pH dependent SERS and solvation studies of tyrosine adsorbed on silver colloidal nano particles combined with DFT calculations

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Chemical Physics 340 (2007) 69–78 www.elsevier.com/locate/chemphys

pH dependent SERS and solvation studies of tyrosine adsorbed on silver colloidal nano particles combined with DFT calculations Animesh K. Ojha

*

Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India Received 22 March 2007; accepted 23 July 2007 Available online 8 August 2007

Abstract The surface enhanced Raman spectra of tyrosine in colloidal Ag solution have been recorded over a range of pH. A line shape analysis of the bands at 1359, 1505 and 1577 cm1 was performed between pH 3.5 and 8.5. The variation of spectral linewidth (FWHM) of the band at 1359 cm1 with pH is explained in terms of two mechanisms in solution: (i) the fluctuation of the pH of a microscopic volume in a solution with an overall uniform pH and/or (ii) the role of changing viscosity and solvation at different pH values due to the intermolecular ionic interactions between different charged states of the tyrosine molecule. The blue shift in three bands with increasing pH has been explained in terms of charge transfer between the different charged states of tyrosine and metal ions upon chemisorption. The experimental spectra are compared with ab initio/DFT calculations of vibrational wavenumbers, bond geometries, binding energy and charge distributions obtained by means of Hartree–Fock (HF) analysis, the nonlocal density functional method (BLYP) and the hybrid functional method (B3LYP). Two basis sets, CEP-31G and lanl2DZ, were used for all calculations. Ó 2007 Elsevier B.V. All rights reserved. Keywords: SERS; Tyr–Ag system; Spectral profile; DFT calculations

1. Introduction Tyrosine (Tyr) is a biologically important molecule. The adsorption of biomolecules on metal substrates has been a subject of great interest for researchers due to its broad potential applications. Surface enhanced Raman scattering (SERS) studies of biological molecules are important to understand the nature of metal adsorbate interactions and to provide more insight into the changing chemical structure of adsorbate molecules during adsorption on rough metal surfaces. SERS has become a valuable and sensitive tool for monitoring changes in the chemical structure of the molecule adsorbed on a metal surface [1–10]. The analysis of spectral profiles of SERS bands provides information about spectral parameters, such as: Raman shift, changes in line width

*

Tel.: +91 9733504202. E-mail address: animesh_r1776@rediffmail.com

0301-0104/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.07.042

and variation in Raman intensities. Shifts in vibrational wavenumbers and relative intensities in the spectra of adsorbed molecular species compared to spectra in solution provide information about the average conformation of different parts of the adsorbed molecules to the surface [11]. However, linewidth changes provide information about the dynamics of the adsorbate molecules. Many studies [12–16] have used SERS to investigate the nature of adsorbate–surface interactions. Some theoretical models [8,9] have also been proposed to explain the enhancement mechanisms in SERS. Compion and Kambhampati [10] have given a good review of SERS. In a recent study [12], the normal Raman and SERS spectra of 5-fluorouracil in water as well as in colloidal Ag solution at different pH values were presented. The experimental results are explained in terms of structural stability and vibrational analysis performed by density functional theory (DFT) calculations. Thomas et al. [13] reported SERS and DFT studies of 5 amino tetrazole (5AT) in a colloidal silver solution at pH 9. SERS band positions and

A.K. Ojha / Chemical Physics 340 (2007) 69–78

intensities have been discussed on the basis of DFT calculations. The significant change between SERS and normal spectra, combined with the theoretical data for Ag–5AT complexes describe the adsorption of 5AT on Ag colloidal particles through the interactions of lone pair electrons of the nitrogen atom. Similarly, a complete computational study [14] has been done on pyridine–metal cluster complexes using different quantum chemical methods. DFT methods are used to determine structural properties such as binding energies, vibrational wavenumber and intensities in the IR spectra of free pyridine and pyridine–metal complexes. Phthalimide (PIM) was studied by Arcoa et al. [15] after adsorption onto a silver electrode or silver colloids. These authors concluded that the SERS spectrum is due to the formation of PIM–Ag complexes on silver surfaces, and that there is no evidence for the formation of new electronic states due to the adsorption of PIM onto colloidal Ag nanoparticles. In a recent study [16], SERS spectra of 4-methyl-pyridine were compared with those obtained in an electrochemical cell, while DFT calculations were also made to explain the experimental results. The band shape analysis of SERS modes may help in understanding the static and dynamic interactions of adsorbed molecules with Ag colloids. The present study carries out a systematic analysis of the pH-dependent changes in the line profile parameters of SERS bands for tyrosine–Ag amino acid–metal complexes. The variation of pH in solution causes different charged states of the Tyr molecule in solution. The different charged states of tyrosine in the presence of Ag+ in solution at different pH values provide the possibility of chemisorptions of different ionic forms of the tyrosine molecule with Ag+. Therefore, this study investigates the interaction between tyrosine and Ag+ using Hartree–Fock (HF) and non local DFT methods [17] (BLYP: Becke exchange functionals and Lee, Yang and Parr correlation functionals), and hybrid DFT methods [18] (B3LYP: Becke’s three parameters and Lee, Yang and Parr correlation functionals). Two basis sets, CEP-31G and Lanl2DZ, were used to perform all calculations. The vibrational analysis using a DFT calculation in various modes is expected to give valuable insight into the interaction between tyrosine and Ag+. The approach used in the theoretical calculations in the present study is based on single atom attachment to the tyrosine molecule. The theoretical portion in the present study is only one approach to explore the coordinative effects of Tyr–Ag+ attachment.

