Hydration of l-glycylvaline and l-glycylvalylglycine zwitterions: Structural and vibrational studies using DFT method

Solvent effects on the structural and vibrational features of l-glycylvaline (L-GV) and l-glycylvalylglycine (L-GVG) in zwitterionic forms have been performed by means of DFT method. Relaxed potential energy surface scans (RPES) performed at B3LYP/6-31++G(d) level were used to map the reaction coordinate surfaces and to identify the geometries corresponding to the minima energy. Explicit solvation model, where L-GV and L-GVG are respectively surrounded by 9 and 11 water molecules interacting with H-donor and H-acceptor sites, as well as the different hybrid solvation models of solvation (explicit/COSMO, explicit/PCM and explicit/CPCM) allowed to analyze the hydration effects. Those number of water molecules are sufficient to fully hydrate carboxyl(C_tOO^-), amine (N_tH₃⁺) and amide (NH, C=O) groups. Harmonic vibrational modes calculated after geometry optimization on each solvated complex are performed at B3LYP/6-31++g(d) and PBE0/6-31++g(d) methods and a post-processing treatment enable us to assign the vibrational modes of LGV and L-GVG. The frequencies of the assigned modes obtained using B3LYP in explicit/COSMO are in good agreement with available IR and Raman values than those found in both explicit/PCM and explicit/CPCM which proved to be very close.