EXPERIMENTAL AND THEORETICAL INVESTIGATION ON L-CYSTEINE IN AQUEOUS MEDIUM. A STUDY COMBINING FTIR AND RAMAN WITH QUANTUM MECHANICAL/MOLECULAR MECHANICS METHODS

KARINA GUZZETTI; LILIAN EMILIA DAVIES; ALISIA YURKINA; ELIDA ROMANO; SILVIA ANTONIA BRANDÁN

Structural Analysis and Modelling: Research and Development

Nova Publishers

Lugar: Los Ángeles; Año: 2013; p. 1 - 36

Quantum mechanical/molecular mechanics (QM/MM) calculations were carried out in order to study the theoretical structures of L-cysteine in gas and in aqueous solution phases and to observe the changes that occur on structural and vibrational properties in both media. L-cystine was characterized by infrared spectra in solid state and Horizontal Attenuated Total Reflectance (HATR) spectroscopies in aqueous solution and by Raman spectroscopy techniques in solid state, in aqueous solution and in an alkaline solution. Optimized geometries and relative stabilities for two monomer zwitterions were calculated at the B3LYP/6-31G* and B3LYP/6-311++G** theory levels taking into account the solvent effects by using the integral equation formalism of the polarizable continuum model (IEF-PCM). For a complete assignment of the IR and Raman spectra of L-cysteine in aqueous solution, density functional theory (DFT) calculations were combined with Pulay´s scaled quantum mechanical force field (SQMFF) methodology in order to fit the theoretical wavenumbers values to the experimental ones. Furthermore, two theoretical molecular structures for two dimeric species of L-cystine by using the MM/AMBER force field were studied and a complete assignment of the 78 vibrational normal modes for both dimers in the solid phase was accomplished. The presence of hydrogen bonding in monomers and dimers of L-cysteine was studied by means of Atoms in Molecules theory (AIM). An agreement between theoretical and available experimental results was found.