characteristic for the deuterated imidazolium ring at 1405 and 1600 cm -1 (Table 1) were well-defined in the SERS spectrum at E = 0.15 V.
H N,~+~N.H:C~ il
..-
When the copper electrode potential is changed from 0.15 to - 0 . 2 and - 0 . 8 V, the inverse relationship between the spectrum intensity and E is observed when compared to the data at pH 1.2. The SERS spectra become very intensive and undergo considerable changes at negative E. For instance, new bands appear at 1109, 1151, 1231, 1319 and 1574 cm -1, the band at 1200 cm -1 disappears and the intensity of the band at 1636 cm -z decreases substantially at E = - 0 . 8 V (Fig. 3). The experiments with the DzO-based solutions have shown that SERS spectra of the deuterated histidine at negative potentials and pH 3.1 possess additional bands at 1320 and 1570 cm -1, whereas the bands at 1410 and 1600 cm -~, both characteristic for the N-deuterated imidazolium ring (Table 1), disappear. An important point is that the new bands listed have their counterparts in the solution Raman spectrum at pH 11.9 (see Table 1). This clearly demonstrates that, when the electrode potential becomes more negative, the deprotonation of the imidazolium ring proceeds at the surface and histidine adsorbs through the neutral imidazole group, even though the pH of the solution equals 3.1. Moreover, the presence of the intensive bands at 1151 and 1574 c m - ' , which are associated with the Im 8(R) + / 3 ( N - H ) and Im z,(R) + / 3 ( N - H ) vibrations, respectively, allows to suppose that the N - H bond of imidazole is close to the copper surface.
Kc,--CW-C.,-CH-COO" .',;.;,.. '.:
H.C,=-- C , H - - C H - - C H-- C O O H /,. .......,~ ~ I
N H a+
H
H 4 H i s =÷
c
-?.-eoo
H3His"
c.w-
CHTCH-COO
H N,',~C~ N, : H
H2His
B
°
HHis-
HC~-- C ~ l l - OHm- C H - COO" /, ......,, I
: N,~, - ,:' %c=/N,: n
275
NH 2 His'-
Fig. 1. Structures of the ionic forms of histidine.
N H2
276
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
~.
I
1900
I
1700
I
1500
~
I
1300
-o.2v
I
11 O0
900
I
700
500 cm"
Fig. 2. SERS spectra of 0.02 M L-histidineon a copper electrode at three different electrode potentials in H20-based acidic solution (0.1 M H2SO4, pH 1.2).
c m - 1 is much more intensive than that in the solution spectrum). Second, the SERS band at 1574 c m - 1 has a high degree of the 1598 c m - 1 component. At E = - 0.2 V the characteristic bands of the neutral imidazole ring are less intensive, whereas the bands at 1598 and 1350 c m - 1 are strongly enhanced. Most likely these two bands refer to the ~(NH 2) and
However, the SERS spectrum of histidine at E =
- 0 . 8 V and the Raman spectrum of the amino acid solution at pH 11.9 are not completely identical. First, the relative intensity of the above-mentioned bands is different (e.g., the SERS band at 1264 cm -1 is very intensive, whereas in the solution spectrum it splits into two components; the SERS band at 1491
00
~ to
~f~.
IV
"
o'J
V
° o
,,...v k,~ ,,
I
1900
1700
I
1500
I
1300
I
11 O0
I
900
I
700
500 cm ~
Fig. 3. SERS spectra of 0.02 M L-histidineon a copper electrode at three different electrode potentials in H20-based acidic solution (0.1 M Na2SO4, pH 3.1).
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
~"
I
1900
1700
I
1500
I
1300
I
1100
277
~
A
I
900
I
700
I
I
500 300
200 am"
Fig. 4. SERS spectra of 0.02 M L-histidine on a copper electrode at two different electrode potentials at pH 3.1. (A,B) Amino acid in H20-based 0.1 M Na2SO 4 solution containing 0.1 M NaCI; (C) amino acid in D20-based 0.1 M Na2SO 4 solution containing 0.1 M NaCI.
to(CH 2) vibrations (see Table 1), respectively, suggesting that histidine at this potential is adsorbed through the deprotonated a-NH 2 group (adsorption state B). Strong enhancement of the neutral imidazole bands at E = - 0 . 8 V enables to conclude that at this potential value histidine adsorbs through the neutral imidazole ring and a-NH 2 group (adsorption
1900
i 1700
I 1500
I 1300
state C). However, it is not unlikely that two adsorption states - adsorption state B and the adsorption only through the neutral imidazole ring - reveal themselves on the surface at E = - 0 . 8 V. The addition of the CI- ions to the histidine solution of pH 3.1 causes different changes in the SERS spectra (Fig. 4) when compared to the spectra
I 1100
I 900
I 700
,500 c m ~
Fig. 5. SERS spectra of 0.02 M L-histidine on a copper electrode at three different electrode potentials in H20-based neutral solution (0.1 M Na2SO4, pH 7.0).