ized water. In the solution, Ag particles are in the ionic Ag+ state because of its low oxidation potential. The pH of this solution was adjusted using 1 M HCl and 1 M NaOH solutions. For SERS measurements, Tyr solutions of different pH values were added to the Ag solution (of pH value 7.86). For all experiments, the volume ratio of Ag to amino acid solution was maintained at 9:1. Different volume ratios were tried, but a significant enhancement in the intensity of the vibrational modes was observed only at the 9:1 ratio. The final pH of the mixed solution did not stabilize, even after 15–20 min. The range of fluctuation in the pH of the solution was 0.1 –0.5. SERS spectra were measured in back-scattering geometry using a 488 nm Argon laser as an excitation source. The spectrometer was equipped with a 1200 grooves/mm holographic grating, a holographic super-notch filter and a Peltier cooled CCD detector. The data acquisition time for each spectrum was 120 s. The expected spectral resolution is around 0.5 cm1 for the current configuration. Fig. 1 shows the experimentally recorded SERS as well as the fitted spectra (baseline subtracted) in the region of 1000–1800 cm1 of 1 mM Tyr at seven different pH values. The broad background due to the vibrational mode of water appears in the range of 1500–1800. In order to subtract the baseline, this broad feature of water was

pH = 10.5

pH = 8.5

pH = 7.5 Raman Intensity

70

pH = 6.5

pH = 5.5

pH = 4.5

2. Experimental details and results Silver nitrate (AgNO3), sodium borohydride (NaBH4), sodium hydroxide (NaOH) and hydrochloric acid (HCl) of analytical reagent grade were used to prepare the colloidal Ag solution and to maintain the pH of the solutions. The colloidal silver solution was prepared in deionized water following the method described by Creighton et al. [19]. A 1 mM solution of tyrosine was prepared in deion-

pH = 3.5 1000

1200

1400

1600

1800

Wavenumber/ cm-1 Fig. 1. Experimental and fitted Raman spectra of Tyr at eight different pH values over the region 1000–1800 cm1.

A.K. Ojha / Chemical Physics 340 (2007) 69–78

-1

1368

36

1366

32

1364

28

1362

24

1360

Linewidth (FWHM) / cm-1

40

1370

Peak Position / cm

20

1358 4

5

7

6

8

9

pH of Tyrosine Fig. 2a. Variation in peak position and linewidth of the 1359 cm1 band of Tyr with pH.

70 60

Raman Intensity / a.u.

normalized at a position where the Raman band was absent and then subtracted from the net SERS spectrum. All spectra in Fig. 1 have two prominent and broadened features. The intensity of the broadened peaks was lower for the lower wavenumber peaks than for the higher wavenumber peaks. The intense peak, which appeared on the higher wavenumber side, shows a composite nature. All these peaks are very weak in a 1 mM solution of Tyr. A new peak begins to appear near a less intense feature from pH 8.5 and continues to increase down to pH 3.5. In order to get the exact values of spectral parameters such as Raman shift, linewidth changes and intensity at different pH values, a rigorous line shape analysis has been carried out. A non-linear fit was performed for the spectra for all pH values using the standard software Spectra-Calc. Curve fitting was carried out considering the band as a mixture of Lorentzian and Gaussian curves, which is essentially as good as a Voigt profile [20]. To check the uniqueness of the fitting results, each spectrum was fitted with different reasonable initial estimates and each time the same fitted profiles and fitting parameters were obtained. The non-linear fit for spectra at pH 3.5 resolves into three peaks at 1290 (~m1 Þ, 1505 (~m2 Þ and 1577 (~m3 Þ cm1. The spectra for pH 8.5 to 4.5 were fit to four peaks, the fourth peak appearing between ~m1 and ~m2 . This weak band starts appearing at 1368 cm1 at pH 8.5, and upon decreasing the pH, it disappears below pH 4.5. All four peaks are not observed in the Raman spectrum of tyrosine solution alone (without Ag colloids) at the same concentration. The experimental as well as analyzed spectra at different pH values are presented together in Fig. 1.

