p-Azidophenyl phosphate is a photoactivatable phosphorylating reagent and p-benzoquinone monoimine precursor

p-Azidophenyl phosphate (I) has been exposed to ultraviolet light (λ=313 nm) in aqueous solution with or without Lys. Analysis of the photoproducts by means of UV–VIS, IR, 1H, 13C and 31P NMR spectroscopy has revealed that under irradiation of I inorganic phosphate (Pi) is released, and p-benzoquinone monoimine (II) and p-benzoquinone (III) have appeared. The electrophilic nature of the intermediate results in a high tendency to react with lysine molecules, whereas the reaction with water is less favourable when I is irradiated in the presence of Lys. The product formed in this case is a phosphoramidate whose structure has been tentatively supported by 31P NMR spectroscopy. These results imply that a p-azidophenyl phosphate is a highly potent aryl nitrene-precursor, which can be transformed easily into p-benzoquinone monoimine and is able to phosphorylate nucleophilic groups of amino acids. This finding is of great importance for the discussions about the chemical nature of protein photomodification products when p-azidophenyl phosphate derivatives are used as modifying reagents.