关键词: 芳香性氨基酸, 二阶极化率, 质子化和去质子化分子, 密度泛函理论, 态求和方法

Abstract:

The geometrical structures of a series of neutral, protonated, and deprotonated aromatic amino acids (Phe, [Phe―H]^-, PheH⁺, Tyr, [Tyr―H]^-, TyrH⁺, Trp, [Trp―H]^-, and TrpH⁺) were optimized using density functional theory (DFT)-B3LYP with a 6-31G^* basis set. Based on the optimized structures, the excited state properties were studied using time-dependent DFT at the B3LYP/6-31G^* level. We calculated the second-order polarizabilities for second harmonic generation with the sum-over-states method. We examined the origins of the nonlinear optical responses and determined the cause for the variation in the second-order polarizabilities. Our calculations show that the second-order polarizabilities for protonated, and deprotonated aromatic amino acids are much higher than those for the neutral aromatic amino acids, with the order Phe < PheH⁺ < [Phe―H]^- and Tyr < TyrH⁺ < [Tyr―H]^-. By analyzing their electronic origins, we find that charge transitions in the side chains (benzene, phenol, and indole) make the main contributions to the second-order polarizability for neutral aromatic amino acids. For protonated and deprotonated aromatic amino acids, π→π^* charge transfers within indole rings, and charge transfers within amino groups and the carboxyl groups attached to alpha-carbon atoms make almost identical contributions to the second-order polarizability.

Key words: Aromatic amino acid, Second-order polarizability, Protonated and deprotonated molecules, Density functional theory, Sum-over-states method