Singlet Oxygen-mediated Photobleaching of the Prosthetic Group in Hemoglobins and C-phycocyanin

Proteins bearing colored prosthetic groups, such as the heme group in hemoglobin or the bilin group in c-phycocyanin, quench singlet oxygen by interactions at the apoprotein and the prosthetic group levels. In both proteins, chemical modification of the chromophore constitutes only a minor reaction pathway. While total deactivation of singlet oxygen takes place with rate constants of 4.0 x 10(9) and 4.2 x 10(8) M-1 s-1 for hemoglobin and phycocyanin, respectively, the bleaching of the chromophore takes place with rate constants of 3.2 x 10(6) and approximately 1 x 10(7) M-1 s-1. Irradiation of phycocyanin with red light bleaches the chromophore with low yields (approximately 0.8 x 10(-4)). Part of this bleaching is mediated by singlet oxygen produced by the irradiation of the bilin group. The low relevance of the singlet oxygen pathway is compatible with a low quantum yield (approximately 10(-3)) of free singlet oxygen production after irradiation of the protein.