CIDNP study of sensitized photooxidation of S-methylcysteine and S-methylglutathione in aqueous solution

Abstract

Time-resolved CIDNP (chemically induced dynamic nuclear polarization) was applied to the investigation of the photo-oxidation of S-methylcysteine, Cys(Me), and S-methylglutathione, GS(Me). It was established that the reaction of Cys(Me) with a photosensitizer, triplet excited 4-carboxybenzophenone or 3,3′,4,4′-carboxybenzophenone, occurs via electron transfer from the sulfur atom with the formation of a sulfur-centered cation radical. From the pH-dependence of nuclear polarization it is established that the pKa value of the terminal amino group of this radical is 4.3, which is much lower than that of the initial compound, pKa = 9.0. At pH below 4.3 the sulfur-centered cation radical of Cys(Me) formed as a result of triplet quenching participates in the degenerate electron exchange reaction with the parent molecule resulting in a fast decay of CIDNP in time. At pH above this pKa, CIDNP is constant in time, which is a result of fast decarboxylation of the cyclic radical with a three-electron bond between sulfur and oxygen atoms that is formed from a linear radical with an uncharged amino group. The involvement of this radical was established by the dependence of CIDNP on the external magnetic field ranging from 0.1 to 7 T. For GS(Me), the formation of the cyclic radical the indication of which is time-independent CIDNP correlates with the deprotonation of the terminal amino group of the glutamate residue of GS(Me). The formation of a cyclic radical with a three-electron bond between sulfur and nitrogen atoms is ruled out because of the absence of polarization of the CH-proton in α-position with respect to the amino group, and also by the CIDNP field dependence which is typical for an (S∴O) radical. Thus, the cyclic radical originating from one-electron removal from sulfur of GS(Me) is the 10-membered one with a three-electron bond between the sulfur atom and the oxygen atom of the carboxylic group of the glutamate residue.