Electronic spectra and structure of allopurinol: A xanthine oxidase inhibitor

Electronic absorption, fluorescence and fluorescence excitation spectra of allopurinol, a well-known inhibitor of the enzyme xanthine oxidase, have been studied in aqueous solution at different pH. The observed spectra have been interpreted in terms of the neutral, anionic and cationic forms of allopurinol with the help of molecular orbital calculations which included optimisation of geometries of the neutral keto and enol forms of the molecule in the lowest singlet excited state. Electrostatic potential mapping has been performed to identify the most probable site of binding of allopurinol with xanthine oxidase.