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Vibrational spectra of l -lysine monohydrochloride dihydrate and its two anhydrous forms
Journal of Molecular Structure 917 (2009) 56–62
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Vibrational spectra of L-lysine monohydrochloride dihydrate and its two anhydrous forms A.M. Petrosyan *, V.V. Ghazaryan Molecule Structure Research Center, NAS of Armenia, 26 Azatutyan Avenue, 0014 Yerevan, Armenia
a r t i c l e
i n f o
Article history: Received 22 May 2008 Accepted 25 June 2008 Available online 4 July 2008 Keywords: ATR IR Raman spectra L-Lysine hydrochloride
a b s t r a c t ATR FTIR and Raman spectra of L-LysHCl2H2O and two crystalline forms of anhydrous L-LysHCl are reported and discussed. Distinction with earlier published IR spectra of L-LysHCl2H2O registered by the KBr pellet method is explained by distortion of the spectra because of possible ionic exchange and partial decomposition. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Crystalline salts of optically active amino acids, in particular and L-histidine, recently are being intensively studied. Salts of L-lysine are studied to a lesser degree [1–12]. Ramesh Babu et al. have grown and investigated crystals of L-lysine mono hydrochloride dihydrate (L-LysHCl2H2O) [8,9]. Crystals of L-LysHCl2H2O have been grown also from aqueous solution by unidirectional method [13]. This simple and effective method proposed by Sankaranarayanan and Ramasamy [14] may have great influence on industrial crystal growing from solutions. IR and Raman spectra of L-LysHCl2H2O are reported in Refs. [8,9]. However, we have noticed that IR spectrum reported in [8] does not coincide with IR spectrum of our L-LysHCl2H2O sample. Interpretation of the spectra in [8,9] is performed irrespective of crystal and molecular structure of L-LysHCl2H2O, which is well studied by X-ray [15] and neutron diffraction methods [16,17]. Raman spectra in [8,9] are not registered in the region of stretching vibrations of water crystallization. Thermal properties, in particular dehydration of L-LysHCl2H2O, have been studied [8]. However, IR and Raman spectra of anhydrous L-LysHCl have not been studied. Meanwhile it is known [18] that there are two crystalline forms of L-LysHCl (a- and b-forms). a-Form has been obtained by dehydration of LLysHCl2H2O at higher temperatures (P115 °C [18]), while commonly known commercially available form (designated as b-form by authors of Ref. [18]) is formed at lower temperatures under atmospheric or preferably under reduced pressure. X-ray powder diffraction patterns and IR spectra of a- and b-forms of L-LysHCl L-arginine
* Corresponding author. Tel.: +374 10 285139; fax: +374 10 282267. E-mail address: [email protected] (A.M. Petrosyan). 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.06.026
registered by KBr pellet method are reported in [18]. IR spectra in Ref. [8] also were registered by KBr pellet method. It is known (see e.g. [19]) that IR spectra registered by KBr pellet method can be distorted in result of ionic exchange or partial decomposition. In addition, recently about synthesis and crystal growth of a new nonlinear optical crystal ‘‘tetra L-lysine alanine monohydrochloride dihydrate” have been reported [20], which according to our data is L-LysHCl2H2O as well. The aim of the present paper is to report the Fourier-transform (FT), Raman and FT, attenuated total reflection infrared (ATR IR) spectra of L-LysHCl2H2O and two crystalline forms of anhydrous L-LysHCl.
2. Experimental As a starting reagent we used L-LysHCl obtained in the Institute of Biotechnology (Yerevan) by vacuum dehydration. IR spectrum of this reagent is in good agreement with the spectrum of commercially available L-LysHCl (see the Sigma Chem. Co. catalog of IR spectra registered by Nujol). This form is designated as b-form by authors of Ref. [18]. Crystals of L-LysHCl2H2O were obtained from aqueous solution of L-LysHCl by slow evaporation. Raman spectrum of L-LysHCl2H2O is in good agreement with Raman spectra obtained in Refs. [8,9]. a-Form of L-LysHCl was obtained by dehydration of L-LysHCl2H2O at 150 °C. We employed a Boëtius type microscope with heating stage (in the range 20–350 °C) for observation of dehydration, melting and decomposition processes. Fourier-transform Raman spectra were registered by a NXP FT-Raman Module of a Nicolet 5700 spectrometer (number of scans 256, laser power at sample 0.01 W, resolution 4 cm 1).
A.M. Petrosyan, V.V. Ghazaryan / Journal of Molecular Structure 917 (2009) 56–62
The same spectrometer was used for measuring IR spectra with Nujol (4000–400 cm 1, number of scans 32, and resolution 2 cm 1). Attenuated total reflection Fourier-transform infrared spectra (FTIR ATR) were registered by the aid of a Nicolet ‘‘Nexus” FTIR spectrometer with ZnSe prism (4000–650 cm 1, Happ-Genzel apodization, ATR distortion is corrected, number of scans 32, and resolution 4 cm 1).
