Rotationally resolved electronic spectroscopy of biomolecules in the gas phase. Melatonin

► Rotationally resolved electronic spectra of the A and B bands of melatonin have been analyzed. ► The A band is due to a gauche structure of the side chain, while the B band is an anti structure. ► The potential energy barriers of the internal rotation of the methyl group was determined in both electronic states. ► The lowest electronically excited state is the Lb state.