Vibrational analysis of amino acids and short peptides in hydrated media. VI. Amino acids with positively charged side chains: L-lysine and L-arginine.

In two recent reports of the same series (J. Phys. Chem. B 2007, 111, 1470-1477 and J. Phys. Chem. B 2009, 113, 3169-3178), we have described the geometrical and vibrational analysis of glycine and amino acids (AAs) with hydrophobic side chains through the joint use of optical spectroscopy and quantum mechanical calculations. Here, we report Raman scattering and Fourier-Transform Infrared (FT-IR) Attenuated Total Reflectance (ATR) spectra measured from the aqueous solutions (H(2)O and D(2)O) of L-lysine and L-arginine, i.e. two alpha-AAs with positively charged hydrophilic side chains. The discussion on the vibrational features of both AAs could be carried out thanks to the theoretical calculations performed by means of the Density Functional Theory (DFT) approach at the B3LYP/6-31++G* level. We have analyzed the influence of implicit (with a polarizable dielectric continuum) and explicit (by means of an H(2)O cluster interacting with H-donor and H-acceptor sites of AAs) hydration models. In addition, through the calculated geometrical parameters and vibrational wavenumbers, a discussion was performed on the effect of the Cl(-) anion interacting with the positively charged side chains of explicitly hydrated AAs.