3. Results and discussion 3.1. IR and Raman spectra of L-LysHCl2H2O Infrared and Raman spectra of L-LysHCl2H2O are shown in Fig. 1, and wavenumbers and assignment of bands in Table 1. For discussion of the spectra it is worth to mention the features of the L-LysHCl2H2O structure. L-LysHCl2H2O crystal belongs to monoclinic system: space group P21, Z = 2, a = 7.492(1) Å, b = 13.320(4) Å, c = 5.879(1) Å, b = 97.79(1)° [15–17]. Unit cell parameters of the crystal obtained in Ref. [8]: a = 5.8840 Å, b = 13.3359 Å, c = 7.5014 Å, a = 89.92°, b = 97.63°, and c = 90.01° are in good accordance with cell parameters of [15–17] if one takes into account different choice of ‘‘a” and ‘‘c” axes and assumes that a and c angles are exactly equal to 90°. Thus, they can be considered as identical. In the structure of L-LysHCl2H2O lysine is present in the form of singly charged cation, in which amino groups are protonated owing to carboxyl group and HCl acid, i.e., +H3N-(CH2)4CH(NH3+)COO Cl 2H2O [15–17]. Chloride ion forms four hydrogen bonds, two of which are NAH. . .Cl type (with each of NH3+ groups) and two are OAH. . .Cl type (with each of H2O molecules). Water molecules as a proton donor form the second hydrogen bond with oxygen atoms: OW1AH1. . .O1 and OW2AH1. . .OW1, where O1 is the oxygen atom of carboxylate group. The oxygen OW2 atom as a proton acceptor forms NAH1. . .OW2 hydrogen bond with a-amino group. The third hydrogen atom of the a-amino group forms NAH3. . .O2 hydrogen bond, while remaining two hydrogen atoms of the terminal amino group form NZAH1. . .O1 and NZAH2. . .O2 hydrogen bonds. There are misprints in the labeling of the atoms in the Table 4 of Ref. [16]. It should be NZAH1. . .O1 instead of NZAH1. . .O and NZAH3. . .Cl instead of NZAH1. . .Cl. On the basis of correlation between vibrational frequencies of NAH and OAH bonds and N. . .O, N. . .Cl, O. . .Cl, O. . .O distances [21,22] one can estimate expected values of wavenumbers of m(NH) and m(OH) vibrations. Protonated a-amino group forms NAH1. . .OW2 and NAH3. . .O2 hydrogen bonds with N. . .O distances 2.824 and 2.795 Å, for which one can expect m(NH) wavenumbers around 3050 cm 1, while for NAH2. . .Cl with N. . .Cl distance 3.217 Å a wavenumber around 3000 cm 1 for m(NH) vibration can be expected. For terminal NH3+ group there are NZAH1. . .O1 (2.887 Å), NZAH2. . .O2 (2.788 Å) hydrogen bonds, for which one can expect m(NH) frequencies around 3200 cm 1 and 3050 cm 1, respectively, while for NZAH3. . .Cl (3.173 Å) expected value of m(NH) is ca. 3000 cm 1. For water molecules there are OW1AH1. . .O1 (2.805 Å), OW1AH2. . .Cl (3.220 Å), OW2AH1. . .OW1 (2.718 Å) and OW2AH2. . .Cl (3.254 Å) hydrogen bonds. For the OW1AH1 and OW2AH2 bonds m(OH) wavenumbers around 3500 cm 1, for OW1AH2 ca. 3400 cm 1 and for OW2AH1 ca. 3250 cm 1 can be expected. On this basis bands with wavenumbers 3498 cm 1, 3473 cm 1, 3358 cm 1 and possibly 3231 cm 1 may be assigned to the stretching vibrations of water molecules. Upon dehydration these bands disappear (see section 3.2). In the Raman spectrum vibrations of water molecules are revealed as a band with peaks at 3497 and 3481 cm 1 and also weak and broad one at 3375 cm 1. Stretching vibrations of NAH bonds in the IR spectrum are revealed as bands at 3231 cm 1, 3174 cm 1 and a shoulder at 3047 cm 1, which is overlapped with strong
57
band in the region of stretching vibrations of CAH bonds. Stretching vibrations of CAH bonds are revealed as intense lines in the Raman spectrum. In the region of asymmetric stretching vibration of carboxylate group and asymmetric deformation vibration of NH3+ groups there are peaks at 1668, 1626, 1576 and 1537 cm 1. Deformation vibration of water molecules may be in the same region. Peak at 1668 cm 1 has no noticeable counterpart in the Raman spectrum and therefore can be caused by scissoring vibration of water molecules. Other peaks have counterparts at 1634, 1582 and 1544 cm 1 in the Raman spectrum. Absorption band at 1500 cm 1 may be assigned to symmetric deformation vibration of NH3+ groups, while peak at 1416 cm 1 to symmetric stretching vibration of carboxylate group. Deformation vibrations of carboxylate group are expected in the 800–500 cm 1 region. Rather intensive absorption band at 481 cm 1, which has a Raman counterpart at 482 cm 1, is significantly altered upon dehydration. Therefore we assume that librational vibrations of water molecules contribute to this band. An interpretation of vibrational and NMR spectra authors of Refs. [8,9] mention also NH2, COOH and even CH3 groups, which actually are absent in the structure of the molecule. Discussing the proton magnetic resonance spectrum of L-LysHCl2H2O dissolved in D2O authors of [8] assign the most intensive peak at 4.7 ppm to NH2 and NH3+ groups. We already mentioned above that in the structure of L-LysHCl2H2O both amino groups are protonated. In addition, it is known that protons in the NH3+ groups rapidly become ND3+ in D2O. So it will be more correctly to interpret this signal as caused by five water molecules resulted in by two dissolved water crystallization molecules and three molecules formed by exchange with two NH3+ groups. When the present article has been already written we have learned about in-press article [23], in which it was set as the aim a complete analysis of IR and Raman spectra of L-LysHCl2H2O in the light of crystal structure. Unfortunately, some critical remarks stated in occasion of Refs. [8,9] can be carried also to this article. Here also IR spectrum is registered by KBr pellet method and also distorted. Raman spectrum also is not registered in the region of stretching vibrations of water molecules. Authors [23] write about presence in the structure of a water molecule, which has not been involved in hydrogen bonds and assign to its vibration a peak at 3927 cm 1, which is obviously the result of poor quality of the IR spectrum. Interpretation of vibrational spectra is performed on the inconsistent analysis of the structure without taking into account Refs. [15–17]. Authors [23] believe that in the structure there are zwitter-ionic and protonated forms of lysine and at interpretation of the spectra mention both carboxylate COO group and carboxyl COOH group assigning some bands to vibrations of CAOH and C@O bonds, which actually are absent in the structure. It is not clear what structure have in view authors speaking about ‘‘CANACAC ring”, ‘‘mring(CAC)” and ‘‘C@O group aromatic ring”. The statement of authors that the asymmetric part of unit cell contains two molecules contradicts to the P21 symmetry at Z = 2. In addition, the statement of authors that ‘‘The title compound is nearly 2.5 times more than that of urea” raises doubts. Reporting of the value of the SHG signal of 62.57 mV, which depend on amplification of the receiving path, without reporting the value of the signal of reference sample measured under the same conditions is not informative. This remark, by the way, concerns also Ref. [8]. 3.2. Dehydration of L-LysHCl2H2O and spectra of a- and b-forms of L-LysHCl
It is worth to discuss the conditions of preparation of a- and b-forms of L-LysHCl. Thermogramms (TG, DTG and DTA) of L-LysHCl2H2O are shown in Ref. [8]. The mass loss caused by dehydration is 16.4%, which well corresponds to calculated value
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Fig. 1. ATR IR (a) and Raman (b) spectra of L-LysHCl2H2O.
