Photo-CIDNP Study of Transient Radicals of Met-Gly and Gly-Met Peptides in Aqueous Solution at Variable pH

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) was applied to the investigation of the photo-oxidation of two sulfur containing peptides, glycylmethionine (Gly-Met) and methionylglycine (Met-Gly). It was established that the reaction of Gly-Met with a photosensitizer, triplet 4-carboxybenzophenone, occurs via electron transfer from the sulfur atom and also from the terminal amino group in its uncharged state. The latter process leads to the formation of nuclear polarization of the a-protons of the glycine residue. The sulfur-centered cation radical of Gly-Met formed as a result of triplet quenching participates in the degenerate electron exchange reaction with the parent molecule. The rate constant of this reaction obtained from a simulation of the CIDNP kinetics is 2 x 10(8) M-1 s(-1). Two channels of triplet quenching were also found for the Met-Gly peptide at pH values above the pK(a) of the terminal amino group: electron transfer from the amino group and from the sulfur atom. On the basis of the analysis of the CIDNP spectra and kinetics, it was found that at pH below pK(a) of the terminal amino group photo-oxidation of Met-Gly leads to the formation of an open-chain S-centered cation radical, which releases a proton from its N-terminal amino group to form a five-membered cyclic radical structure with a three electron bond between the S and N atoms. The rate constant of deprotonation obtained to be 1.8 x 10(5) s(-1) is in agreement with the pK(a) = 4.7 of the S-centered radical of Met-Gly determined from the pH dependence of nuclear polarization. At pH > pK(a), the aminium radicals formed in both peptides as a result of electron transfer from the lone pair of N-terminal amino group undergo deprotonation to the neutral aminyl radical on the submicrosecond time scale. The involvement of the different radicals was confirmed by the dependence of CIDNP on the external magnetic field ranging from 0.1 T to 7 T.