71

50 40 30 20 10 4

5

6

7

8

9

pH of Tyrosine

3. Variation in the spectral parameters of the SERS band at 1359 cm1 with pH

Fig. 2b. Variation in the intensity of the 1359 cm1 band of Tyr with pH.

This section discusses the pH dependence of spectral parameters, such as Raman shift, linewidth change and intensity variation for the SERS band which appears at 1359 cm1 at pH 4.5. These parameters show different trends with pH of the Tyr solution when the Raman line profile is carefully analyzed. This may be attributed to the roles of different mechanisms in the solution upon changing the pH. The variation of peak positions and linewidths with pH is presented in Fig. 2. The peak position of the SERS band at 1359 cm1 shows a very interesting variation with pH. Over the entire range of pH, it shows a blue shift of nearly 10 cm1. From pH 4.5 to 5.5, the peak position increases by 1 cm1. This value then remains constant for intermediate pH values from 5.5 to 7.5. There is a sudden increase of 9 cm1 in the peak position of the SRES band on changing the pH from 7.5 to 8.5; this slight change in the nature of the solution, from acidic to basic, results in a large increase in the peak position of the SERS band. It is quite evident that this blue shift is caused by an increase in the force constant of the corresponding vibrational mode. This may be a consequence of the transfer of the electron charge cloud as a result of the adsorption

of the tyrosine molecule in different charge states. Another explanation for this blue shift may be given in terms of a repulsive interaction between tyrosine molecules, which are present in different charge states. This essentially shortens the bond length and leads to a blue shift. The blue shift of the SERS band at 1359 cm1 with increasing pH can be explained in the following two ways: (i) as the pH of a Tyr solution changes, the charge states of the amino acid change, as argued in our earlier study for Phe [11]. The different ionic forms of Tyr molecules lead to a net electrostatic repulsion between them, which in turn increases the force constant for the corresponding mode, causing a blue shift. For pH values above 4.5, the –NH2 group of the Tyr molecule changes to an NHþ 3 (cationic) form, and in the intermediate pH range from 4.5 to 9.5, both NHþ 3 (cationic) and COO (anionic) forms coexist in the solution. On further increasing the pH, there is a transition in the form of the Tyr molecule from cationic to zwitterionic (neutral) and from zwitterionic to anionic. At lower pH values, the slight increase in the wavenumber of the SERS band is due to the dominant nature of repulsive interac-

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A.K. Ojha / Chemical Physics 340 (2007) 69–78

tions. However, for intermediate pH values from 5.5 to 7.5, the presence of both the forms results in competition between attractive and repulsive interactions, leading to almost insignificant changes in the wavenumber of the SERS band. Furthermore, when the pH is changed from 7.5 to 8.5, the repulsive interaction once again dominates over the attractive interaction due to the greater number of Tyr molecules in the anionic form, which essentially increases the force constant and results in a significant increase in the wavenumber of the SERS band. (ii) As discussed above, with the variation of the pH of Tyr solutions, the Tyr molecule exists in different charge states due to changes in the functional groups; NH2 ! NHþ 3 , COOH ! COO and OH ! O. These ionic forms of the molecule tend to bind Ag+ at their ionic sites. During the course of the attachment of silver nanoparticles at ionic sites of Tyr molecules, there is a possibility of the exchange of electron charge clouds between silver nanoparticles and Tyr molecules, which in turn may increase the force constant, leading to a blue shift in the SERS band with increasing pH. A nonlinear fit using a Gaussian function has been made for the linewidth of the SERS band at 1359 cm1. Fig. 2 shows the variation of linewidth with pH, and demonstrates that the experimental points match well with the fitted results. The error bars show the standard deviation of the experimental data from the fitted results. The variation in the linewidth of vibrational modes in solution depends upon several factors, including electrostatic interactions, the viscosity of the solution , and density. This type of variation of linewidth with the relative concentrations of solute and solvent has been explained in terms of fluctuation around a uniform concentration within a microvolume of the solution [21–23]. The variation of the linewidth of the SERS band at 1359 cm1 with pH can be explained by arguments similar to those presented in our recent study [11] for phenylalanine (Phe): (a) similar to the concentration fluctuation model, we propose that the random motion of solute molecules throughout the volume of the colloidal solution causes pH to fluctuate around a mean value of pH in a Gaussian manner. This explanation is also supported by the fact that during our experiment, the pH of the Tyr + Ag complex did not stabilize, even after 15– 20 min [11]. (b) In addition to pH fluctuations, two other effects seem to play important roles in causing the variation of linewidth: (i) the transition of tyrosine from one ionic form to another with increasing pH results in a higher mobility [24] due to the decrease in the intermolecular interactions owing to the increased abundance of the zwitterionic form of the Tyr molecule. The vibrational relaxation time of the molecule is, therefore, expected to decrease for higher pH values. Thus, one can expect an increase in the linewidth with the increase of the pH of the Tyr solution, and (ii) with an increase in pH, the change of the charge state NHþ 3 ! NH2 causes a decrease in solvation [24], and consequently an increase in the viscosity of the solution. This, in turn, decreases the mobility of the

