Mössbauer Spectroscopic Studies of the Nature of Centre X of Photosystem I Reaction Centres from the Cyanobacterium Chlorogloea fritschii.

Abstract

Reduced photosystem I samples, which give the EPR signals associated with A, A and B, and A, B and X centers, were studied using Mossbauer spectroscopy. The Mossbauer spectra obtained from each type of sample is different, which indicates that Fe is associated with all 3 centers. The spectra are similar to those obtained from ferredoxins with 4Fe-4S centers and were fitted with oxidized and reduced components, the relative proportions depending on the degree of reduction of the sample as monitored by EPR. The sample which gave only the A EPR signal showed about 26% of the reduced component, the sample which gave A and B EPR signals showed about 48% of the reduced component, while the sample which gave A, B and X EPR signals showed about 65% of the reduced component. The measurements are consistent with X being a 4Fe-4S center.