Cysteine Thioaldehydes: Photolytic Generation, Reactivity, and Biological Implications

Cysteine thioaldehydes are putative intermediates in sulfatase-maturing enzyme catalysis and in the biosynthesis of penicillin, coenzyme A and families of ribosomally synthesized and posttranslationally modified peptides (RiPPs). Norrish type-II photolysis of phenacylsulfides gives cysteine thioaldehydes but can be inefficient in aqueous solution owing to competing β-scission. We show that irradiation of phenacylsulfides bearing non-conjugating electron-withdrawing substituents leads to efficient Norrish type-II fragmentation. In aqueous buffer, photolysis of cysteine phenacylsulfides leads to high conversion into cysteine enethiolate and isothiazolone, which can be interconverted with an oxidant (Ellman's reagent) or reductant (phosphine). Cysteine enethiolates are relatively stable in aqueous buffer at neutral-to-high pH but undergo cyclocondensation to cysteine thiazole at low pH. Cysteine thioaldehydes undergo appreciable levels of hydrolysis to give formylglycine in the absence of buffer and for longer peptides that adopt enabling secondary structure. Nucleophilic (n → π*) participation of the proximal backbone carbonyl group on cysteine thioaldehydes can lead to peptide oxazoles. These results suggest viable oxidation-before-cyclocondensation biosynthetic pathways for peptide azoles and that the hydrolysis of cysteine thioaldehydes into formylglycine, as proposed for sulfatase activation, is not trivial and relies on a peptide environment that promotes the desired hydration at the expense of other similarly efficient thiocarbonyl-quenching processes.