S. Martusevi6ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
278
obtained at pH 1.2. At E = - 0 . 2 V the SO42- ions desorb and the CI- ions adsorb on the surface (the u(Cu-CI) band at 290 cm -1 is clearly defined). However, the characteristic SERS bands of the adsorbed histidine at 1350 and 1598 cm-1 disappear, indicating the desorption of the a-NH 2 group from the surface. The appearance of the band at 1413 cm -1 may be associated with the participation of COO- in the adsorption process. At the more negative potential ( - 0.8 V) sulfate and chloride ions are desorbed from the copper surface, intensities of the SERS bands of histidine are substantially increased, and the relative changes of the intensity in comparison with the SERS spectrum without CI- ions are observed. For instance, the ratio 11264//11320 equals to 2.9 in the presence of CI- ions, whereas without C1- it is equal to 1.5. The band at 1010 cm -1 is much broader and has a shoulder at 1028 cm-1 that could be assigned to the ~,(C-N) vibrational mode. The new band at 1542 cm -t also appears in the SERS spectrum when CI- ions are present. From the data of the IR spectrum of histidine this band may be assigned to the vS(NH~) vibrational mode [21]. This assignment is supported by the fact that in the SERS spectrum of the D20-based solution (Fig. 4) this band is absent and a new weak band appears at 1160-1180 c m - 1 {~,S(ND~-) [21]}. From these data
to
the assumption may be made that the geometry of the adsorbed histidine in the presence of CI- ions changes, the desorbed a-NH 2 group becomes protonated and forms the ion pair NH~ • • • C1- at significantly negative potentials of the copper electrode. Most probably this ion pair is oriented with its positively charged end towards the surface.
3.4. SERS spectra of L-histidine in neutral solution (pH 7.0) The SERS spectra of histidine in the N a 2 S O 4solution of pH 7.0 (Fig. 5) undergo significant changes when compared to the spectra in the acidic solutions. The predominant form of histidine in a solution of pH 7.0 is HzHis ° (90%). As evident from Fig. 5, the SERS spectrum of histidine is very intensive at a potential of - 0 . 2 V, however, its intensity substantially decreases at more negative potentials. The intensive band at 1641 c m - 1 could be assigned at a first glance to the protonated imidazole group. However, this assumption must be ruled out, since this band is conserved in the SERS spectrum obtained from the DEO-based solution (Fig. 6). What is more, if the latter is the case, the characteristic bands of the N-deuterated imidazolium group at 1400 and 1600 c m - 1 a r e also missing (see based
,~.
v
v -0.8V I
1900
1700
I
1500
I
1300
I
1100
I
900
I
700
500cm +
Fig. 6. SERS spectra of 0.02 M L-histidine on a copper electrode at three different electrode potentials in DzO-based neutral solution (0.1 M Na2SO4, pH 7.0).
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
state D into adsorption state C at E < - 0 . 2 V and pH 7.0. Interestingly, the introduction of CI- ions into the solution of pH 7.0 at E = - 0 . 2 V (C1- ions adsorbed on the surface), as well as at more negative potentials (C1- ions desorbed from the surface), does not induce changes of the histidine spectral features.
Table 1 and Fig. 6). It is seen from Fig. 5 that the bands involving the in-plane vibration of the N - H bond at 1165 and 1573 cm -1 are rather weak, while the band associated with the in-plane vibration of C2-H bond at 1288 cm -1 is very intensive. This suggests that the imidazole ring of histidine is adsorbed through the deprotonated nitrogen atom. Besides, intensive bands at 1345 and 1381 cm -1 (in the H20-based solution, Fig. 5), and at 1334 and 1383 cm -1 (in the D20-based solution, Fig. 6) are also present in the SERS spectrum of histidine at E = - 0 . 2 V. It seems reasonable to assign the bands at 1381 and 1383 cm -1 to the ~s(COO-) vibrational mode, whereas the lower-frequency bands may be attributed to the wagging mode of the CH 2 group (1355 cm-1 in the H20-based solution spectra at pH 11.9, see Table 1). These facts suggest that the carboxylate group at pH 7.0 also participates in the anchoring of histidine to the copper surface. We denote this adsorption state as adsorption state D. When employing more negative potentials ( - 0.6 and - 0 . 8 V), the SERS spectra of histidine become much weaker ( Figs. 5 and 6). Moreover, they closely resemble the spectra obtained in the solutions of pH 3.1 at the same electrode potential ( Figs. 3 and 4), suggesting the transformation of adsorption
I
1900
1700
I
1500
I
1300
279
3.5. SERS spectra of L-histidine in alkaline solution (pHil.9) The predominant form of histidine in a solution of pH 11.9 is HHis- (99%). The histidine SERS spectra obtained in the NaOH-based solution at this pH value are given in Fig. 7. When the potential of the copper electrode is changed from - 0.6 to - 1.0 V, the intensity of the spectrum does not change significantly. The most distinct bands at 1009, 1270, 1320 and 1572 cm -1 correspond to the vibrations of the neutral imidazole ring (Table 1). The intensity of the band of the Im (ring breathing) vibrational mode at 1320 cm -1 increases when the electrode potential becomes more negative; this is also true for the ratio 11320///1270, where the band at 1270 cm-1 corresponds to the Im fl(C2-H) vibrational mode. It is not unlikely that
I
1100
I
900
I
700
500 cm "1
Fig. 7. SERS spectra of 0.02 M L-histidine on a copper electrode at three different electrode potentials in H20-based basic solution (0.1 M NaOH, pH 11.9).
280
S. Martusevi6ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
these results are related to the change of the adsorbate orientation with respect to the surface. However, from the data of Fig. 7 it is difficult to draw conclusions about the participation of the a-NH 2 and a - C O O - groups in the anchoring of histidine to the surface.
(4) The adsorption of histidine is most likely to occur through the neutral imidazole ring provided that the amino acid is dissolved in the NaOH-based solution at pH 11.9.