Tyr molecule and subsequently leads to a decrease in its linewidth. Competition between the above two effects determines the ultimate variation of linewidth of the SERS band with pH. The intensity vs. pH plot of the SERS band at 1359 cm1 as shown in Fig. 2b again shows a Gaussian type variation with a maximum at pH 6.5. Initially, with an increase of pH, the intensity of the band increases and attains a maximum value (60) at pH 6.5. On increasing pH further, the intensity starts to decrease and reaches 20 at pH 8.5. The pH values 4.5 and 8.5 are the two limiting values; no SERS signal is observed beyond this pH range. It is obvious that the intensity of the SERS band is decided by several factors, one of them being the number of hot sites [9] present in the solution which basically generate a high local optical field and consequently enhance the SERS signal. This study attempts to explain the variation of the intensity of the SERS band in a similar manner to the study [25], with pH thought of as a controlling factor for the different possible sites for the chemisorption of the Tyr molecule on the Ag+ metal surface. The adsorption of different charged states of tyrosine to Ag+ may give rise to orientation effects, which essentially change the symmetry of the adsorbed molecules, resulting in the SERS mode at 1359 cm1 becoming Raman active. With an increase in pH, the number of adsorbed Raman active species in one favorable site increases, and consequently, the intensity of the SERS mode at 1359 cm1 also increases. However, on a further increase of pH from 6.5 to 8.5, we expect that the number of adsorbed Raman active species falls, and thereby the intensity of the mode at 1359 cm1 also decreases. Thus, we can expect that the variation of the number of adsorbed Raman active species with pH may be one factor responsible for a Gaussian type variation of the intensity of the SERS band with pH. 4. pH dependent variation of the peak position of the 1505 and 1577 cm1 bands The positions of the vibrational bands at 1505 and at 1577 cm1 (at pH 3.5) show a steady increase with increasing pH. The change in the peak positions of these bands is presented in Figs. 3a and 3b, respectively. It is evident from Fig. 3 that both modes show an upshift upon increasing the pH of the tyrosine solution. In both cases, the amplitude of the Raman shift between pH values is quite significant. It can be easily estimated from Fig. 3 that the values of the shift for the bands at 1505 and 1577 cm1 with pH are 21 and 23 cm1, respectively. The non-linear behavior of the variation of peak positions for both modes results in an exponential dependence of tyrosine on pH in solution, where a least squares fit of the experimental data was performed. The dependence of the peak position on pH is expressed by the following relation: vðpHÞ ¼ v0 expðApHÞ;

A.K. Ojha / Chemical Physics 340 (2007) 69–78

1530

Peak Position / cm

-1

1525 1520 1515 1510 1505 1500 2

3

4

5

6

7

8

9

10

11

pH of Tyrosine Fig. 3a. Variation in the peak position of the 1505 cm1 band of Tyr with pH.

1600

Peak Position / cm

-1

1595

1590

1585

1580

1575 3

4

5

6

7

8

9

10

11

pH of Tyrosine Fig. 3b. Variation in the peak position of the 1577 cm1 band of Tyr with pH.

where v0 is the initial value (at pH ! 0), and A is a fit parameter. The blue shift of the modes at 1505 and 1577 cm1 with pH of tyrosine in solution can be understood as a pH dependent ionic interaction among the Tyr molecules as well as Ag colloid and Tyr molecules. The variation of the different charged states of Tyr molecules with pH is supposed to modify the interaction between the ionic and cationic forms of tyrosine molecules. 5. Computational details and results The ab initio and DFT calculations of [Tyr + Ag]+ complexes were performed using Gaussian 98 [26]. Theoretical calculations for optimized geometries and vibrational wavenumbers of [Tyr + Ag]+ complexes were made using the HF method with two different basis sets, CEP-31 G

