KINETICS OF SOME ELECTRON-TRANSFER REACTIONS IN BIOLOGICAL PHOTOSYSTEMS .1. PULSE-RADIOLYSIS STUDY OF SPINACH FERREDOXIN REDUCTION BY THE HYDRATED ELECTRON AND CO2- RADICAL

The reduction of spinach ferredoxin by the CO2- radical and the hydrated electron (eaq-) was studied by pulse radiolysis in the pH range between 5.05-9.67. The reduction of oxidized spinach ferredoxin by both CO2- and eaq- was essentially quantitative. The CO2- radical reduced spinach ferredoxin by a single 2nd-order process at a rate k5 = (6.2 .+-. 0.6) .cntdot. 107 M-1 .cntdot. s-1. Reduction by eaq- followed a biphasic pathway. The 1st phase obeyed 2nd-order kinetics for the reduction of the cluster. The 2nd phase followed an intramolecular 1st-order reaction which was observed as a further reduction of the active site. Spectral changes accompanying the reduction of oxidized spinach ferredoxin in the UV and visible range were discussed.