References 4. Conclusions In conclusion, the results of the SERS studies of L-histidine adsorption on the copper surface may be summarized as follows: (1) The imidazole ring of histidine adsorbed on the copper surface remains protonated and is not involved in the chemical coordination with the surface, providing the adsorption process takes place from the H2SO4-based solution at pH 1.2. The plane of the imidazolium ring is mainly oriented perpendicularly to the surface (adsorption state A). The sulfate and chloride ions compete with histidine for the adsorption sites. (2) Three potential-dependent adsorption states of histidine are observed, if the adsorption process takes place from the NazSO4-based solution at pH 3.1. Adsorption state A is realized on the copper surface at E > - 0 . 2 V. At E = - 0 . 2 V histidine adsorbs through the a-NH 2 group (adsorption state B), whereas at E = - 0 . 8 V the amino acid adsorbs through the neutral imidazole ring and a-NH 2 group (adsorption state C). The adsorbed C1- ions cause detachment of the a-NH 2 group from the surface and formation of the ion pair NH~ . . . C1- is observed at E < - 0 . 6 V. (3) Strong adsorption of histidine proceeds through the deprotonated nitrogen atom of the imidazole ring and a - C O O - group (adsorption state D) provided that the amino acid is dissolved in the neutral Na2SO4-based solution and E > - 0 . 2 V. Adsorption state D is transformed into adsorption state C at more negative potentials of the copper electrode. The presence of CI- ions does not cause significant changes in the SERS spectra.
[1] G. Hue, O. Dai and S. Jiang, J. Am. Chem. Soc., 110 (1988) 2393. [2] G. Hue, J. Ding, P. Wu and G. Ji, J. Electroanal. Chem., 270 (1989) 162. [3] S. Yoshida and H. Ishido, J. Chem. Phys., 78 (1983) 6960. [4] J. Bukowska, A. Kudelski and K. Jackowska, J. Electroanal. Chem., 309 (1991) 251. [5] J.S. Suh and M. Moskovits, J. Am. Chem. Soc., 108 (1986) 4711. [6] S.K. Kim, M.S. Kim and S.W. Suh, J. Raman Spectrosc., 18 (1987) 171. [7] I.R. Nabiev, V.A. Savchenko and E.S. Efremov, J. Raman Spectrosc., 14 (1983) 375. [8] M. Picquart, G. Lacrampe and M. Jafraim, Spectroscopy of Biological Molecules, Wiley, Chichester, 1985, p. 190. [9] V.J. Razumas, J.D. Kazlauskaite, G.-J.A. Vidugiris and J.J. Kulys, Bioorgan. Khim., 15 (1989) 40 (in Russian). [10] G. Niaura and A. Malinauskas, Chem. Phys. Lett., 207 (1993) 455. [11] D. Gaffinkel and J.T. Edsall, J. Am. Chem. Soc., 80 (1958) 3807. [12] D.S. Caswell and T.G. Spiro, J. Am. Chem. Soc., 108 (1986) 6470. [13] M. Tasumi, I. Harada and T. Takamatsu, J. Raman Spectrosc., 12 (1982) 149. [14] H. Takeuchi, Y. Kimura, I. Koitabashi and I. Harada, J. Raman Spectrosc., 22 (1991) 233. [15] F.R. Dollish, W.G. Fateley and F.F. Bentley, Characteristic Raman Frequencies of Organic Compounds, Wiley, New York, 1974. [16] M. Takeda, R. Ivazzo and D. Garfinkel, J. Am. Chem. Soc., 80 (1958) 3813. [17] S.A. Asher and J.L. Murtaugh, Appl. Spectrosc., 42 (1988) 83. [18] T.M. Herne, A.M. Ahem and R.L. Garrell, J. Am. Chem. Soc., 113 (1991) 846. [19] N. Tanaka, H. Kitano and N. Ise, J. Phys. Chem., 94 (1990) 6290. [20] G. Niaura and A. Malinauskas, Chemija, 4 (1991) 14. [21] S. Pinchas and I. Laulicht, Infrared Spectra of Labelled Compounds, Academic Press, London, 1971, p. 155.
,
L . .
J~
•
,.
VIBRATIONAL SPECTROSCOPY
,."
ELSEVIER
Vibrational Spectroscopy 10 (1996) 271-280
Adsorption of L-histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy Saulius Martusevi~ius a, Gediminas Niaura b, Zita Talaikyt6 Valdemaras Razumas a
a, *
a Institute of Biochemistry, Mokslininlat 12, 2600 Vilnius, Lithuania b Institute of Chemistry, A. GoJtauto 9, 2600 Vilnius, Lithuania
Received 1 January 1995
Abstract The adsorption of L-histidine on a copper electrode from H20-and D20-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO2- and C1- ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO2 - and C l ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than - 0 . 2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the a - N n 2 group and the neutral imidazole ring. The C1- ions cause the protonation and detachment of the a-NH 2 group from the surface and the formation of the ion pair NH~- • • • C l can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the a - C O O - group at E >_ - 0 . 2 V. However, this adsorption state is transformed into the adsorption state in which the a-NH 2 group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring. Keywords: Adsorption; Raman spectrometry; Raman scattering, surface enhanced; Histidine; Copper electrode
I. Introduction L-Histidine, one of the natural amino acids, plays an important role in various biological processes. There are numerous indications of the significance of
* Corresponding author.