73

and lanl2DZ. DFT calculations for different configurations of [Tyr + Ag]+ complexes were also carried out employing the B3LYP and BLYP functions using the CEP-31G and lanl2DZ basis sets. The basis sets chosen for calculation were the Stephence/Basch/Krauss ECP Split-valence CEP-31G and lanl2DZ sets. These basis sets were chosen for this study since in an earlier study [27], they yielded theoretical results in good agreement with the experimental values for metal– molecule complexes. The CEP-31G basis set is also known as the SBKJC pseudopotential basis set, and has been reported to provide reliable results on transition metal binding, structure and reactivity [27]. This basis set consists of the effective core pseudo potentials in conjunction with a double f contraction for the valence electrons. The lanl2DZ basis set consists of the Dunning–Huzinaga full double f contraction on first row elements, and the Los Alamos pseudopotential for core electrons in conjunction with a double f contraction for the other elements. The calculated wavenumbers have been uniformly scaled using scaling factors of 0.983 for the CEP-31G basis set and 0.972 [28] for lanl2DZ basis set. The goal of selecting different functions, namely BLYP and B3LYP, and two different basis sets, CEP-31G and lan2DZ, for the optimization and vibrational analysis of different configurations of [Tyr + Ag]+ complexes was to present a comparative and detailed overview of Tyr–Ag+ interactions by vibrational analysis performed at different levels of theory. We are also interested to assess which method yields results that closer to the experimental observations. The basic objective behind theoretical calculations is two fold. First, the possible configurations for Tyr and its metal complexes will be calculated to get the active sites for chemisorptions, where the possibility of attachment with Ag+ is maximal. Second, the vibrational spectral parameters will also be calculated, such as vibrational wavenumber, Raman activity, vibrational force constant and charge on different atoms of the reference molecule. The present theoretical calculations yield a complete picture of the X–Y–Z motion of each atom away from equilibrium during different vibrational modes. The utility of different methods for calculation is judged by the ability to predict the wavenumbers accurately. In an earlier study [11], we reported that the different charge states of the Phenylalanine (Phe) molecules reveal different [Phe + Ag]+ complexes in Ag colloidal solution. Tyr molecules exist in different ionic forms depending on the pH of the colloidal solution. Thus, the Tyr molecules are expected to be adsorbed on the silver surface through the charged end groups of the Tyr molecule. In order to find the possible sites for attachment of the Tyr molecule to Ag+, the calculations were carried out considering five possible configurations of the [Tyr + Ag]+ complex. These five configurations of [Tyr + Ag]+ complexes were chosen on the basis of possible ionic forms of the Tyr molecule at different pH values. The theoretical calculations yield a number of putative binding arrangements for [Tyr + Ag]+

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A.K. Ojha / Chemical Physics 340 (2007) 69–78

which have very similar calculated energies. The optimized possible configurations of [Tyr + Ag]+ complexes along with the isolated Tyr molecule are shown in Fig. 4. The calculations for [Tyr + Ag]+ complexes were performed keeping Ag+ at different sites closer to N/O/R (II) N/OH/R (III) and O/R (IV) of the tyrosine molecule. The configurations N/O/R (II) N/OH/R (III) and O/R (IV) show charge solvation (CS) geometries similar to earlier studies [29,30]. However, the other two configurations, O/O (V) and /OH (VI) show salt-bridge solvation (SB) geometry. The standard counterpoise method was used to analyze the effect of the basis-set superposition error (BSSE) [31] in the optimized structure. The complex binding energy is defined as BE ¼ ½EðAÞ þ EðBÞ  EðA þ BÞþ  þ EBSSE ; where E(A), E(B)+ and E(A+B)+ represent the energy of [Tyr], Ag+ and [Tyr + Ag]+, respectively. The BSSE corrected energies of the [Tyr+Ag]+ complexes (II)–(VI)

presented in Fig. 4 are: 261.22803825 (II), 261.22076836 (III), 261.20495421 (IV), 261.20009932 (V) and 261.195449111 (VI) hartree. The relative energies of the [Tyr + Ag]+ complexes (II)–(VI) are 0 (II), 4.5 (III), 14.5 (IV), 17.5 (V) and 20.4 (VI) kcal mol1. The BEs of the [Tyr + Ag]+ complexes were also calculated using the above relationship. The BEs calculated using the B3LYP function and the CEP-31G basis set for [Tyr + Ag]+ complexes (II)–(VI) presented in Fig. 4 are: 72 (II), 66 (III), 56 (IV), 53 (V) and 50 (VI) kcal mol1. The BE of the [Tyr + Ag]+ complex in configuration N/O/R (II) is 72 kcal mol1. This BE is the highest among all possible configurations. Configuration (II) shows a charge solvation feature and has a 22 kcal mol1 greater binding energy compared to the BE of the salt-bridge (SB) configuration (VI). A similar kind of trend was observed by Polfer et al. [29] for [Phe + Ag]+ complexes. This observation is well supported by earlier studies [32,33]. Looking at the

Fig. 4. Optimized structures of isolated Tyr (I) and [Tyr + Ag]+ complexes: Charge solvation (CS) N/O/R (II), Charge solvation (CS) N/OH/R (III), Charge solvation (CS) O/R (IV), Salt bridge solvation (SB) O/O (V) and Salt bridge solvation (SB)/OH (VI) and binding energy (BE) of each complex calculated using the B3LYP/CEP-31G level of theory. Blue ball: nitrogen, green: carbon, white ball: hydrogen, red ball: oxygen, and sky blue ball: silver atom. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