16.476% for two water molecules. By the way, the molecular mass 182.65 g/mol indicated in Ref. [8] for dihydrate actually concerns anhydrous L-LysHCl. It is worth noting that in the re-
gion of dehydration there is a double peak on the DTA curve (at ca. 71 °C and ca. 98 °C), while corresponding peak on DTG curve is single (at ca. 88 °C) [8]. One may assume that in
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A.M. Petrosyan, V.V. Ghazaryan / Journal of Molecular Structure 917 (2009) 56–62 Table 1 Wavenumbers of the infra-red and Raman spectra of L-lysine hydrochloride dihydrate and two (a and b) forms of anhydrous L-lysine hydrochloride L-LysHCl2H2O
L-LysHCl
ATR IR
Raman
3498 3473 3358 3231 3174 3047sh 3011 2950 2936
3497 3481sh 3375w
ATR IR
L-LysHCl
Raman
3146 3021sh 3001; 2977sh 2960 2922 2901 2889; 2865
2865 2824sh 2788 2729 2636 2604 2085 1990 1946vw 1668 1626 1576 1537 1500 1470 1458sh 1443sh 1416 1349 1307 1290 1261 1244 1204 1161 1148
1634sh 1582 1544wsh 1506 1487; 1474 1462 1442 1425 1363; 1348 1323 1295vw 1264 1248 1207 1166 1152
1086 1048
1088 1055
1007 961vw 937 914 866
1012 965 941 921 869
789 736 685; 650sh 558 481
794; 769
414
(a-form)
2784; 2763 2717 2641 2581vw
688 559 482 416 385 323 214
2983; 2964 2944; 2937sh 2933sh; 2913
2901 2869 2816sh
2870 2785
Raman
2988; 2961 2947 2911
2941; 2924 2908 2889; 2871sh 2860sh 2774sh
2874 2780
2645 2601 2098 1992
2602 2111 2008
1467sh 1449 1425; 1405 1361; 1347 1320 1281 1262 1242vw 1221 1183 1150 1137sh 1101; 1087 1050 1030 999 976; 957 936 909 863 804 784; 764 744; 733 709; 668 563; 552 502vw; 479 431 414
ATR IR
Assignment
3126; 3115
2933; 2916
1695sh 1613 1583 1564 1515; 1503
(b-form)
1612 1592vw 1563 1504sh 1472 1454 1442 1430; 1412 1364; 1351 1323; 1313 1266vw 1226vw 1186 1152 1138 1087 1058 1032wsh 1003 980; 958 930 917sh 867 808vw 786; 772 712; 671 555 474 433 416 350 331; 292 187
1623 1599 1543sh; 1527 1497 1488sh; 1471sh 1464 1439 1420 1379; 1360 350;1340 1330sh; 1321 1286 1265 1241 1224; 1191 1158 1144 1128 1102; 1073 1056; 1049 1030 1011; 1001sh 985; 957 936 905 858 803 787; 764 743; 734 704; 661 558; 550 474 435 418
1627 1578 1548; 1534vw 1476sh 1465sh; 1458 1444 1425 1363; 1355 1343; 1336 1311 1268vw 1245vw 1226vw; 1194 1161 1147 1131 1083 1059 1033 1015 988; 960 937 909 866vw 806 790; 769 745vw 707; 664 554 474 440 420 394vw 327; 301; 279 201
m(OH) H2O m(OH) H2O m(OH) H2O m(NH) NH3+ m(NH) NH3+ m(NH) NH3+ m(NH) NH3+ mas(CH) CH2 mas(CH) CH2 mas(CH) CH2 mas(CH) CH2 ms(CH) CH2 m(CH) CH Overtones Overtones Overtones Overtones Overtones Overtones Overtones Overtones mas(COO ) das(NH3+) das(NH3+) ds(NH3+) d(CH2) d(CH2) d(CH2) ms(COO ) x(CH2) x(CH2) s(CH2) s(CH2) s(CH2) s(CH2) q(NH3+) q(NH3+) q(NH3+) m(CAN) m(CAN) mCAN) m(CAC) m(CAC) m(CAC)
and and and and and and and and
combinational combinational combinational combinational combinational combinational combinational combinational
bands bands bands bands bands bands bands bands
d(COO )
q(CH2) x(COO ) q(COO )
sh, shoulder; w, weak; v, very; m, stretching; d, scissoring; x, wagging; s, twisting; q, rocking; as, asymmetric; s, symmetric.
addition to dehydration melting also occurs as in the case of L-Lys2H2C2O4H2O [12]. However, our direct thermo-microscopic observation of the dehydration process has not confirmed this assumption. There is a wide plateau on the TG curve after dehydration. Anhydrous L-LysHCl sample obtained by heating of L-LysHCl2H2O in the 130–200 °C range proved to be a-form by IR spectra. Authors of Ref. [8] have concluded that the crystal decomposes without melting. However, our observation showed that anhydrous a-form L-LysHCl melts at 260 °C before decomposition. This well corresponds to endothermic DTA peak at ca. 261 °C just in the end of plateau [8]. One may assume that the
first peak at 71 °C is caused by dehydration with formation of b-form L-LysHCl, while the second peak corresponds to transformation of b-form L-LysHCl into a-form. This is in qualitative accordance with data of Ref. [18] that at lower temperatures b-form is formed, while at higher temperatures (P115 °C) a-form is formed. For checking this we have heated the sample of L-LysHCl2H2O at 75 °C during 1 h. The sample obtained in these conditions proved to be b-form L-LysHCl by its IR spectrum. However, heating at 80 °C and above led to formation of a-form L-LysHCl. We found, however, that addition of 20% powdered b-form L-LysHCl to L-LysHCl2H2O assists to transformation
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of L-LysHCl2H2O into b-form L-LysHCl at heating at 80, 90, 100 and 110 °C. It is worth noting that b-form L-LysHCl obtained at
low temperatures by vacuum drying and also by dehydration at 75 °C and cooled up to a room temperature at the subsequent
Fig. 2. ATR IR (a) and Raman (b) spectra of a-form L-LysHCl.
A.M. Petrosyan, V.V. Ghazaryan / Journal of Molecular Structure 917 (2009) 56–62
heating does not transform into a-form L-LysHCl and remains stable up to melting and decomposition. According to Ref. [18]
a-form L-LysHCl completely transforms into b-form during 15 h at 40 °C and 76% humidity.
Fig. 3. ATR IR (a) and Raman (b) spectra of b-form L-LysHCl.