histidine in enzyme action and in the binding of metal ions to proteins. The imidazole group of histidine is ionized at physiological pH values, thus playing a major role in the buffer action of proteins. In the context of investigations of protein adsorption on different surfaces it is of interest to obtain information about the interfacial behavior of L-histidine per se. Surface-enhanced Raman scattering (SERS) spec-
0924-2031/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0924-2031(95)00025-9
272
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
troscopy appears to be a powerful tool in the investigation of the adsorption behavior of molecules on the surfaces of SERS-active metals because the vibrations of molecules may change significantly at solid-liquid interfaces. It is possible to determine the geometry of the adsorbate and the groups being bonded to the surface. The groups that are directly involved in adsorption in general could be recognized by the sufficiently large A v value (difference in frequencies between SERS and solution Raman bands) and also by enhancement of the intensity AI (the ratio of the intensity of the SERS band to the intensity of the same band in the solution spectrum). The latter parameter increases most significantly if the vibrational mode of the adsorbate has a large polarizability component perpendicular to the surface. There are grounds to believe that the imidazole group of histidine is highly surface active. For instance, imidazole can react with colloidal silver forming polymeric surface complexes [1]. The formation of the polymeric film on a copper electrode immersed in a benzimidazole solution was also reported [2]. The adsorption of imidazole and its 3-alkyl derivatives on copper was studied by Fourier transform infrared (FI'-IR) spectroscopy [3]. The adsorption of imidazole on a roughened polycrystalline silver electrode has been studied by SERS spectroscopy [4]. The SERS results indicated that different forms of imidazole (anionic, neutral and probably ylide form) can be observed at the Ag surface. The relative amounts of these forms depend on the electrode potential and on the pH of the solution. On the other hand, from the data of SERS spectra of aliphatic amino acids and dipeptides it was established that they may adsorb on colloidal silver through the a-carboxylic and possibly a-NH 2 groups [5,6]. SERS spectra of histidine adsorbed on colloidal silver were presented in Refs. [7-9], but the results were not discussed in sufficient detail. In the present work the dependence upon the pH of the solution and electrode potential over the interval from - 1 . 0 to 0.15 V (versus the normal hydrogen electrode) of the adsorption of L-histidine on a polycrystalline copper electrode was studied. As far as we know, the copper electrode and histidine do
not exhibit electrochemical activity over this potential range. Thus, the effect of potential on the SERS spectra of histidine, if any, should be determined by the adsorptive behaviour of the amino acid.
2. Experimental 2.1. Reagents and solutions
Sulfuric acid of chemically pure grade was doubly distilled at 240°C. Sodium sulfate, chemically pure, was recrystallized once from triply distilled water and annealed at 350°C. L-Histidine (Reanal, Hungary) was recrystallized twice from triply distilled water. For deuteration experiments, 99.8% D20 was used as received from Sigma. Raman and SERS spectra of 0.2 and 0.02 M L-histidine, respectively, were measured in the triply distilled water-or D20-based solutions of 0.1 M H2SO 4 (pH 1.2), 0.1 M Na2SO 4 (pH 3.1 and 7.0) or 0.1 M NaOH (pH 11.9). The pH values of the NazSOa-based solutions were attained by using H 2 S O 4 or NaOH. The pH values presented in the text constitute the function of the pH glass electrode. In specific cases 0.1 M NaCI was added to the standard histidine solution. The Raman spectrum of the 0.02 M L-histidine solution does not differ from that of the background. Before use all solutions were deoxygenated by bubbling ultra-pure argon for about lh. 2.2. Equipment and procedures
Raman spectra of solutions were recorded in a rectangular quartz cell. SERS measurements were performed in a cylindrical three-electrode electrochemical cell. The counter electrode was a Pt wire. Its compartment was separated from the working electrode compartment by glass frits. The SERS-active copper electrode (geometric surface area of 0.07 cm 2) was prepared according to the previously described method [10]. The surface of the working electrode was placed at 5 mm distance from the cell window. The potential of the working electrode was measured versus the saturated Ag/AgC1 reference electrode supplied with a Luggin capillary to minimize the influence of the
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
CI- ions on the adsorption of L-histidine at the copper electrode. The potential of the working electrode was controlled using an OH-105 polarograph (Radelkis, Hungary). All potentials in the text are given versus the normal hydrogen electrode (NHE). An argon laser was used for solution Raman excitation at 488 nm (power approx. 40 mW). The 632.8-nm line of the H e - N e laser was used in the SERS experiments as the excitation source with approx. 10 mW power at the sample. The laser beam was incident on the surface at an angle of 61 ° with respect to the surface normal and focused to a line with a cross-area of approx. 1 mm 2. The scattered Raman light was analyzed using a double monochromator with 1200 lines mm-1 gratings and detected by a cooled photomultiplier and a photon-counting system. The instrumental parameters were as follows: spectral slit width S = 5 cm -1, integration time ~-= 0.5 s.
3. Results and discussion
3.1. Raman spectra Of L-histidine solutions To gain a better insight into the SERS spectra of the adsorbed histidine, in the first stage of the investigations the Raman spectra of the amino acid solutions have been studied at three different pH values: 1.2, 3.1 and 11.9. The Raman spectra were recorded in H 2 0 - a n d D20-based solutions. The band frequencies observed and possible assignments of the vibrational modes on the basis of the literature data [11-19] are listed in Table 1. In aqueous solutions, depending on the pH, histidine may exist in five ionic forms: p K a = 1.8
H4His 2+
~
p K a = 6.0
H3His +
~
COOH pK a=9.1
'~" NH~
H2His °
Im pK a= 14
HHis-
~
His 2-
(1)
Im
where COOH, Im and NH~- denote the a-carboxylic, imidazole and a-amino groups, respectively. The structures of the corresponding forms presented in Eq. 1 are shown in Fig. 1. According to the given p K a values of histidine
273
(Eq. 1), the amino acid solutions at pH 1.2, 3.1 and 11.9 comprise approximately 80% of H4His 2÷ and 20% of H3His +, 5% of H4His 2+ and 95% of H3His +, and 99% of H H i s - and 1% of His 2-, respectively. Hence the data of Table 1 reveal that the bands most characteristic for the positively charged imidazolium ring in the forms of HaHis 2+ and H3His + are located, on the average, at 1194, 1490 and 1630 cm -1 (for the N-deuterated imidazolium ring at 1110, 1408 and 1602 cm-1). On the other hand, the neutral imidazole ring in HHis- is characterized by the presence of strong bands at 1158, 1230, 1320 and 1570 cm -1.