A.K. Ojha / Chemical Physics 340 (2007) 69–78

BE of SB structures (V) and (VI), SB structure (V) has more binding energy compared to SB structure (VI) by an amount of 3 kcal mol1. The BEs of all CS and SB [Tyr + Ag]+ complexes are close to each other. Therefore, it is very difficult to point out which configuration of the [Tyr + Ag]+ complex would be the most probable configuration. The difference between the BEs of CS configurations N/O/R (II) and N/OH/R (III) is 6 kcal mol1. The difference between the BE of these two configurations is small, and therefore, no strong statement can be made about a single [Tyr + Ag]+ complex being most probable among the two configurations (II) and (III) . The values of binding energy and bond length corresponding to different configurations obtained with the B3LYP method using the CEP-31 G/Lanl2DZ basis set are presented in Table 1. The larger change in the bond lengths can be explained in terms of stronger binding interactions between different complexes [34]. In configurations (II) and (III), Ag+ forms a tridentate bond with N and O atoms and p electrons of the benzene ring. However, in configuration (IV), Ag+ formed two bonds, one with an O atom and the other with p electrons of the benzene. In SB solvated structure (V), the O–H bond of the –COOH group breaks and the H atom goes to the –NH2 group, making it NHþ 3 . On the other hand, Ag+ has formed a permanent bond with the O atom of the –OH group of the Tyr molecule in configuration (VI). The bond length between the Ag+ and N atoms (Ag+  N) in the CS N/O/R (II) complex is shorter compared to the bond length of (Ag+  N) in CS N/OH/R (III). Thus, the Ag+  N bond of the CS N/O/R (II) complex is stronger than in the CS N/OH/R (III) complex. The results calculated for three CS complexes using the B3LYP/ CEP-31G basis set show an increase in the Ag  O bond length and a consistent decrease in the Ag+  p bond length from structure (II) to (IV) in Fig. 4. The order of bond length for tridenate binding of Ag+ with O, N and p electrons of the ring in the CS N/O/R (II) structure is

75

Tyr p  Ag+ > Tyr O  Ag+ > Tyr N  Ag+. We believe that the different values of electronegativity and the atomic radii of the O and N atoms are responsible for the above changes in bond length in CS N/O/R complexes. The ˚ . This bond is stronger length of the Ag  N bond is 2.316 A than any other bond present in all possible [Tyr + Ag]+ complexes. A similar observation has been reported for the Ag–N bond distance in the pyridine–Ag+ complex ˚ ) [35], while the calculated Ag–N distance in the (2.17 A pyridine–Ag(0) complexes was longer. Table 2 presents the electronic charges, q, residing in each atom of an uncoordinated Tyr molecule as obtained by DFT calculations using the B3LYP/CEP-31G basis set with the Mulliken method, along with the charge (Dq) on each atom of a Tyr molecule after coordination with Ag+. In most cases, precise determination of the adsorption site on the Ag+ surface is very difficult. The best way of treating this problem is to calculate the negative charge density on each of these possible active sites. Atoms with higher negative charge densities have a higher probability of acting as adsorptive sites for the Ag substrate. As is evident from the values of Table 2 for the CS N/O/R (II) [Tyr + Ag]+ complexes, a larger change in electron density occurs for the carbon atoms of the ring C1, C2 and C3. These carbon atoms gain electron density, and the other three carbon atoms of the ring C4, C5 and C6 lose electron density. The electron densities of the O and N atoms involved in tridenate binding (configuration (II)) with Ag+ increase relative to their densities in a free Tyr molecule. The change in electron density can be understood in terms of a polarization of charge around the atom due to the Ag+ interaction. The decrease of electron density at the C4 atom causes a longer Ag+  p bond length due to a weak interaction between p electrons of the ring and Ag+. The electron density of the N atom increases by 0.044 (in electron units). The increase in electron density causes a strong interaction between Ag+ and the N atom,

Table 1 Variation of Tyr N  Ag, Tyr O  Ag and Tyr p  Ag bond lengths (BL) and binding energies (BE) of five [Tyr + Ag]+ complexes calculated using the B3LYP function with two basis sets CEP-31G and lanl2DZ Complexes

Bonds

B3LYP CEP-31G

CS N/O/R (II)

CS N/OH/R (III)

CS O/R (IV) SB O/O (V) SB/OH (VI)

Lanl2DZ

BE, kcal mol1

˚ BL, A

BE, kcal mol1

˚ BL, A

Tyr N  Ag Tyr O  Ag Tyr p  Ag

72

2.316 2.391 2.504

60

2.421 2.462 2.567

Tyr N  Ag Tyr O  Ag Tyr p  Ag

66

2.344 2.603 2.464

55

2.342 2.614 2.512

Tyr O  Ag Tyr p  Ag

56

2.262 2.399

46

2.284 2.429

Tyr O  Ag Tyr O  Ag

53

2.463 2.350

42

2.470 2.365

Tyr O  Ag

50

2.216

40

2.29

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A.K. Ojha / Chemical Physics 340 (2007) 69–78

