Diazonium modification of Photosystem I. A specific effect on iron-sulfur Center B

Modification of chloroplast membranes with diazonium benzene sulfonate (DABS) leads to a loss of Photosystem I-dependent ferredoxin reduction but not methyl viologen reduction. EPR studies of DABS-modified membranes show no inhibition of P-700⁺ formation at cryogenic temperatures, but iron-sulfur Center A photoreduction is markedly inhibited. Iron-sulfur Center B photoreduction at physiological temperatures in DABS-modified membranes is also markedly inhibited and little Center B can be detected after dark chemical reduction. These results indicate DABS specifically modifies iron-sulfur Center B of the spinach chloroplast Photosystem I electron acceptor complex and that Center B is obligately required for the reduction of Center A at cryogenic temperatures. Possible electron transport pathways at physiological temperatures are also considered.