3.2. SERS spectra of L-histidine at a copper electrode in acidic solutions (pH 1.2) SERS spectra of histidine obtained at pH 1.2 in the potential (E) range of the copper electrode from 0.15 to - 0 . 4 V (vs. NHE) are shown in Fig. 2. As noted above, the predominant (80%) ionic form of histidine at this pH is HaHis e÷ (Eq. 1, Fig. 1). First, it is worth noting that the pronounced intensive band at 972 cm -1 is assigned to the totally symmetric vibration of the adsorbed SO 2- ions on the copper surface [20], whereas the others should be assigned to the vibrations of the adsorbed histidine. When E becomes more negative the intensity of the SO 2- band increases, as the intensity of the histidine bands decreases, indicating the desorption of histidine from the surface. The change of intensity of the 972 cm -1 mode of adsorbed SO42- on the copper surface was studied earlier and a parabolic curve of intensity versus potential was observed with a maximum at - 0 . 4 V in 0.1 M n 2 s o 4 solution [20]. From the data of Fig. 2 it is evident that at E > - 0.4 V the histidine ions compete in the adsorption with the SO42- ions, whereas the latter anions are the main adsorbable species at E = - 0 . 4 V. To confirm the assumption of competitive adsorption between the anions and histidine the experiments were performed using chloride ions as additional anions in the solution. The addition of 0.1 M NaCI to the standard histidine solution causes complete disappearance of the SO 2- and histidine bands, regardless of the electrode potential. However, the ~,(Cu-C1) band at 290 cm-1 was clearly defined in all spectra.
S. Martusevirius et al. / Vibrational Spectroscopy 10 (1996) 271-280
274
These results prove conclusively the above-mentioned assumption. The comparison of the frequencies of the adsorbed histidine vibrations (Fig. 2) with those observed in the solution Raman spectrum at pH 1.2 (Table 1) shows that A v of the most prominent bands is small. The same is observed when SERS and Raman spectra obtained in D20-based solutions were compared. The frequencies of the intensive imidazolium ring vibrations - v ( N 1 - C 2 - N 3 ) + 8 ( N - D ) at 1410 cm -1 and v(C4-C 5) at 1603 cm -1
- and the width values at half maximum of these bands in SERS and solution Raman spectra were in close agreement. These results indicate that the imidazole ring of the adsorbed histidine remains protonated and the nitrogen atoms of the heterocycle are not involved in the chemical coordination with the copper surface. If the assumption is made that the electromagnetic mechanism is mainly responsible for the intensity enhancement in this system, it is likely that the plane of the imidazolium ring of H4His 2+ is mainly oriented perpendicularly to the surface, since
Table 1 Raman frequencies (cm -1 ) of histidine in the H20-and D20-based solutions of different pH pH 1.2
pH 3.1
pH 11.9
Assignment
Ref.
H20
D20
HzO
D20
H20
D20
517 w
506 w 595 v w 637 m
518 v w
509 m 597 m 628 m
528 m
522 m
8 ( C O O - ), ¢(NH 2)
[151
621 661 717 769 811
618 663 719 773 814 835 866 919
Im %(R) 6 ( C O O - ) , Im x ( R ) l m (ring breathing) Im x ( C - H ) Im (ring breathing) Im / / ( C - H ) Im fl(R), v ( C - C ) Im x(R), v ( C - C O O - ) Im ~(R) v(C-C) 8(C-CH) Im 8(R) v(C-N) Im 8(R) + / 3 ( C 2 - H ) ~(C-H) Im ~5(R) + 13(C5-H) , 8 ( N - D ) + v(N1C2N3) Im 8(R) + f l ( N l - n ) Im 8(R) Im (ring breathing) Im /3(C2-H) Im 8(R) + / 3 ( C 2 - H ) Im (ring breathing) to(Ell 2)
[15] [15,16] [15] [15] [15] [15] [16] [17,18] [12] [15] [19] [12] [15,16] [15,16] [15] [12,13,15,17] [12,15,17]
v(NIC2N3) + tS(ND), v s ( C O O - ) 8(CH2), 8 ( N ~ - H )
[13-16,18] [12,15,17,18]
Im v(R) + / 3 ( C 2 - H ) Im v(g) + f l ( N 1 - H ) v(C 4 =C5), 8(NH2),Vas(COO-) v(C=O)
[12,15,17] [11,15,17] [11,14,151 [15]
633 664 700 770 812
vw sh vw sh m
854 m 917 m
705 w 773 v w
860 sh 911 m
964 sh
627 664 707 775 812
m vw w w w
854 m 918 m
860 w 903 w
963 sh
975 sh 1004 s 1040 w 1074 vw
993 m
999 m
992 m
1063 sh 1090 w
1074 vw
1070 sh 1092 m 1108 vs
1112 vs 1194 vs
704 w 778 sh 812 s
1104 m
m m w vw m
856 m 918 v w 935 w 962 w 989 m 1008 sh 1044 v w 1068 sh 1088 sh 1096 s 1158 s
m m w vw sh s m vw
994 s 1012 s 1064 w
1168 sh
1194 vs
1270 vs
1264 vs
1270 vs
1254 m 1271 w
1363 w
1357 m
1329 sh 1360 m
1321 w 1355 w
1233 1265 1284 1321 1355
s s vs vs vs
1438 w
1410 vs 1443 w
1406 m 1441 m
1405 vs 1441 m
1407 s 1442 vs
1491 vs
1477 w
1489 vs
1631 vs 1734 m
1603 vs 1734 m
1629 vs
1476 v w 1562 w 1600 vs
1491 s 1571 vs 1637 s,br
1230 1258 1274 1317 1349 1373 1407 1439
vs sh s vs sh s m m
1482 m 1566 vs
[17] [12,15,17] [12,15] [12,15,17] [15]
Intensities: w = weak, m = medium, s = strong, v = very, sh = shoulder, br = broad. Assignments: v = stretch, /~ = deformation, /3 = inplane bend, X = out-of-plane bend, to = wag, ¢ = torsion, s = symmetric, as = antisymmetric.
