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Print version ISSN 0103-9733
Braz. J. Phys. vol.40 no.3 São Paulo Sept. 2010
J.L.B. FariaI; P.T.C. FreireII,*; R.O. GonçalvesII; F.E.A. MeloII; J. Mendes FilhoII; R.J.C. LimaIII; A.J.D. MorenoIII
IDepartamento de Física, Universidade Federal de Mato Grosso, Cuiabá-MT, 78060-900, Brazil
IIDepartamento de Física, Universidade Federal do Ceará, Fortaleza-CE, 60455-760, Brazil
IIICentro de Ciências Sociais, Saúde e Tecnologia, Universidade Federal do Maranhão, Imperatriz-MA, 65900-410, Brazil
Polarized Raman spectra of L-arginine hydrochloride monohydrated single crystal in nine different scattering geometries of the two irreducible representations of factor group C2 were studied at room temperature. The experimental wavenumber values are compared with those obtained from ab-initio calculation and the assignment of the Raman bands to the respective molecular vibrations is also given. Finally, a discussion related to a previously reported phase transition undergone by L-arginine hydrochloride monohydrated single crystal at low temperature is furnished.
Amino acids (NH2 CH COOH R, where R is a radical) are the basic units of proteins and peptides of all living beings. For an unknown reason Nature has choose 20 of these special molecules, differing in the R part, to form the impressive number of proteins found in our planet. L-arginine, among other amino acids, and its salts, including L-arginine.HCl, is known to protect protein from inactivation in frozen solutions, during freeze-drying , during spray-drying  and in the storage of lyophilized solids . Beyond this, L-arginine and acid combinations have been extensively used in the last years to assist in the recovery of chemically unfolded proteins and recombinant proteins expressed in inclusion bodies .
Beyond these biological aspects related to amino acid in general, and L-arginine in particular, it is also possible to found interesting physical properties related to this amino acid. For example, the search for new non-linear optical materials with high non-linear coefficients and high damage threshold has lead to the discovery of L-arginine phosphate monohydrate (LAP) . These studies also allowed the discovery of new materials having L-arginine as the main substance. Among these material is L-arginine hydrochloride monohydrate (LAHW) whose several physical properties as thermal expansion, elastic properties and dielectric characteristics were presented in Ref. .
Some works deal with the temperature behavior of L-arginine.HCl.H2O monocrystals. It was observed that under high temperature conditions LAHW begins to lost water of crystallization at 70ºC and at 200ºC about two-third of it are eliminated . Under low temperature conditions, on the other hand, it was observed through Raman scattering technique, evidence of a phase transition undergone by LAHW between 100 and 110 K . Although in the paper of Ref.  some Raman spectra are presented, up to now, there is no complete investigation of the polarized Raman spectra for LAHW, as well as, an assignment of the observed modes. Such assignment is very important because it helps us to understand what vibrations are related to the crystal modifications, in particular under temperature and pressure changes.
The objective of this paper is twofold: (i) to present the Raman spectra in the spectral region 20 3700 cm-1 for nine scattering geometries of the two irreducible representations of the C2 factor group; (ii) to give a tentative assignment of the observed Raman bands based on ab-initio calculations.
Single crystals of LAHCL were grown from aqueous solution containing L-arginine hydrochloride powder, C6H14N4O2.HCl, from Sigma by the slow evaporation method at controlled temperature (293 K). In order to characterize and to do the orientation of the crystals we used X-ray diffraction patterns obtained from a Rigaku DMAX diffractometer using Cu Kα radiation monochromated with a graphite crystal. The Raman spectra were obtained with a Jobin Yvon T64000 micro-Raman system equipped with an N2-cooled charge coupled device system. The samples were polished with diamond paste with granulations with 10, 3 and 1 µm. The slits were set for a 2 cm-1 spectral resolution. Excitation was effected with the 514.5 nm radiation from an argon ion laser. The incident laser had a power less than 5 mW on the surface of the sample.
