PHOTOINDUCED ELECTRON-TRANSFER, DECARBOXYLATION, AND RADICAL FRAGMENTATION OF CYSTEINE DERIVATIVES - A CHEMICALLY-INDUCED DYNAMIC NUCLEAR-POLARIZATION STUDY

The photoreactions of cysteine derivatives I with 4-carboxybenzophenon e in D2O were investigated by measurements of chemically induced dynam ic nuclear polarization (CIDNP). The quenching mechanism is electron t ransfer from sulfur at every pH; even if the amino group of I is depro tonated, electron transfer from nitrogen does not participate. Decarbo xylation of I-.+ to give alpha-aminoalkyl radicals V-. occurs on the C IDNP time scale and causes strong cooperative effects. The decarboxyla tion rate is increased significantly by deprotonation of the amino fun ction; this is due to product control. V-. decays by two competing pat hways. Fragmentation of the C-beta-S bond in V-. yields vinylamine, wh ich is hydrolyzed to acetaldehyde at pH less than or similar to 7.25, and thiyl radicals, which then attack the sensitizer to give combinati on products. Oxidation of V-. by ground-state sensitizer leads to sulf ur-containing aldehydes or other products, depending on pH. Relative r ates of fragmentation and oxidation were determined from CIDNP signal intensities. From the temperature dependence of the polarizations, the activation energy of beta-fragmentation was estimated to be 54 kJ mol (-1).