Structural and vibrational study on zwitterions of l-threonine in aqueous phase using the FT-Raman and SCRF calculations

Quantum mechanical/molecular mechanics (QM/MM) calculations were performed to study the most stable theoretical structure of l-threonine in an aqueous solution phase and to observe the changes occurring in the structural and vibrational properties of l-threonine in the aqueous media. l-threonine was characterized by Raman spectroscopies in the solid state and in aqueous solution. Optimized geometries and relative stabilities for the monomer and two dimers were calculated using the hybrid B3LYP level taking into account the solvent effects by means of the integral equation formalism of the polarizable continuum model (IEF-PCM). The obtained results in the aqueous solution were compared with those calculated for the monomer in the gas phase. The presence of hydrogen bonding in the monomer of l-threonine was studied by means of the Atoms in Molecules theory (AIM) and natural bond orbital (NBO) studies. The complete assignments of the vibrational spectra of l-threonine in the aqueous solution was performed by using the Pulay’s scaled quantum mechanical force field (SQMFF) methodology. Additionaly, hydrated complexes of the monomer considering the interaction with one, two, three and four discrete water molecules were also studied taking into account the solvent effect by using the IEFPCM method. An agreement between theoretical and available experimental results was found.