61 L-LysHCl
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IR and Raman spectra of a- and b-forms L-LysHCl are shown in Figs. 2 and 3 and assignments of the bands in Table 1. If one ignores some details, the spectra of a- and b-forms L-LysHCl seem similar enough. In high-frequency region (around 2900 cm 1) of the spectra there are broad strong bands caused by stretching vibrations of NH and CH bonds. In the Raman spectra in this region the strongest peaks caused by stretching vibrations of CH bonds are dominating. Stretching vibrations of NH bonds of NH3+ groups are revealed in the form of broad bands in the 3400–2500 cm 1 range, which are overlapped with strong peaks caused by vibrations of CH bonds. Most easily the spectra of a- and b-forms L-LysHCl can be distinguished by absorption bands in the field of asymmetric stretching vibration of carboxylate group and asymmetric deformation vibration of NH3+ groups. a-Form has in this region three peaks with 1613, 1583 and 1564 cm 1 wavenumbers, while b-form has two peaks at 1623 and 1599 cm 1. Strengthening of NAH. . .O hydrogen bonds of NH3+ groups should lead to shifting of absorption bands of stretching m(NH) vibrations towards low-frequency region and accordingly to shifting towards high-frequency region of bands caused by das(NH3+) deformation vibrations. On this basis one can suppose that hydrogen bonds in the structure of b-form are stronger than that of a-form L-LysHCl. IR spectra of a- and b-forms LLysHCl registered by KBr pellet method [18] are in good correspondence with ATR spectra. However, the spectrum of a-form [18] contains a small peak in the region of stretching vibrations of crystallization water (ca. 3375 cm 1), which is absent in our spectrum. Assignments of other bands are indicated in Table 1. 3.3. On ‘‘tetra L-lysine alanine monohydrochloride dihydrate” Recently a paper [20] has been published, in which authors report on obtaining of a new crystal ‘‘tetra L-lysine alanine monohydrochloride dihydrate (TLAMHCl)” from aqueous solution containing L-LysHCl2H2O and L-alanine in the 4:1 mole ratio. Though in the paper it is spoken about determination of the crystal structure, however, somewhat definite data about structure has not been reported. Moreover, it is difficult to understand what structure of TLAMHCl the authors have in view at interpretation of the IR spectrum when they write that «The FTIR spectra of TLAMHCl crystal was recorded...in order to study the metal complex coordination”, and also ‘‘A peak observed at about 2112 cm 1 in TLAMHCl is because of more available group of AC„CA stretching vibration”. There is also a strange sentence ‘‘The peaks presented at 907 and 861 cm 1 (overtones) are assigned for d(CAH) stretch”. We have obtained crystals from aqueous solutions containing L-LysHCl2H2O and L-alanine in mole ratios 5:1, 4:1, 3:1, 2:1, 1:1, and also from solution containing 4(L-LysHCl2H2O) + L-Ala + HCl. In all cases the IR spectra of obtained crystals exactly coincided with IR spectrum of L-LysHCl2H2O. Therefore we consider that ‘‘tetra L-lysine alanine monohydrochloride dihydrate” represents actually L-LysHCl2H2O. IR spectrum reported in [20] however does not coincide with the spectrum shown in Fig. 1. In the 500–1600 cm 1 range the spectrum [20] have certain similarity with the spectrum reported in [8]. We believe that difference of the spectra [8] and [20] from the ATR IR spectrum registered by us (Fig. 1) is caused by an ionic exchange (Cl ? Br) and partial decomposition in result of registration of the spectrum by KBr pellet method. The TGA/DTA data for ‘‘tetra L-lysine alanine monohydrochloride dihydrate” [20] satisfactorily correspond to respective data of L-LysHCl2H2O [8]. The shifting of the peaks may be caused by different heating rates (20°/min and 10°/min, respectively). However, interpretation of the data of thermal analysis is contradictory. On the one hand they write
that ‘‘The calorimetric measurements clearly show an existence of four solid to solid reversible phase transitions” and on the other hand in the conclusion they write that ‘‘there is no phase transitions and decomposition till 266 °C”. Also on the one hand they write that the sample loses two water crystallization molecules below 133.2 °C and on the other hand that ‘‘the material is thermally stable in the working temperature range that is 30– 266 °C”. Actually above dehydration temperature the LLysHCl2H2O crystal as such does not exist any more and the working temperature range is limited, as authors of Ref. [8] rightly noted, by dehydration temperature (650 °C). 4. Conclusions ATR FTIR and Raman spectra of L-LysHCl2H2O are reported and discussed taking into consideration its crystal and molecular structure. Conditions of obtaining two forms of anhydrous L-LysHCl and their ATR FTIR and Raman spectra are reported and discussed. Previously reported ‘‘tetra L-lysine alanine monohydrochloride dihydrate” [20] according to our data actually is L-lysine monohydrochloride dihydrate L-LysHCl2H2O. Acknowledgements The present work was supported by the Government of Armenia (Grant # 0111). Authors thank Dr. Aghajanyan A.E. for providing with L-lysine hydrochloride reagent and also Dr. Martirosyan G.G. for the help in registration of the spectra. References [1] Suzuki, Y. Matsuoka, Jpn. Kokai Tokkyo koho JP 03 03, 834 [91 33, 834] (Cl. G02F1/35) 14 February 1991, appl. 89/168, 872, 4 pp. [2] A.M. Petrosyan, R.P. Sukiasyan, S.S. Terzyan, V.M. Burbelo, Z. Naturforschung 53a (1998) 528. [3] A.M. Petrosyan, R.P. Sukiasyan, S.S. Terzyan, V.M. Burbelo, Acta Cryst. B55 (1999) 221. [4] A.M. Petrosyan, V.M. Burbelo, R.A. Tamazyan, H.A. Karapetyan, R.P. Sukiasyan, Z. Naturforschung 55a (2000) 199. [5] M.K. Marchewka, S. Debrus, H. Ratajczak, Cryst. Growth Des. 3 (2003) 587. [6] S. Debrus, M.K. Marchewka, J. Baran, M. Drozd, R. Czopnik, A. Pietraszko, H. Ratajczak, J. Solid State Chem. 178 (2005) 2880. [7] R.P. Sukiasyan, Proc. Conf. Laser Physics-2004, October 12–15, 2004, Ashtarak, Armenia, 2005, pp.105–108. [8] R. Ramesh Babu, N. Vijayan, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (2006) 405. [9] R. Ramesh Babu, K. Sethuraman, N. Vijayan, G. Bhagavannarayana, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (2006) 906. [10] V. Mathivanan, T. Raghavalu, M. Kovendhan, K. Suriya Kumar, S. Gokul Raj, G. Ramesh Kumar, R. Mohan, Cryst. Res. Technol. 43 (2008) 248. [11] Z.H. Sun, G.H. Zhang, X.Q. Wang, X.F. Cheng, X.J. Liu, L.Y. Zhu, H.L. Fan, G. Yu, D. Xu, J. Cryst. Growth 310 (2008) 2842. [12] R.P. Sukiasyan, H.A. Karapetyan, A.M. Petrosyan, J. Mol. Struct. 888 (2008) 230. [13] R. Ramesh Babu, K. Sethuraman, R. Gopalakrishnan, P. Ramasamy, J. Cryst. Growth 297 (2006) 356. [14] K. Sankaranarayanan, P. Ramasamy, J. Cryst. Growth 280 (2005) 467. [15] D.A. Wright, R.E. Marsh, Acta Cryst. 15 (1962) 54 (Correction 16 (1963) 431). [16] T.F. Koetzle, M.S. Lehmann, J.J. Verbist, W.C. Hamilton, Acta Cryst. B28 (1972) 3207. [17] R.R. Bugayong, A. Sequeira, R. Chidambaram, Acta Cryst. B28, 1972, 3214. [18] Y. Takayanagi, K. Iizumi, M. Miyazawa, N. Yamaya, H. Tamura, US Patent 4256917, 1981. [19] A.M. Petrosyan, Vibrational Spectrosc. 43 (2007) 284. [20] V. Sivashankar, R. Sankar, R. Siddheswaran, R. Jayavel, P. Murugakoothan, Mat. Chem. Phys.109 (2008) 119. [21] K. Nakamoto, M. Margoshes, R.E. Rundle, J. Amer. Chem. Soc. 77 (1955) 6480. [22] A. Novak, in: Infrared and Raman Spectroscopy of Biological Molecules. Proc. NATO Advanced Study Institute, Athens, Greece, August 22–31, 1978, Dordrecht 1979, p. 279. [23] V. Krishnakumar, R. Nagalakshmi, S. Monohar, L. Kocsis, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. (2008), doi:10.1016/j.saa.2007.11.035.