3. RESULTS AND DISCUSSION
Fig. 1 presents the zwitterion form of an arginine molecule (the number associated to the atoms will be used in the assignment of the normal modes). Single crystal data of LAHW crystal confirms that the compound grows with a monoclinic lattice belonging to the P21 () space group with two molecules of arginine (C6H14O2N4), two units of HCl and two units of H2O per unit cell. In the primitive cell of LAHW, all the atoms occupy sites with symmetry C1 and the 186 vibrations can be decomposed into the irreducible representations of the factor group C2 as Γ = 93A + 93B, with one A and two B belonging to the acoustic branch and all the other being Raman and infra red active. In Figs. 2 - 5, Z represents an axis parallel to the  direction, X represents an axis parallel to the  direction and Y is an axis perpendicular to the X and Z axes.
In order to give insights about the assignment of the normal modes of the crystal we have performed ab initio calculations. All calculations were carried out using Gaussian98 (by computational resources in CENAPAD's facilities) and the results were viewed by MOLEKEL programme packages. Geometry optimization and frequency calculation for L-arginine in the gas phase were performed with Self-Consistent methods and Hartree-Fock (HF) level of theory, with 6 31 + G(d,p) and 6 311 ++ G(d,p) basis sets. The molecular framework was designed in Gchempaint (gnome chemical software packages) and the atomic coordenates of isolated l-arginine was used in input file. This structure was optimized using δE < 108 as convergence parameter and the vibrational wavenumbers were then calculated. The output file contained the optimized structure, the Raman and IR intensities and frequencies, and the atomic displacements for each mode. At the optimized structure of the molecule, no imaginary frequency was obtained, proving that a true minimum of the potential energy surface was found.
In all spectra presented in this paper we use the conventional Porto notation a(bc)d, where a and d represent the directions of the incident and the scattered light and b and c represent the directions of polarizations of the incident and the scattered light. The scattering geometries in all figures are related to the irreducible representations of the factor group C2 as follows: x(yy)x, z(yy)z, y(xx)y, z(xx)z, x(zz)x, y(zz)y and z(yx)z are from A irreducible representation and x(yz)x and y(zx)y are from the B irreducible representation.
Figure 2 presents the Raman spectra of LAHW for nine different scattering geometries in the spectral region 25 225 cm1 at room temperature. This region is known to have the lattice modes of the crystal (ω < 200 cm1), although some internal modes can also be observed with low wavenumber, as occurs with the torsional vibration of CO2, τ(CO2), for L-asparagine monohydrated , L-valine  and L-isoleucine .
In fact, the ab initio calculations performed in the present work show the occurrence of vibrations with ω < 200 cm1 that can be associated to internal modes. As an example, using the 6 311 ++ G** base set, it is possible to assign the in-plane torsion of , τ(NH3+)ip, as the mode appearing with wavenumber of 63 cm1.
Related to the torsional vibration of , τ(CO2), our calculations were not able to identify it precisely. However, it is possible to assign the band observed at ~ 189 cm1 as the τ(CO2). This is based on the studies performed on other hydrogenated amino acid crystals [9 - 11] as well as in deuterated one . Such an assignment is important because under extreme conditions torsional vibrations of CO2 can present particular behavior and furnish insights about the hydrogen bonds, as those presented by L-alanine-d7 under pressure variation .
Fig. 3 presents the Raman spectra of LAHW for nine different scattering geometries in the spectral region 200-650 cm1. In this region it is found the bands associated to torsional vibration of , τ(). In general, the band associated to this vibration has a low intensity, at least as suggested by former works on L-alanine  and L-leucine . Other important vibration is associated to the rocking of NH2, r(NH2), which ab initio calculations show to be between 445 and 465 cm1. Bending vibration of NH are also found in this spectral region, at about 530 cm1, while the bending of N C N, δ(NCN), was identified as the peak at ~ 551 cm1, a band with small linewidth as can be observed in Fig. 3 for some scattering geometries.
