Photo-oxidation of seven cyclic dipeptides containing methionine, Met, and/or S-methyleysteine, Cys(Me) by-electron transfer from the sulfur atom was studied in aqueous solution by time-resolved and field dependent CIDNP (chemically induced dynamic nuclear polarization). Hyperpolarized high resolution NMR spectral patterns of the starting peptides detected immediately after pulsed laser excitation show signals of all protons that are bound to carbons neighboring the sulfur atom, thus proving the involvement of sulfur-centered cation radicals. The magnetic field dependence Of CIDNP shows a pronounced maximum that is determined by the g-factors and hyperfine coupling constants. of the transient radical species. From simulation of the experimental data obtained for the magnetic field dependences of CIDNP, three types of radical structures were characterized: (1) a linear sulfur-centered cation radical of the methionine (Met) residue (g = 2.0107 +/- 0.0010) for cyclo-(D-Met-L-Met) (transconfiguration), cyclo-(D-Met-L-CysMe)) (trans-configuration), and Cyclo-(Gly-Met); (2) a cyclic radical (S therefore O)(+) (g = 2.0088 +/- 0.0010) with a two-center three-electron bond (2c=3e) structure between the sulfur atom of the Cys(Me) residue and the oxygen atom: of cyclo-(D-Met-L-Cys(Me))and cyclo-(Gly-Cys(Me)); (3) a cyclic radical (S therefore S)(+) (g = 2.013 +/- 0.0020) with a two-center, three-electron bond structure between the two sulfur atoms of the peptides cyclo-(L-Met-L-Met), cyclo-(L-Met-L-Cys(Me)), and cyclo-(L-Cys(Me)-L-Cys(Me)). In contrast, no indication of any type of cyclic radicals with a two-center three-electron bond between sulfur and nitrogen atoms was found. In, addition, the hyperfine coupling constants (HFCCs) were determined.