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
the enhanced bands at 864 and 1270 cm-1 (Fig. 2) are determined by in-plane vibrations. This adsorption state will further be defined as "adsorption state A". It should be mentioned that the potential-dependent spectral changes, described in this and following Sections 3.3-3.5, are totally reversible under changes of potential. 3.3. SERS spectra of L-histidine at a copper electrode in acidic solutions (pH 3.1) SERS spectra of histidine obtained at pH 3.1 in the E range of the copper electrode from 0.15 to - 0 . 8 V are shown in Fig. 3. The predominant ionic form of histidine in a solution of pH 3.1 is H3His ÷ (95%). As can be seen from the data in Fig. 3, the SERS spectrum of histidine at E = 0.15 V and pH 3.1 is very much like the spectrum obtained at pH 1.2 (Fig. 2). Hence there are grounds to believe that the H3His ÷ form adsorbs on the copper surface through the protonated imidazole ring as does the H4His 2+ form (adsorption state A). The above conclusions are supported by the SERS spectrum of histidine in the D20-based solution of pH 3.1, i.e., the bands characteristic for the deuterated imidazolium ring at 1405 and 1600 cm -1 (Table 1) were well-defined in the SERS spectrum at E = 0.15 V.
H N,~+~N.H:C~ il
..-
When the copper electrode potential is changed from 0.15 to - 0 . 2 and - 0 . 8 V, the inverse relationship between the spectrum intensity and E is observed when compared to the data at pH 1.2. The SERS spectra become very intensive and undergo considerable changes at negative E. For instance, new bands appear at 1109, 1151, 1231, 1319 and 1574 cm -1, the band at 1200 cm -1 disappears and the intensity of the band at 1636 cm -z decreases substantially at E = - 0 . 8 V (Fig. 3). The experiments with the DzO-based solutions have shown that SERS spectra of the deuterated histidine at negative potentials and pH 3.1 possess additional bands at 1320 and 1570 cm -1, whereas the bands at 1410 and 1600 cm -~, both characteristic for the N-deuterated imidazolium ring (Table 1), disappear. An important point is that the new bands listed have their counterparts in the solution Raman spectrum at pH 11.9 (see Table 1). This clearly demonstrates that, when the electrode potential becomes more negative, the deprotonation of the imidazolium ring proceeds at the surface and histidine adsorbs through the neutral imidazole group, even though the pH of the solution equals 3.1. Moreover, the presence of the intensive bands at 1151 and 1574 c m - ' , which are associated with the Im 8(R) + / 3 ( N - H ) and Im z,(R) + / 3 ( N - H ) vibrations, respectively, allows to suppose that the N - H bond of imidazole is close to the copper surface.
Kc,--CW-C.,-CH-COO" .',;.;,.. '.:
H.C,=-- C , H - - C H - - C H-- C O O H /,. .......,~ ~ I
N H a+
H
H 4 H i s =÷
c
-?.-eoo
H3His"
c.w-
CHTCH-COO
H N,',~C~ N, : H
H2His
B
°
HHis-
HC~-- C ~ l l - OHm- C H - COO" /, ......,, I
: N,~, - ,:' %c=/N,: n
275
NH 2 His'-
Fig. 1. Structures of the ionic forms of histidine.
N H2
276
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
~.
I
1900
I
1700
I
1500
~
I
1300
-o.2v
I
11 O0
900
I
700
500 cm"
Fig. 2. SERS spectra of 0.02 M L-histidineon a copper electrode at three different electrode potentials in H20-based acidic solution (0.1 M H2SO4, pH 1.2).
c m - 1 is much more intensive than that in the solution spectrum). Second, the SERS band at 1574 c m - 1 has a high degree of the 1598 c m - 1 component. At E = - 0.2 V the characteristic bands of the neutral imidazole ring are less intensive, whereas the bands at 1598 and 1350 c m - 1 are strongly enhanced. Most likely these two bands refer to the ~(NH 2) and
However, the SERS spectrum of histidine at E =
- 0 . 8 V and the Raman spectrum of the amino acid solution at pH 11.9 are not completely identical. First, the relative intensity of the above-mentioned bands is different (e.g., the SERS band at 1264 cm -1 is very intensive, whereas in the solution spectrum it splits into two components; the SERS band at 1491
00
~ to
~f~.
IV
"
o'J
V
° o
,,...v k,~ ,,
I
1900
1700
I
1500
I
1300
I
11 O0
I
900
I
700
500 cm ~
Fig. 3. SERS spectra of 0.02 M L-histidineon a copper electrode at three different electrode potentials in H20-based acidic solution (0.1 M Na2SO4, pH 3.1).
