Photoinduced Electron Transfer, Decarboxylation, and Radical Fragmentation of Cysteine Derivatives:  A Chemically Induced Dynamic Nuclear Polarization Study

• Published in: Journal of the American Chemical Society Vol. 118; no. 12; pp. 2882 - 2891
• Main Authors: Goez, Martin, Rozwadowski, Jaroslaw, Marciniak, Bronislaw
• Format: Journal Article
• Language: English
• Published: American Chemical Society 27.03.1996
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja9536678

The photoreactions of cysteine derivatives I with 4-carboxybenzophenone in D2O were investigated by measurements of chemically induced dynamic nuclear polarization (CIDNP). The quenching mechanism is electron transfer from sulfur at every pH; even if the amino group of I is deprotonated, electron transfer from nitrogen does not participate. Decarboxylation of I •+ to give α-aminoalkyl radicals V • occurs on the CIDNP time scale and causes strong cooperative effects. The decarboxylation rate is increased significantly by deprotonation of the amino function; this is due to product control. V • decays by two competing pathways. Fragmentation of the Cβ−S bond in V • yields vinylamine, which is hydrolyzed to acetaldehyde at pH ≲ 7.25, and thiyl radicals, which then attack the sensitizer to give combination products. Oxidation of V • by ground-state sensitizer leads to sulfur-containing aldehydes or other products, depending on pH. Relative rates of fragmentation and oxidation were determined from CIDNP signal intensities. From the temperature dependence of the polarizations, the activation energy of β-fragmentation was estimated to be 54 kJ mol-1.