Vacuum-ultraviolet electronic circular dichroism of L-alanine in aqueous solution investigated by time-dependent density functional theory.

The electronic circular dichoism (ECD) of L-alanine in the vacuum-ultraviolet region was calculated for various optimized structures using time-dependent density functional theory (TDDFT) to assign the CD spectrum observed experimentally in aqueous solution down to 140 nm [Matsuo, et al. Chem. Lett. 2002, 826]. The structure of L-alanine in vacuo was optimized using density functional theory (DFT) at the B3LYP/6-31G* level. Its hydrated structure was optimized with nine water molecules (six and three around carboxyl and amino groups, respectively) using DFT and a continuum model (Onsager model). The dihedral angles of carboxyl and amino groups in the optimized hydrated structure differed greatly from those in the crystal and in nonhydrated structures optimized using a continuum model only. The ECD spectrum calculated for the hydrated structure had two successive positive peaks with molar ellipticities of about 2000 deg cm2 dmol(-1) at around 205 and 185 nm, which were close to those observed experimentally. These positive peaks were attributable to n pi* transitions of the carboxyl group, with the latter peak also influenced by the pi pi* transition of the carboxyl group that originates below 175 nm. A small negative peak observed at around 252 nm was also predicted from the hydrated structure. These results demonstrate that the hydrated water molecules around the zwitterions play a crucial role in stabilizing the conformation of L-alanine in aqueous solution and that TDDFT is useful for the ab initio assignment of ECD spectra down to the vacuum-ultraviolet region.