Theoretical Thermodynamic Study of Arginine and Lysine Amino Acids at different Solvents

The thermodynamic functions such as enthalpy, H°, Gibbs free energy, G°, and entropy, S°, of Arginine and Lysine amino acids were theoretically studied at different polar solvents by using ²Gaussian o3², software. First, the structural optimization of isolated Arginine and Lysine were done in the gas phase by applying the Density Functional Theory (B3LYP) level with 3-21G, 6-31G and 6-311G basis sets. Moreover, vibrational frequencies were calculated in gas phase on the optimized geometries at the same level of theory to obtain enthalpy, H°, Gibbs free energy, G°, and entropy, S°. Then, the calculation about the solvent effects on the thermodynamic functions of Arginine and lysine amino acids were performed for the various polar solvents (Water, Methanol and Ethanol) by using self-consistent Reaction-Field (SCRF=PCM) model at B3LYP/6-311G. Thermodynamically analysis shows the relative enthalpy changes, DH°, Gibbs free energy changes, DG°, are negative values but the entropy changes, DS°, are positive values for Arginine and Lysine molecules. Also, the results shows, with increasing dielectric constant of solvents the stability of considered molecules increases.