theoretical study of the solvent effects on the thermodynamic functions of alanine and valine amino acids

using gaussian 03, software the thermodynamic functions such as gibbs free energy, g, enthalpy, h, and entropy, s, of alanine and valine amino acids were theoretically studied at different solvents. first, the density functional theory (b3lyp) level with 3-21g, 6-31g and 6-31+g basis sets were employed to optimization of isolated alanine and valine amino acids in the gas phase. moreover, vibrational frequencies were calculated in gas phase on the optimized geometries at the same level of theory to obtain the thermodynamic functions such as gibbs free energy, g, enthalpy, h, and entropy, s,. then, the calculation about the solvent effects on the thermodynamic functions of alanine and valine amino acids were performed for the various solvents (water, methanol and ethanol) by using self consistent reaction-field (scrf=pcm) model at b3lyp/6-31+g. thermodynamically analysis shows the relative gibbs free energy changes, g, enthalpy changes, h, are negative values but the entropy changes, s, are positive values for alanine and valine amino acids. also, the results shows, with increasing of dielectric constant of solvents the stability of considered amino acids increases.