Table 2 Calculation of charge transfer at the B3LYP/CEP-31G level of theory for the plausible [Tyr + Ag]+ complexes (II)–(VI) presented in Fig. 4 Atom

Uncoord. charge q

C1 C2 C3 C4 C5 C6 C7 C8 N9 O10 C11 O12 O13 Ag14 H15 H16 H17 H18 H19 H20 H21 H22 H23 H24 H25

0.124 0.056 0.176 0.013 0.213 0.315 0.452 0.097 0.573 0.300 0.137 0.115 0.275 0 0.299 0.307 0.275 0.337 0.351 0.199 0.161 0.189 0.281 0.290 0.391

B3LYP/CEP-31G CS N/O/R (II) Dq

CS N/OH/R (III) Dq

CS O/R (IV) Dq

SB O/O (V) Dq

SB/OH (VI) Dq

0.039 0.191 0.064 0.217 0.000 0.086 0.021 0.088 0.044 0.031 0.032 0.007 0.020 0.449 0.006 0.007 0.009 0.031 0.023 0.033 0.048 0.060 0.034 0.030 0.024

0.042 0.205 0.057 0.210 0.001 0.095 0.012 0.081 0.049 0.031 0.038 0.106 0.098 0.471 0.005 0.009 0.013 0.019 0.024 0.041 0.041 0.062 0.039 0.033 0.027

0.018 0.135 0.108 0.161 0.010 0.047 0.080 0.067 0.381 0.042 0.009 0.083 0.016 0.504 0.012 0.017 0.004 0.039 0.023 0.036 0.051 0.021 0.034 0.010 0.023

0.013 0.213 0.039 0.098 0.081 0.009 0.058 0.052 0.067 0.011 0.019 0.065 0.029 0.756 0.001 0.002 0.009 0.034 0.013 0.010 0.037 0.057 0.049 0.022 0.029

0.152 0.289 0.040 0.006 0.054 0.017 0.012 0.010 0.004 0.216 0.005 0.015 0.001 0.786 0.018 0.003 0.013 0.032 0.079 0.015 0.021 0.031 0.002 0.008 0.003

A negative Dq indicates a gain of electron density and a positive Dq represents a decrease in electron density.

˚ . On resulting in a shorter Ag+  N bond of length 2.316 A other hand, the electron density on the O atom decreases, ˚ ) commaking the Ag+  O bond slightly longer (2.391 A pared to the bond length of Ag+  N. In the case of Ag+ and the p electrons of the ring, the electron density decreases near the site where Ag+ is located, resulting in ˚ compared to a weaker Ag+  p bond of length 2.504 A the Ag+  N and Ag+  O bonds. To understand the metal dependent behavior on vibrational features of the tyrosine molecule, Table 3 summarizes the wavenumber shifts (WN), force constant (FC) and Raman activity for the three experimentally observed Raman modes obtained through different DFT calculations for the tridenate [Tyr + Ag]+ (II) complex. These modes exhibit significant wavenumber shifts and sensitivity to metal surface binding. It is still not clear whether the shift in the wavenumbers of the SERS bands occurs due to the direct interaction between the molecule and the metal surface, or if it can be accounted for in terms of the inductive effect of tyrosine. In DFT studies [32,33], a shift has been reported in the wavenumber of the vibrational mode of the molecule upon coordination with metals. In the present study, all three modes show blue shifts with increasing pH. The calculated wavenumber of the three modes under study corresponding to the CS N/O/R (II) structure is given in Table 3. All the vibrational modes are more or less perturbed by the presence of the metal ion. Theoretically calculated wavenumbers of the three SERS modes are 1378, 1508 and 1607 cm1. In addition to

the wavenumbers of the three SERS modes, the force constants and Raman activity for these three modes are also given in Table 3. The values of Raman activity for different modes can be used to explain the enhancement in the intensity of different modes upon chemisorption of Tyr at the Ag+ surface. However, the wavenumber shift in SERS modes can be considered in terms of a redistribution of electron density during chemisorption of the molecule on a metal surface, which essentially modifies the corresponding force constant leading to shifts in the wavenumbers of vibrational modes. The values of Raman activity for the SERS modes at 1378, 1508 and 1607 cm1 are higher than their values in an isolated Tyr molecule, 7 ! 13.5, 10.14 ! 15 and 12.88 ! 20.7, respectively. The calculated theoretical values of wavenumber, Raman activity and force constant of three SERS modes of the [Tyr + Ag]+ complex in the CS N/O/R (II) configuration at the B3LYP/CEP-31G level of theory nicely match the experimental results. 6. Conclusions The present study provides a clear understanding of SERS with tyrosine and explores the effect of metal dependent bonding of complex biomolecules on spectral parameters such as wavenumber, intensity and linewidth. In this study, a particular emphasis has been given to understanding the change in line profile parameters such as intensity, wavenumber and linewidth with pH. The varia-

5.33 6.59 10.44 Calc. A = B3LYP/CEP-31G, Calc. B = BLYP/CEP-31G, Calc. C = B3LYP/lanl2DZ, Calc. D = BLYP/lanl2DZ, Calc. E = HF/CEP-31G and Calc. F = HF/lanl2DZ.

