On the biological activity of drug molecules: Busulfan and nabumetone

► Electronic structure of drug molecules busulfan and nabumetone has been determined by UV photoelectron spectroscopy and high-level ab initio calculations. ► The electronic structure of busulfan has been related to its biological activity as an alkylating agent. ► The electronic structure of nabumetone has been related to its metabolic transformation with the aid of molecular docking study. ► The electronic structure of mesyl group which is commonly encountered in organic chemistry has been studied for the first time.