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Vibrational spectra of L-lysine monohydrochloride dihydrate and its two anhydrous forms A.M. Petrosyan *, V.V. Ghazaryan Molecule Structure Research Center, NAS of Armenia, 26 Azatutyan Avenue, 0014 Yerevan, Armenia
a r t i c l e
i n f o
Article history: Received 22 May 2008 Accepted 25 June 2008 Available online 4 July 2008 Keywords: ATR IR Raman spectra L-Lysine hydrochloride
a b s t r a c t ATR FTIR and Raman spectra of L-LysHCl2H2O and two crystalline forms of anhydrous L-LysHCl are reported and discussed. Distinction with earlier published IR spectra of L-LysHCl2H2O registered by the KBr pellet method is explained by distortion of the spectra because of possible ionic exchange and partial decomposition. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Crystalline salts of optically active amino acids, in particular and L-histidine, recently are being intensively studied. Salts of L-lysine are studied to a lesser degree [1–12]. Ramesh Babu et al. have grown and investigated crystals of L-lysine mono hydrochloride dihydrate (L-LysHCl2H2O) [8,9]. Crystals of L-LysHCl2H2O have been grown also from aqueous solution by unidirectional method [13]. This simple and effective method proposed by Sankaranarayanan and Ramasamy [14] may have great influence on industrial crystal growing from solutions. IR and Raman spectra of L-LysHCl2H2O are reported in Refs. [8,9]. However, we have noticed that IR spectrum reported in [8] does not coincide with IR spectrum of our L-LysHCl2H2O sample. Interpretation of the spectra in [8,9] is performed irrespective of crystal and molecular structure of L-LysHCl2H2O, which is well studied by X-ray [15] and neutron diffraction methods [16,17]. Raman spectra in [8,9] are not registered in the region of stretching vibrations of water crystallization. Thermal properties, in particular dehydration of L-LysHCl2H2O, have been studied [8]. However, IR and Raman spectra of anhydrous L-LysHCl have not been studied. Meanwhile it is known [18] that there are two crystalline forms of L-LysHCl (a- and b-forms). a-Form has been obtained by dehydration of LLysHCl2H2O at higher temperatures (P115 °C [18]), while commonly known commercially available form (designated as b-form by authors of Ref. [18]) is formed at lower temperatures under atmospheric or preferably under reduced pressure. X-ray powder diffraction patterns and IR spectra of a- and b-forms of L-LysHCl L-arginine
* Corresponding author. Tel.: +374 10 285139; fax: +374 10 282267. E-mail address: [email protected] (A.M. Petrosyan). 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.06.026
registered by KBr pellet method are reported in [18]. IR spectra in Ref. [8] also were registered by KBr pellet method. It is known (see e.g. [19]) that IR spectra registered by KBr pellet method can be distorted in result of ionic exchange or partial decomposition. In addition, recently about synthesis and crystal growth of a new nonlinear optical crystal ‘‘tetra L-lysine alanine monohydrochloride dihydrate” have been reported [20], which according to our data is L-LysHCl2H2O as well. The aim of the present paper is to report the Fourier-transform (FT), Raman and FT, attenuated total reflection infrared (ATR IR) spectra of L-LysHCl2H2O and two crystalline forms of anhydrous L-LysHCl.
2. Experimental As a starting reagent we used L-LysHCl obtained in the Institute of Biotechnology (Yerevan) by vacuum dehydration. IR spectrum of this reagent is in good agreement with the spectrum of commercially available L-LysHCl (see the Sigma Chem. Co. catalog of IR spectra registered by Nujol). This form is designated as b-form by authors of Ref. [18]. Crystals of L-LysHCl2H2O were obtained from aqueous solution of L-LysHCl by slow evaporation. Raman spectrum of L-LysHCl2H2O is in good agreement with Raman spectra obtained in Refs. [8,9]. a-Form of L-LysHCl was obtained by dehydration of L-LysHCl2H2O at 150 °C. We employed a Boëtius type microscope with heating stage (in the range 20–350 °C) for observation of dehydration, melting and decomposition processes. Fourier-transform Raman spectra were registered by a NXP FT-Raman Module of a Nicolet 5700 spectrometer (number of scans 256, laser power at sample 0.01 W, resolution 4 cm 1).
A.M. Petrosyan, V.V. Ghazaryan / Journal of Molecular Structure 917 (2009) 56–62
The same spectrometer was used for measuring IR spectra with Nujol (4000–400 cm 1, number of scans 32, and resolution 2 cm 1). Attenuated total reflection Fourier-transform infrared spectra (FTIR ATR) were registered by the aid of a Nicolet ‘‘Nexus” FTIR spectrometer with ZnSe prism (4000–650 cm 1, Happ-Genzel apodization, ATR distortion is corrected, number of scans 32, and resolution 4 cm 1).
3. Results and discussion 3.1. IR and Raman spectra of L-LysHCl2H2O Infrared and Raman spectra of L-LysHCl2H2O are shown in Fig. 1, and wavenumbers and assignment of bands in Table 1. For discussion of the spectra it is worth to mention the features of the L-LysHCl2H2O structure. L-LysHCl2H2O crystal belongs to monoclinic system: space group P21, Z = 2, a = 7.492(1) Å, b = 13.320(4) Å, c = 5.879(1) Å, b = 97.79(1)° [15–17]. Unit cell parameters of the crystal obtained in Ref. [8]: a = 5.8840 Å, b = 13.3359 Å, c = 7.5014 Å, a = 89.92°, b = 97.63°, and c = 90.01° are in good accordance with cell parameters of [15–17] if one takes into account different choice of ‘‘a” and ‘‘c” axes and assumes that a and c angles are exactly equal to 90°. Thus, they can be considered as identical. In the structure of L-LysHCl2H2O lysine is present in the form of singly charged cation, in which amino groups are protonated owing to carboxyl group and HCl acid, i.e., +H3N-(CH2)4CH(NH3+)COO Cl 2H2O [15–17]. Chloride ion forms four hydrogen bonds, two of which are NAH. . .Cl type (with each of NH3+ groups) and two are OAH. . .Cl type (with each of H2O molecules). Water molecules as a proton donor form the second hydrogen bond with oxygen atoms: OW1AH1. . .O1 and OW2AH1. . .OW1, where O1 is the oxygen atom of carboxylate group. The oxygen OW2 atom as a proton