+11  39 44 18.3 9.0 8.6 1.8 9.8 29.7 2.11 1.41 7.95 +01 52 16 13.5 15.0 20.7 1.76 1.47 9.25 +19 +03 +30 1359 1505 1577

FC

+27 01 +26

1.5 1.48 9.37

2.1 1.46 1.7

1.6 9.2 7.7

6.44 5.30 11.10

+33 +26 +69 3.53 1.85 1.73 +28 +15 +59

2.92 1.86 1.75

RA WS, cm1

FC Calc. F

WS, cm1

RA FC Calc. E

RA FC WS, cm1

Calc. D

RA FC WS, cm1

Calc. C

RA FC Calc. B

WS, cm1 WS, cm1

RA Calc. A Obs. cm1

Table 3 Experimentally observed (Obs.) wavenumber and the difference between computed and observed wavenumber shift (WS), force constant (FC) in mDyne/A unit and Raman activity (RA) of SERS bands

A.K. Ojha / Chemical Physics 340 (2007) 69–78

77

tion in the spectral parameters of the 1359 cm1 band with pH has been explained in more detail. The SERS band at 1359 cm1 shows a blue shift with an increase in pH from 4.5 to 8.5. The blue shift of the SERS band with pH has been explained in two ways: (i) electronic charge transfer between Ag+ and different charge states of the Tyr molecule and (ii) the nature of ionic interactions between different charge states of the Tyr molecule. The linewidth of the SERS band at 1359 cm1 shows a Gaussian type of variation with pH. This type of variation in linewidth is explained in terms of two possible mechanisms, namely pH fluctuation in a microvolume and a combined effect of viscosity and solvation at different pH values. The structural and binding properties of [Tyr + Ag]+ complexes have been investigated using different quantum chemical methods such as B3LYP, BLYP and HF employing CEP-31G and lanl2DZ basis sets. Acknowledgements The author thanks CSIR for financial support. I wish to thank Prof. A. Roy in whose laboratory the author got initiated in the field of research reported in this paper. Prof. B. P. Asthana and Dr. Ranjan K. Singh are acknowledged for stimulating discussion during the preparation of the manuscript. We also thank the reviewers for valuable suggestions. References [1] M. Fleischman, P.J. Hendra, A. McQuillian, Chem. Phys. Lett. 26 (1974) 163. [2] K. Kneipp, H. Kneipp, V.B. Kartha, R. Manoharan, G. Deinum, I. Itzkan, R.R. Dasari, M.S. Feld, Phys. Rev. E 57 (1998) R6281. [3] S. Nie, S.R. Emory, Science 275 (1997) 1102. [4] P. Kambhampati, C.M. Child, A. Campion, J. Chem. Phys. 108 (1998) 5013. [5] A. Rasmussen, V. Deckert, J. Raman Spectrosc. 37 (2006) 311. [6] A. Sackmann, A. Materny, J. Raman Spectrosc. 37 (2006) 305. [7] B.H. Loo, K. Tse, K. Parsons, C. Adelman, A. EI-Hage, Y.J. Lee, J. Raman Spectrosc. 37 (2006) 299. [8] P.W. Barber, R.K. Chang, H. Massoudi, Phys. Rev. Lett. 50 (1983) 997. [9] H. Xu, J. Aizpurua, J.M. Kall, P. Appel, Phys. Rev. E 62 (2000) 4318. [10] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241, and references cited therein. [11] A.K. Ojha, A. Singha, S. Dasgupta, R.K. Singh, A. Roy, Chem. Phys. Lett. 431 (2006) 121. [12] I. Pavel, S. Cota, S. Cinta-Pinzaru, W. Kiefer, J. Phys. Chem. A 109 (2005) 9945. [13] S. Thomas, N. Biswas, S. Venkateswaran, S. Kapoor, S. Naumov, T. Mujherjee, J. Phys. Chem. A 109 (2005) 9928. [14] D.Y. Wu, M. Hayashi, Y.J. Shiu, K.K. Liang, C.H. Chang, Y.L. Yeh, S.H. Lin, J. Phys. Chem. A 107 (2003) 9568. [15] R.F. Froaca, R.E. Clavijo, M.D. Halls, H.B. Schlegel, J. Phys. Chem. A 104 (2000) 9500. [16] G. Cardini, M. Muniz-Miranda, V. Schettino, J. Phys. Chem. B 108 (2004) 17007. [17] A.D. Becke, J. Chem. Phys. 97 (1992) 9173; A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [18] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.

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