title: Experimental and theoretical studies of vacuum-ultraviolet electronic circular dichroism of hydroxy acids in aqueous solution.
abstract: The electronic circular dichroism (ECD) spectra of three L-hydroxy acids (L-lactic acid, (+)-(S)-2-hydroxy-3-methylbutyric acid, and (-)-(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 nπ* transition of the carboxyl group, with the latter peak also being influenced by the ππ* 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.
authors.affiliation: Department of Mathematical and Life Sciences, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Fukuyama
authors.forename: Koichi
authors.initials: K
authors.lastname: Matsuo
authors.forename: Kunihiko
authors.initials: K
authors.lastname: Gekko
mesh_terms: D000409:Alanine / Q000737:chemistry; D002621:Chemistry / Q000379:methods*; D002942:Circular Dichroism; D004563:Electrochemistry / Q000379:methods; D004581:Electronics; D006880:Hydroxy Acids / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D015203:Reproducibility of Results; D013237:Stereoisomerism; D017356:Synchrotrons; D013696:Temperature; D013997:Time Factors; D014466:Ultraviolet Rays; D014618:Vacuum; D014867:Water / Q000737:chemistry*
chemical_list: D006880:Hydroxy Acids; D014867:Water; D000409:Alanine