acceptor forms NAH1. . .OW2 hydrogen bond with a-amino group. The third hydrogen atom of the a-amino group forms NAH3. . .O2 hydrogen bond, while remaining two hydrogen atoms of the terminal amino group form NZAH1. . .O1 and NZAH2. . .O2 hydrogen bonds. There are misprints in the labeling of the atoms in the Table 4 of Ref. [16]. It should be NZAH1. . .O1 instead of NZAH1. . .O and NZAH3. . .Cl instead of NZAH1. . .Cl. On the basis of correlation between vibrational frequencies of NAH and OAH bonds and N. . .O, N. . .Cl, O. . .Cl, O. . .O distances [21,22] one can estimate expected values of wavenumbers of m(NH) and m(OH) vibrations. Protonated a-amino group forms NAH1. . .OW2 and NAH3. . .O2 hydrogen bonds with N. . .O distances 2.824 and 2.795 Å, for which one can expect m(NH) wavenumbers around 3050 cm 1, while for NAH2. . .Cl with N. . .Cl distance 3.217 Å a wavenumber around 3000 cm 1 for m(NH) vibration can be expected. For terminal NH3+ group there are NZAH1. . .O1 (2.887 Å), NZAH2. . .O2 (2.788 Å) hydrogen bonds, for which one can expect m(NH) frequencies around 3200 cm 1 and 3050 cm 1, respectively, while for NZAH3. . .Cl (3.173 Å) expected value of m(NH) is ca. 3000 cm 1. For water molecules there are OW1AH1. . .O1 (2.805 Å), OW1AH2. . .Cl (3.220 Å), OW2AH1. . .OW1 (2.718 Å) and OW2AH2. . .Cl (3.254 Å) hydrogen bonds. For the OW1AH1 and OW2AH2 bonds m(OH) wavenumbers around 3500 cm 1, for OW1AH2 ca. 3400 cm 1 and for OW2AH1 ca. 3250 cm 1 can be expected. On this basis bands with wavenumbers 3498 cm 1, 3473 cm 1, 3358 cm 1 and possibly 3231 cm 1 may be assigned to the stretching vibrations of water molecules. Upon dehydration these bands disappear (see section 3.2). In the Raman spectrum vibrations of water molecules are revealed as a band with peaks at 3497 and 3481 cm 1 and also weak and broad one at 3375 cm 1. Stretching vibrations of NAH bonds in the IR spectrum are revealed as bands at 3231 cm 1, 3174 cm 1 and a shoulder at 3047 cm 1, which is overlapped with strong
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band in the region of stretching vibrations of CAH bonds. Stretching vibrations of CAH bonds are revealed as intense lines in the Raman spectrum. In the region of asymmetric stretching vibration of carboxylate group and asymmetric deformation vibration of NH3+ groups there are peaks at 1668, 1626, 1576 and 1537 cm 1. Deformation vibration of water molecules may be in the same region. Peak at 1668 cm 1 has no noticeable counterpart in the Raman spectrum and therefore can be caused by scissoring vibration of water molecules. Other peaks have counterparts at 1634, 1582 and 1544 cm 1 in the Raman spectrum. Absorption band at 1500 cm 1 may be assigned to symmetric deformation vibration of NH3+ groups, while peak at 1416 cm 1 to symmetric stretching vibration of carboxylate group. Deformation vibrations of carboxylate group are expected in the 800–500 cm 1 region. Rather intensive absorption band at 481 cm 1, which has a Raman counterpart at 482 cm 1, is significantly altered upon dehydration. Therefore we assume that librational vibrations of water molecules contribute to this band. An interpretation of vibrational and NMR spectra authors of Refs. [8,9] mention also NH2, COOH and even CH3 groups, which actually are absent in the structure of the molecule. Discussing the proton magnetic resonance spectrum of L-LysHCl2H2O dissolved in D2O authors of [8] assign the most intensive peak at 4.7 ppm to NH2 and NH3+ groups. We already mentioned above that in the structure of L-LysHCl2H2O both amino groups are protonated. In addition, it is known that protons in the NH3+ groups rapidly become ND3+ in D2O. So it will be more correctly to interpret this signal as caused by five water molecules resulted in by two dissolved water crystallization molecules and three molecules formed by exchange with two NH3+ groups. When the present article has been already written we have learned about in-press article [23], in which it was set as the aim a complete analysis of IR and Raman spectra of L-LysHCl2H2O in the light of crystal structure. Unfortunately, some critical remarks stated in occasion of Refs. [8,9] can be carried also to this article. Here also IR spectrum is registered by KBr pellet method and also distorted. Raman spectrum also is not registered in the region of stretching vibrations of water molecules. Authors [23] write about presence in the structure of a water molecule, which has not been involved in hydrogen bonds and assign to its vibration a peak at 3927 cm 1, which is obviously the result of poor quality of the IR spectrum. Interpretation of vibrational spectra is performed on the inconsistent analysis of the structure without taking into account Refs. [15–17]. Authors [23] believe that in the structure there are zwitter-ionic and protonated forms of lysine and at interpretation of the spectra mention both carboxylate COO group and carboxyl COOH group assigning some bands to vibrations of CAOH and C@O bonds, which actually are absent in the structure. It is not clear what structure have in view authors speaking about ‘‘CANACAC ring”, ‘‘mring(CAC)” and ‘‘C@O group aromatic ring”. The statement of authors that the asymmetric part of unit cell contains two molecules contradicts to the P21 symmetry at Z = 2. In addition, the statement of authors that ‘‘The title compound is nearly 2.5 times more than that of urea” raises doubts. Reporting of the value of the SHG signal of 62.57 mV, which depend on amplification of the receiving path, without reporting the value of the signal of reference sample measured under the same conditions is not informative. This remark, by the way, concerns also Ref. [8]. 3.2. Dehydration of L-LysHCl2H2O and spectra of a- and b-forms of L-LysHCl
It is worth to discuss the conditions of preparation of a- and b-forms of L-LysHCl. Thermogramms (TG, DTG and DTA) of L-LysHCl2H2O are shown in Ref. [8]. The mass loss caused by dehydration is 16.4%, which well corresponds to calculated value
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Fig. 1. ATR IR (a) and Raman (b) spectra of L-LysHCl2H2O.
