Experimental and Theoretical Studies of Vacuum-Ultraviolet Electronic Circular Dichroism of Hydroxy Acids in Aqueous Solution

The electronic circular dichroism (ECD) spectra of three L-hydroxy acids (L-lactic acid, (1)-(S)-2-hydroxy-3-methylbutyric acid, and (2)-(S)-2-hydroxyisocaproic acid) were measured down to 160 nm in aqueous solution using a vacuum-ultraviolet ECD spectrophotometer. To assign the two positive peaks around 210 and 175 nm and the one negative peak around 190 nm in the observed spectra, the ECD spectrum of L-lactic acid was calculated using time-dependent density functional theory (DFT) for the optimized structures by DFT and a continuum model. The observed ECD spectrum was successfully reproduced as the average spectrum for four optimized structures with seven water molecules that localized around the COO- and OH groups of L-lactic acid. The positive peak around 210 nm and the negative peak around 185 nm in the calculated spectrum were attributable to the n pi* transition of the carboxyl group, with the latter peak also being influenced by the pi pi* transition of the carboxyl group; however, the positive peak around 165 nm involved unassignable higher energy transitions. The comparison of the calculated ECD spectra for L-lactic acid and L-alanine revealed that the network with loose hydrogen bonding around the COO- and OH groups is responsible for the flexible conformation of hydroxy acids and complicated side-chain dependence of ECD spectra relative to amino acids. Chirality 23: E52-E58, 2011. (C) 2011 Wiley Periodicals, Inc.