Fig. 4 presents the Raman spectra of LAHW in the spectral region 650-1750 cm1 for nine different scattering geometries. This region presents a series of bands and the ab initio calculation was fundamental to assign in a most precise manner. The peak observed at about 678 cm1 was associated to a C2 C3 stretching vibration, ν(C2 C3). Here we need to state the following observation: both, C2 and C3 carbon atoms are hold with other atoms (NH3 for C2 and O2 for C3) in such a way that we do not have a C C "pure" vibration, as occurs with diamond; this explains why for diamond the ν(C C) vibration is observed at 1332 cm1. A wagging vibration related to the N C N unit was associated to the band observed at 751 cm1. The bending of unit, δ(), was observed at 843 cm1, as pointed out by our calculations. The bands observed at about 930 and 940 cm1 were associated to the wagging vibration of NH2, w(NH2). The stretching vibration of C2 N1, ν(C2 N1), was associated to the band observed at ~ 1052 cm1, and an out-of-plane rocking vibration, r(), was assigned as the band observed at about 1090 cm1. An out-of-plane vibration was assigned as the band observed at ~ 1250 cm1.
Many bands are observed between 1300 and 1400 cm1, among them the twist vibration of CH2, t(CH2). It is worth to note that the 6-311G basis of calculation suggests that the C2 C3 stretching vibration should be observed at 1388 cm1. However, because the reason given previously (carbon atoms bonded to other atoms) we believe that the suggestion is not correct. The band observed at 1406 cm1 was associated with the bending vibration of , δ(), and a rocking vibration of the same unit, r(), was associated to the band observed at ~ 1472 cm1. Finally, in this spectral region we also observed bending vibrations of the H N C unit, between 1600 and 1650 cm1.
Fig. 5 presents the Raman spectra of LAHW crystal in the spectral region 2800-3600 cm1 for nine different scattering geometries. The profile between 2800 and 3000 cm1 is very rich, originated mainly from stretching vibrations of CH. However, our calculations were not able to assign separately all bands in this region. Finally, the bands with high linewidths centered at ~ 3200 cm1 and 3400 cm1 can be associated to the stretching vibration of water, ν(H2O), as it is expected from crystals with structural water .
With this assignment we can both (i) through light on the phase transition undergone by LAHW at low-temperatures and (ii) to give insights on future works on vibrational properties of the crystal under high pressure conditions. Related to the first point we remember that Raman spectroscopy study have suggested the occurrence of a phase transition undergone by LAHW crystal at about 110 K . This phase transition was inferred mainly by the change of band profiles in the low-wavenumber region of the spectra. Obviously, as stated previously, these modifications are associated with lattice mode vibrations. Interesting enough is that we have also observed modifications in bands observed at 1088-1100 cm1. The assignment of the present study confirms the identification of such bands as rocking vibrations of NH3 units. In other words, our study reinforces the interpretation that the low temperature phase transition undergone by LAHW can be understood mainly as conformational change of the L-arginine molecules, mainly consequence of modifications on hydrogen bonds N H ... O, with the oxygen atom belong to a water molecule or to the COO group of another arginine molecule. This is in good accordance with experimental facts already presented by L-cisteine  and L-serine  crystals when submitted to high pressure conditions - the change of molecular conformation, due to variations in the dimensions of hydrogen bonds, modifies the symmetry of the unit cell of the crystal.
In this paper we presented the polarized Raman spectra of LAHW in the spectral region between 25 and 3600 cm1. Ab initio calculations were used to assign most of the bands observed in all nine scattering geometries. With such identification we gave insights about a previously observed phase transition undergone by LAHW at low temperatures. Finally, the work will be useful to understand eventual modifications in the Raman spectra of LAHW, for example, when submitted to high pressure conditions in future works.
The authors acknowledge CNPq, CAPES and FUNCAP for partial financial support. The authors also acknowledge Dr. J.M. Sasaki for X-ray diffraction measurements and CENAPAD-SP for the use of the GAUSSIAN 98 software package and for computational facilities through the project reference "proj373".
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(Received on 16 October, 2009)