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
~"
I
1900
1700
I
1500
I
1300
I
1100
277
~
A
I
900
I
700
I
I
500 300
200 am"
Fig. 4. SERS spectra of 0.02 M L-histidine on a copper electrode at two different electrode potentials at pH 3.1. (A,B) Amino acid in H20-based 0.1 M Na2SO 4 solution containing 0.1 M NaCI; (C) amino acid in D20-based 0.1 M Na2SO 4 solution containing 0.1 M NaCI.
to(CH 2) vibrations (see Table 1), respectively, suggesting that histidine at this potential is adsorbed through the deprotonated a-NH 2 group (adsorption state B). Strong enhancement of the neutral imidazole bands at E = - 0 . 8 V enables to conclude that at this potential value histidine adsorbs through the neutral imidazole ring and a-NH 2 group (adsorption
1900
i 1700
I 1500
I 1300
state C). However, it is not unlikely that two adsorption states - adsorption state B and the adsorption only through the neutral imidazole ring - reveal themselves on the surface at E = - 0 . 8 V. The addition of the CI- ions to the histidine solution of pH 3.1 causes different changes in the SERS spectra (Fig. 4) when compared to the spectra
I 1100
I 900
I 700
,500 c m ~
Fig. 5. SERS spectra of 0.02 M L-histidine on a copper electrode at three different electrode potentials in H20-based neutral solution (0.1 M Na2SO4, pH 7.0).
S. Martusevi6ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
278
obtained at pH 1.2. At E = - 0 . 2 V the SO42- ions desorb and the CI- ions adsorb on the surface (the u(Cu-CI) band at 290 cm -1 is clearly defined). However, the characteristic SERS bands of the adsorbed histidine at 1350 and 1598 cm-1 disappear, indicating the desorption of the a-NH 2 group from the surface. The appearance of the band at 1413 cm -1 may be associated with the participation of COO- in the adsorption process. At the more negative potential ( - 0.8 V) sulfate and chloride ions are desorbed from the copper surface, intensities of the SERS bands of histidine are substantially increased, and the relative changes of the intensity in comparison with the SERS spectrum without CI- ions are observed. For instance, the ratio 11264//11320 equals to 2.9 in the presence of CI- ions, whereas without C1- it is equal to 1.5. The band at 1010 cm -1 is much broader and has a shoulder at 1028 cm-1 that could be assigned to the ~,(C-N) vibrational mode. The new band at 1542 cm -t also appears in the SERS spectrum when CI- ions are present. From the data of the IR spectrum of histidine this band may be assigned to the vS(NH~) vibrational mode [21]. This assignment is supported by the fact that in the SERS spectrum of the D20-based solution (Fig. 4) this band is absent and a new weak band appears at 1160-1180 c m - 1 {~,S(ND~-) [21]}. From these data
to
the assumption may be made that the geometry of the adsorbed histidine in the presence of CI- ions changes, the desorbed a-NH 2 group becomes protonated and forms the ion pair NH~ • • • C1- at significantly negative potentials of the copper electrode. Most probably this ion pair is oriented with its positively charged end towards the surface.
3.4. SERS spectra of L-histidine in neutral solution (pH 7.0) The SERS spectra of histidine in the N a 2 S O 4solution of pH 7.0 (Fig. 5) undergo significant changes when compared to the spectra in the acidic solutions. The predominant form of histidine in a solution of pH 7.0 is HzHis ° (90%). As evident from Fig. 5, the SERS spectrum of histidine is very intensive at a potential of - 0 . 2 V, however, its intensity substantially decreases at more negative potentials. The intensive band at 1641 c m - 1 could be assigned at a first glance to the protonated imidazole group. However, this assumption must be ruled out, since this band is conserved in the SERS spectrum obtained from the DEO-based solution (Fig. 6). What is more, if the latter is the case, the characteristic bands of the N-deuterated imidazolium group at 1400 and 1600 c m - 1 a r e also missing (see based
,~.
v
v -0.8V I
1900
1700
I
1500
I
1300
I
1100
I
900
I
700
500cm +
Fig. 6. SERS spectra of 0.02 M L-histidine on a copper electrode at three different electrode potentials in DzO-based neutral solution (0.1 M Na2SO4, pH 7.0).
S. Martusevi~ius et al. / Vibrational Spectroscopy 10 (1996) 271-280
state D into adsorption state C at E < - 0 . 2 V and pH 7.0. Interestingly, the introduction of CI- ions into the solution of pH 7.0 at E = - 0 . 2 V (C1- ions adsorbed on the surface), as well as at more negative potentials (C1- ions desorbed from the surface), does not induce changes of the histidine spectral features.
Table 1 and Fig. 6). It is seen from Fig. 5 that the bands involving the in-plane vibration of the N - H bond at 1165 and 1573 cm -1 are rather weak, while the band associated with the in-plane vibration of C2-H bond at 1288 cm -1 is very intensive. This suggests that the imidazole ring of histidine is adsorbed through the deprotonated nitrogen atom. Besides, intensive bands at 1345 and 1381 cm -1 (in the H20-based solution, Fig. 5), and at 1334 and 1383 cm -1 (in the D20-based solution, Fig. 6) are also present in the SERS spectrum of histidine at E = - 0 . 2 V. It seems reasonable to assign the bands at 1381 and 1383 cm -1 to the ~s(COO-) vibrational mode, whereas the lower-frequency bands may be attributed to the wagging mode of the CH 2 group (1355 cm-1 in the H20-based solution spectra at pH 11.9, see Table 1). These facts suggest that the carboxylate group at pH 7.0 also participates in the anchoring of histidine to the copper surface. We denote this adsorption state as adsorption state D. When employing more negative potentials ( - 0.6 and - 0 . 8 V), the SERS spectra of histidine become much weaker ( Figs. 5 and 6). Moreover, they closely resemble the spectra obtained in the solutions of pH 3.1 at the same electrode potential ( Figs. 3 and 4), suggesting the transformation of adsorption
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1300
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3.5. SERS spectra of L-histidine in alkaline solution (pHil.9) The predominant form of histidine in a solution of pH 11.9 is HHis- (99%). The histidine SERS spectra obtained in the NaOH-based solution at this pH value are given in Fig. 7. When the potential of the copper electrode is changed from - 0.6 to - 1.0 V, the intensity of the spectrum does not change significantly. The most distinct bands at 1009, 1270, 1320 and 1572 cm -1 correspond to the vibrations of the neutral imidazole ring (Table 1). The intensity of the band of the Im (ring breathing) vibrational mode at 1320 cm -1 increases when the electrode potential becomes more negative; this is also true for the ratio 11320///1270, where the band at 1270 cm-1 corresponds to the Im fl(C2-H) vibrational mode. It is not unlikely that
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500 cm "1
Fig. 7. SERS spectra of 0.02 M L-histidine on a copper electrode at three different electrode potentials in H20-based basic solution (0.1 M NaOH, pH 11.9).
