Conformation specific and charge directed reactivity of radical cation intermediates of α-substituted (amino, hydroxy, and keto) bioactive carboxylic acids

Abstract

The reactivity of radical cation carboxylic acids is investigated on the basis of mass spectrometry, infrared-vacuum ultraviolet (IR-VUV) photoionization spectroscopy, and high level correlated ab initio calculations. Their reactivity is found to be highly conformation specific and is governed by their initial charge distribution following ionization. In the present work, the radical cations of lactic acid, pyruvic acid, glycine, and valine are studied to probe their stability and conformation specific reactivity following single photon, vertical ionization at 10.5 eV. For lactic acid, glycine, and valine, the localization site of the hole following sudden removal of an electron depends on their specific intramolecular hydrogen bonding network. Lactic acid, glycine, and valine undergo complete fragmentation following vertical ionization at 10.5 eV; however, pyruvic acid does not completely dissociate following vertical ionization. Only 45% of the pyruvic acid parent ions undergo C_α–C_carboxylic bond dissociation. If the hole is localized on the COOH moiety of glycine, valine, and lactic acid, a hydrogen transfer is favored from the COOH to the α-substituent. If the hole is localized on the α-hydroxy or -amine substituent and the singly occupied molecular orbital (SOMO, where the hole resides) is parallel to the C_α–C_carboxylic bond, C_α–C_carboxylic bond dissociation occurs through charge transfer from the α-substituent to the C_α–C_carboxylic bond. The present study reveals that the specific conformations of α-substituted carboxylic acids govern their radical cationic reactivity. The radical cation of pyruvic acid exhibits a special stability due to enolization of the α-keto form on the cationic surface.