466 RAMAN SPECTRA OF L-ISOLEUCINE CRYSTALS F.M. Almeida, J.H. da Silva, P.T.C. Freire, V. Lemos * , F.E.A. Melo, and J. Mendes Filho Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, Campus do Pici, 60455—970 Fortaleza, Ceará, Brazil; E-Mail:tarso@fisica.ufc.br, * volia@fisica.ufc.br Keywords: amino acid crystal, Raman scattering Abstract: This contribution reports on experimental work of polarized Raman scattering from L- isoleucine crystals. From the room temperature spectra several vibrations are assigned. The Raman spectra at low temperatures point to the stability of the orthorhombic D 2 2 structure down to 10 K. Amino acid crystals can be used as simple systems to model low-frequency dynamics of proteins and to investigate phonon dynamics and structural instabilities [1]. Of particular interest in the study of amino acid crystals is the temperature effects on the vibrational properties by Raman spectroscopy to gain information on: (i) the temperature coefficients of normal mode wavenumbers (ii) the stability of the structure with variation of temperature, and (iii) the temperature effects on various types of intermolecular forces, such as the hydrogen bond [2 - 3]. Isoleucine (L-2-amino-3-methylopentanoic acid) is an aliphatic amino acid, which crystallizes in an orthorhombic structure belonging to P222 1 (D 2 2 ) space group [4]. X-ray measurements show that L-isoleucine, similar to L-valine, has two crystallographically independent molecules with trans and gauche I conformations in the unit cell [5]. Fig. 1 shows the Raman spectra in the low wavenumber region in two scattering geometries. Both geometries allows for the appearance of modes from the irreducible representation A of the factor group D 2 . The bands with wavenumber ν < 150 cm -1 are assigned to the translational lattice vibrations, and those with ν > 150 cm -1 to motions associated with out-of-plane vibrations γ(N - H … O), C α - C - C bending, C O - C α - C bending, C α - H torsion, among others. 200 400 600 536 489 446 389 346 314 204 177 129 82 38 556 556 536 491 445 425 389 342 278 203 142 171 88 40 intensity c(bb)c c(aa)c L-isoleucine Wavenumber / cm -1 Fig. 1. Room-temperature Raman spectra of L-isoleucine crystal recorded in two scattering geometries in the [0 – 600] cm -1 region. Fig. 2 shows the Raman spectra of the L-isoleucine crystal in the c(bb)c scattering geometry for selected temperatures, in the low wavenumber range where the bands associated with the lattice