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these results are related to the change of the adsorbate orientation with respect to the surface. However, from the data of Fig. 7 it is difficult to draw conclusions about the participation of the a-NH 2 and a - C O O - groups in the anchoring of histidine to the surface.
(4) The adsorption of histidine is most likely to occur through the neutral imidazole ring provided that the amino acid is dissolved in the NaOH-based solution at pH 11.9.
References 4. Conclusions In conclusion, the results of the SERS studies of L-histidine adsorption on the copper surface may be summarized as follows: (1) The imidazole ring of histidine adsorbed on the copper surface remains protonated and is not involved in the chemical coordination with the surface, providing the adsorption process takes place from the H2SO4-based solution at pH 1.2. The plane of the imidazolium ring is mainly oriented perpendicularly to the surface (adsorption state A). The sulfate and chloride ions compete with histidine for the adsorption sites. (2) Three potential-dependent adsorption states of histidine are observed, if the adsorption process takes place from the NazSO4-based solution at pH 3.1. Adsorption state A is realized on the copper surface at E > - 0 . 2 V. At E = - 0 . 2 V histidine adsorbs through the a-NH 2 group (adsorption state B), whereas at E = - 0 . 8 V the amino acid adsorbs through the neutral imidazole ring and a-NH 2 group (adsorption state C). The adsorbed C1- ions cause detachment of the a-NH 2 group from the surface and formation of the ion pair NH~ . . . C1- is observed at E < - 0 . 6 V. (3) Strong adsorption of histidine proceeds through the deprotonated nitrogen atom of the imidazole ring and a - C O O - group (adsorption state D) provided that the amino acid is dissolved in the neutral Na2SO4-based solution and E > - 0 . 2 V. Adsorption state D is transformed into adsorption state C at more negative potentials of the copper electrode. The presence of CI- ions does not cause significant changes in the SERS spectra.
[1] G. Hue, O. Dai and S. Jiang, J. Am. Chem. Soc., 110 (1988) 2393. [2] G. Hue, J. Ding, P. Wu and G. Ji, J. Electroanal. Chem., 270 (1989) 162. [3] S. Yoshida and H. Ishido, J. Chem. Phys., 78 (1983) 6960. [4] J. Bukowska, A. Kudelski and K. Jackowska, J. Electroanal. Chem., 309 (1991) 251. [5] J.S. Suh and M. Moskovits, J. Am. Chem. Soc., 108 (1986) 4711. [6] S.K. Kim, M.S. Kim and S.W. Suh, J. Raman Spectrosc., 18 (1987) 171. [7] I.R. Nabiev, V.A. Savchenko and E.S. Efremov, J. Raman Spectrosc., 14 (1983) 375. [8] M. Picquart, G. Lacrampe and M. Jafraim, Spectroscopy of Biological Molecules, Wiley, Chichester, 1985, p. 190. [9] V.J. Razumas, J.D. Kazlauskaite, G.-J.A. Vidugiris and J.J. Kulys, Bioorgan. Khim., 15 (1989) 40 (in Russian). [10] G. Niaura and A. Malinauskas, Chem. Phys. Lett., 207 (1993) 455. [11] D. Gaffinkel and J.T. Edsall, J. Am. Chem. Soc., 80 (1958) 3807. [12] D.S. Caswell and T.G. Spiro, J. Am. Chem. Soc., 108 (1986) 6470. [13] M. Tasumi, I. Harada and T. Takamatsu, J. Raman Spectrosc., 12 (1982) 149. [14] H. Takeuchi, Y. Kimura, I. Koitabashi and I. Harada, J. Raman Spectrosc., 22 (1991) 233. [15] F.R. Dollish, W.G. Fateley and F.F. Bentley, Characteristic Raman Frequencies of Organic Compounds, Wiley, New York, 1974. [16] M. Takeda, R. Ivazzo and D. Garfinkel, J. Am. Chem. Soc., 80 (1958) 3813. [17] S.A. Asher and J.L. Murtaugh, Appl. Spectrosc., 42 (1988) 83. [18] T.M. Herne, A.M. Ahem and R.L. Garrell, J. Am. Chem. Soc., 113 (1991) 846. [19] N. Tanaka, H. Kitano and N. Ise, J. Phys. Chem., 94 (1990) 6290. [20] G. Niaura and A. Malinauskas, Chemija, 4 (1991) 14. [21] S. Pinchas and I. Laulicht, Infrared Spectra of Labelled Compounds, Academic Press, London, 1971, p. 155.