16.476% for two water molecules. By the way, the molecular mass 182.65 g/mol indicated in Ref. [8] for dihydrate actually concerns anhydrous L-LysHCl. It is worth noting that in the re-
gion of dehydration there is a double peak on the DTA curve (at ca. 71 °C and ca. 98 °C), while corresponding peak on DTG curve is single (at ca. 88 °C) [8]. One may assume that in
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A.M. Petrosyan, V.V. Ghazaryan / Journal of Molecular Structure 917 (2009) 56–62 Table 1 Wavenumbers of the infra-red and Raman spectra of L-lysine hydrochloride dihydrate and two (a and b) forms of anhydrous L-lysine hydrochloride L-LysHCl2H2O
L-LysHCl
ATR IR
Raman
3498 3473 3358 3231 3174 3047sh 3011 2950 2936
3497 3481sh 3375w
ATR IR
L-LysHCl
Raman
3146 3021sh 3001; 2977sh 2960 2922 2901 2889; 2865
2865 2824sh 2788 2729 2636 2604 2085 1990 1946vw 1668 1626 1576 1537 1500 1470 1458sh 1443sh 1416 1349 1307 1290 1261 1244 1204 1161 1148
1634sh 1582 1544wsh 1506 1487; 1474 1462 1442 1425 1363; 1348 1323 1295vw 1264 1248 1207 1166 1152
1086 1048
1088 1055
1007 961vw 937 914 866
1012 965 941 921 869
789 736 685; 650sh 558 481
794; 769
414
(a-form)
2784; 2763 2717 2641 2581vw
688 559 482 416 385 323 214
2983; 2964 2944; 2937sh 2933sh; 2913
2901 2869 2816sh
2870 2785
Raman
2988; 2961 2947 2911
2941; 2924 2908 2889; 2871sh 2860sh 2774sh
2874 2780
2645 2601 2098 1992
2602 2111 2008
1467sh 1449 1425; 1405 1361; 1347 1320 1281 1262 1242vw 1221 1183 1150 1137sh 1101; 1087 1050 1030 999 976; 957 936 909 863 804 784; 764 744; 733 709; 668 563; 552 502vw; 479 431 414
ATR IR
Assignment
3126; 3115
2933; 2916
1695sh 1613 1583 1564 1515; 1503
(b-form)
1612 1592vw 1563 1504sh 1472 1454 1442 1430; 1412 1364; 1351 1323; 1313 1266vw 1226vw 1186 1152 1138 1087 1058 1032wsh 1003 980; 958 930 917sh 867 808vw 786; 772 712; 671 555 474 433 416 350 331; 292 187
1623 1599 1543sh; 1527 1497 1488sh; 1471sh 1464 1439 1420 1379; 1360 350;1340 1330sh; 1321 1286 1265 1241 1224; 1191 1158 1144 1128 1102; 1073 1056; 1049 1030 1011; 1001sh 985; 957 936 905 858 803 787; 764 743; 734 704; 661 558; 550 474 435 418
1627 1578 1548; 1534vw 1476sh 1465sh; 1458 1444 1425 1363; 1355 1343; 1336 1311 1268vw 1245vw 1226vw; 1194 1161 1147 1131 1083 1059 1033 1015 988; 960 937 909 866vw 806 790; 769 745vw 707; 664 554 474 440 420 394vw 327; 301; 279 201
m(OH) H2O m(OH) H2O m(OH) H2O m(NH) NH3+ m(NH) NH3+ m(NH) NH3+ m(NH) NH3+ mas(CH) CH2 mas(CH) CH2 mas(CH) CH2 mas(CH) CH2 ms(CH) CH2 m(CH) CH Overtones Overtones Overtones Overtones Overtones Overtones Overtones Overtones mas(COO ) das(NH3+) das(NH3+) ds(NH3+) d(CH2) d(CH2) d(CH2) ms(COO ) x(CH2) x(CH2) s(CH2) s(CH2) s(CH2) s(CH2) q(NH3+) q(NH3+) q(NH3+) m(CAN) m(CAN) mCAN) m(CAC) m(CAC) m(CAC)
and and and and and and and and
combinational combinational combinational combinational combinational combinational combinational combinational
bands bands bands bands bands bands bands bands
d(COO )
q(CH2) x(COO ) q(COO )
sh, shoulder; w, weak; v, very; m, stretching; d, scissoring; x, wagging; s, twisting; q, rocking; as, asymmetric; s, symmetric.
addition to dehydration melting also occurs as in the case of L-Lys2H2C2O4H2O [12]. However, our direct thermo-microscopic observation of the dehydration process has not confirmed this assumption. There is a wide plateau on the TG curve after dehydration. Anhydrous L-LysHCl sample obtained by heating of L-LysHCl2H2O in the 130–200 °C range proved to be a-form by IR spectra. Authors of Ref. [8] have concluded that the crystal decomposes without melting. However, our observation showed that anhydrous a-form L-LysHCl melts at 260 °C before decomposition. This well corresponds to endothermic DTA peak at ca. 261 °C just in the end of plateau [8]. One may assume that the
first peak at 71 °C is caused by dehydration with formation of b-form L-LysHCl, while the second peak corresponds to transformation of b-form L-LysHCl into a-form. This is in qualitative accordance with data of Ref. [18] that at lower temperatures b-form is formed, while at higher temperatures (P115 °C) a-form is formed. For checking this we have heated the sample of L-LysHCl2H2O at 75 °C during 1 h. The sample obtained in these conditions proved to be b-form L-LysHCl by its IR spectrum. However, heating at 80 °C and above led to formation of a-form L-LysHCl. We found, however, that addition of 20% powdered b-form L-LysHCl to L-LysHCl2H2O assists to transformation
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of L-LysHCl2H2O into b-form L-LysHCl at heating at 80, 90, 100 and 110 °C. It is worth noting that b-form L-LysHCl obtained at
low temperatures by vacuum drying and also by dehydration at 75 °C and cooled up to a room temperature at the subsequent
Fig. 2. ATR IR (a) and Raman (b) spectra of a-form L-LysHCl.
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heating does not transform into a-form L-LysHCl and remains stable up to melting and decomposition. According to Ref. [18]
a-form L-LysHCl completely transforms into b-form during 15 h at 40 °C and 76% humidity.
Fig. 3. ATR IR (a) and Raman (b) spectra of b-form L-LysHCl.
