Photolysis and quantum-chemical calculations of the nalidixic acid radical states

Abstract

The nalidixic acid transient spectra were analyzed and the scheme of photo-conversion of the acid was suggested. At a large delay of laser pulses the transient absorption spectra contain the bands due to the dissociation of the products of the cation-and anion-radical photodecomposition in the micro-and millisecond range. By quantum-chemical calculations we revealed that at the transition to the ion-radical state a proton migration occurs subsequent to the movement of the center of gravity of the negative charge. The phototransformation of nalidixic was found to proceed with a TICT-effect.The antibiotics of quinolone class (fqH) have found wide application in medicine. However, the tendency of some antibiotics to phototoxicity [1] requires the study of the influence of light on the composition and phototransformations of the shortliving intermediates. This paper presents the results of laser flash photolysis and quantum chemical calculations of the anionic, neutral and cationic protolytic forms of nalidixic acid (nlqH). The numbering of atoms in the structure of nlqH is shown in Fig. 1.The primary photochemical processes at the interaction of photons with the compounds of fluoroquinolones is intra-or intermolecular electron transfer between the fragments with the transition of nalidixic acid into the singlet S 1 -excited state.The stepwise photophysical and photochemical processes within nanosecond and microsecond ranges are accompanied by competing processes that include energy migration, nonradiative energy loss, electron and proton transport, and luminescence. The primary photochemical process in the case of nalidixic acid is commonly regarded as the intramolecular electron transfer.We believe that the processes of protonation or deprotonation of quinolones occurring at the intra-or intermolecular electron transfer in the course of the photoexcitation play a key role in the acid-base catalysis of antibiotics. It was of interest to follow the spectral-kinetic and photochemical properties of nalidixic acid in acidic and basic environments for the cationic, neutral and anionic forms. In the study of photolysis of nalidixic acid we found the qualitative Fig. 1. Structure of nalidixic acid with numbering of its atoms (Arabic numerals) and fragments (Roman numerals).