CROSSOVER OF A HIGH-SPIN (S=7 2, 3/2) TO A LOW-SPIN (S=1/2) IRON-SELENIUM CLUSTER IN F-A AND F-B OF PHOTOSYSTEM-I ON REBINDING OF PSAC ONTOP700-F-X CORES/

PsaC is a tightly bound ferredoxin in the Photosystem I (PSI) reaction center which contains two [4Fe-4S] clusters named F-A and F-B. We rec ently proposed that the mixed-ligand F-B cluster in C14D(PsaC) and the mixed-ligand F-A cluster in C51D(PsaC) exist in a spin state of S=3/2 , and that a spin state crossover to S=1/2 occurs when the PsaC mutant s are rebound onto P700-F-x cores. Since EPR signals from a highly rho mbic S=3/2 spin state can be difficult to study, wild-type PsaC was re constituted with iron and selenium to introduce an easily detected S=7 /2 spin state similar to that shown for Clostridial ferredoxin. When t he unbound [4Fe-4Se] PsaC was chemically reduced, a sharp derivative r esonance was found at g=5.171 attributed to the excited +/-3/2 doublet from an S=7/2 spin multiplet. An additional peak was found at g=5.616 attributed to the superimposed +/-1/2 and +/-3/2 doublets from a high ly rhombic S=3/2 spin multiplet, and an axial set of resonances found around g=2.0 attributed, in part, to a classical S=1/2 spin state. Whe n the [4Fe-4Se] PsaC was rebound onto P700-F-x cores, the spin populat ion derived from the S=7/2 and 3/2 spin states was negligible. Illumin ation of the rebuilt PS I complex at 15 K resulted in two rhombic sets of resonances, one with g values of 2.043, 1.941 and 1.854, diagnosti c of F-A, and the other with g values of 2.067, 1.941 and 1.878, diagn ostic of F-B. Chemical reduction with sodium dithionite at pH 10.5 or photoaccumulation by freezing during illumination resulted in a set of resonances with g values of 2.046, 1.938, 1.920 and 1.883, characteri stic of a spin-coupled F-A(-)/F-B(-) pair. The spin state crossover in this iron chalcogenide cluster is the first known to be induced by pr otein-protein association and reinforces the hypothesis that an S=3/2 to 1/2 crossover occurs in the PS I-rebound mutants C14D(PsaC) and C51 D(PsaC).