61 L-LysHCl
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IR and Raman spectra of a- and b-forms L-LysHCl are shown in Figs. 2 and 3 and assignments of the bands in Table 1. If one ignores some details, the spectra of a- and b-forms L-LysHCl seem similar enough. In high-frequency region (around 2900 cm 1) of the spectra there are broad strong bands caused by stretching vibrations of NH and CH bonds. In the Raman spectra in this region the strongest peaks caused by stretching vibrations of CH bonds are dominating. Stretching vibrations of NH bonds of NH3+ groups are revealed in the form of broad bands in the 3400–2500 cm 1 range, which are overlapped with strong peaks caused by vibrations of CH bonds. Most easily the spectra of a- and b-forms L-LysHCl can be distinguished by absorption bands in the field of asymmetric stretching vibration of carboxylate group and asymmetric deformation vibration of NH3+ groups. a-Form has in this region three peaks with 1613, 1583 and 1564 cm 1 wavenumbers, while b-form has two peaks at 1623 and 1599 cm 1. Strengthening of NAH. . .O hydrogen bonds of NH3+ groups should lead to shifting of absorption bands of stretching m(NH) vibrations towards low-frequency region and accordingly to shifting towards high-frequency region of bands caused by das(NH3+) deformation vibrations. On this basis one can suppose that hydrogen bonds in the structure of b-form are stronger than that of a-form L-LysHCl. IR spectra of a- and b-forms LLysHCl registered by KBr pellet method [18] are in good correspondence with ATR spectra. However, the spectrum of a-form [18] contains a small peak in the region of stretching vibrations of crystallization water (ca. 3375 cm 1), which is absent in our spectrum. Assignments of other bands are indicated in Table 1. 3.3. On ‘‘tetra L-lysine alanine monohydrochloride dihydrate” Recently a paper [20] has been published, in which authors report on obtaining of a new crystal ‘‘tetra L-lysine alanine monohydrochloride dihydrate (TLAMHCl)” from aqueous solution containing L-LysHCl2H2O and L-alanine in the 4:1 mole ratio. Though in the paper it is spoken about determination of the crystal structure, however, somewhat definite data about structure has not been reported. Moreover, it is difficult to understand what structure of TLAMHCl the authors have in view at interpretation of the IR spectrum when they write that «The FTIR spectra of TLAMHCl crystal was recorded...in order to study the metal complex coordination”, and also ‘‘A peak observed at about 2112 cm 1 in TLAMHCl is because of more available group of AC„CA stretching vibration”. There is also a strange sentence ‘‘The peaks presented at 907 and 861 cm 1 (overtones) are assigned for d(CAH) stretch”. We have obtained crystals from aqueous solutions containing L-LysHCl2H2O and L-alanine in mole ratios 5:1, 4:1, 3:1, 2:1, 1:1, and also from solution containing 4(L-LysHCl2H2O) + L-Ala + HCl. In all cases the IR spectra of obtained crystals exactly coincided with IR spectrum of L-LysHCl2H2O. Therefore we consider that ‘‘tetra L-lysine alanine monohydrochloride dihydrate” represents actually L-LysHCl2H2O. IR spectrum reported in [20] however does not coincide with the spectrum shown in Fig. 1. In the 500–1600 cm 1 range the spectrum [20] have certain similarity with the spectrum reported in [8]. We believe that difference of the spectra [8] and [20] from the ATR IR spectrum registered by us (Fig. 1) is caused by an ionic exchange (Cl ? Br) and partial decomposition in result of registration of the spectrum by KBr pellet method. The TGA/DTA data for ‘‘tetra L-lysine alanine monohydrochloride dihydrate” [20] satisfactorily correspond to respective data of L-LysHCl2H2O [8]. The shifting of the peaks may be caused by different heating rates (20°/min and 10°/min, respectively). However, interpretation of the data of thermal analysis is contradictory. On the one hand they write
that ‘‘The calorimetric measurements clearly show an existence of four solid to solid reversible phase transitions” and on the other hand in the conclusion they write that ‘‘there is no phase transitions and decomposition till 266 °C”. Also on the one hand they write that the sample loses two water crystallization molecules below 133.2 °C and on the other hand that ‘‘the material is thermally stable in the working temperature range that is 30– 266 °C”. Actually above dehydration temperature the LLysHCl2H2O crystal as such does not exist any more and the working temperature range is limited, as authors of Ref. [8] rightly noted, by dehydration temperature (650 °C). 4. Conclusions ATR FTIR and Raman spectra of L-LysHCl2H2O are reported and discussed taking into consideration its crystal and molecular structure. Conditions of obtaining two forms of anhydrous L-LysHCl and their ATR FTIR and Raman spectra are reported and discussed. Previously reported ‘‘tetra L-lysine alanine monohydrochloride dihydrate” [20] according to our data actually is L-lysine monohydrochloride dihydrate L-LysHCl2H2O. Acknowledgements The present work was supported by the Government of Armenia (Grant # 0111). Authors thank Dr. Aghajanyan A.E. for providing with L-lysine hydrochloride reagent and also Dr. Martirosyan G.G. for the help in registration of the spectra. References [1] Suzuki, Y. Matsuoka, Jpn. Kokai Tokkyo koho JP 03 03, 834 [91 33, 834] (Cl. G02F1/35) 14 February 1991, appl. 89/168, 872, 4 pp. [2] A.M. Petrosyan, R.P. Sukiasyan, S.S. Terzyan, V.M. Burbelo, Z. Naturforschung 53a (1998) 528. [3] A.M. Petrosyan, R.P. Sukiasyan, S.S. Terzyan, V.M. Burbelo, Acta Cryst. B55 (1999) 221. [4] A.M. Petrosyan, V.M. Burbelo, R.A. Tamazyan, H.A. Karapetyan, R.P. Sukiasyan, Z. Naturforschung 55a (2000) 199. [5] M.K. Marchewka, S. Debrus, H. Ratajczak, Cryst. Growth Des. 3 (2003) 587. [6] S. Debrus, M.K. Marchewka, J. Baran, M. Drozd, R. Czopnik, A. Pietraszko, H. Ratajczak, J. Solid State Chem. 178 (2005) 2880. [7] R.P. Sukiasyan, Proc. Conf. Laser Physics-2004, October 12–15, 2004, Ashtarak, Armenia, 2005, pp.105–108. [8] R. Ramesh Babu, N. Vijayan, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (2006) 405. [9] R. Ramesh Babu, K. Sethuraman, N. Vijayan, G. Bhagavannarayana, R. Gopalakrishnan, P. Ramasamy, Cryst. Res. Technol. 41 (2006) 906. [10] V. Mathivanan, T. Raghavalu, M. Kovendhan, K. Suriya Kumar, S. Gokul Raj, G. Ramesh Kumar, R. Mohan, Cryst. Res. Technol. 43 (2008) 248. [11] Z.H. Sun, G.H. Zhang, X.Q. Wang, X.F. Cheng, X.J. Liu, L.Y. Zhu, H.L. Fan, G. Yu, D. Xu, J. Cryst. Growth 310 (2008) 2842. [12] R.P. Sukiasyan, H.A. Karapetyan, A.M. Petrosyan, J. Mol. Struct. 888 (2008) 230. [13] R. Ramesh Babu, K. Sethuraman, R. Gopalakrishnan, P. Ramasamy, J. Cryst. Growth 297 (2006) 356. [14] K. Sankaranarayanan, P. Ramasamy, J. Cryst. Growth 280 (2005) 467. [15] D.A. Wright, R.E. Marsh, Acta Cryst. 15 (1962) 54 (Correction 16 (1963) 431). [16] T.F. Koetzle, M.S. Lehmann, J.J. Verbist, W.C. Hamilton, Acta Cryst. B28 (1972) 3207. [17] R.R. Bugayong, A. Sequeira, R. Chidambaram, Acta Cryst. B28, 1972, 3214. [18] Y. Takayanagi, K. Iizumi, M. Miyazawa, N. Yamaya, H. Tamura, US Patent 4256917, 1981. [19] A.M. Petrosyan, Vibrational Spectrosc. 43 (2007) 284. [20] V. Sivashankar, R. Sankar, R. Siddheswaran, R. Jayavel, P. Murugakoothan, Mat. Chem. Phys.109 (2008) 119. [21] K. Nakamoto, M. Margoshes, R.E. Rundle, J. Amer. Chem. Soc. 77 (1955) 6480. [22] A. Novak, in: Infrared and Raman Spectroscopy of Biological Molecules. Proc. NATO Advanced Study Institute, Athens, Greece, August 22–31, 1978, Dordrecht 1979, p. 279. [23] V. Krishnakumar, R. Nagalakshmi, S. Monohar, L. Kocsis, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. (2008), doi:10.1016/j.saa.2007.11.035.