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 onto P700-F(x) cores

PsaC is a tightly bound ferredoxin in the Photosystem I (PS I) reaction center which contains two [4Fe-4S] clusters named F(A) and F(B). We recently 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 mutants are rebound onto P700-F(x) cores. Since EPR signals from a highly rhombic S = 3/2 spin state can be difficult to study, wild-type PsaC was reconstituted with iron and selenium to introduce an easily detected S = 7/2 spin state similar to that shown for Clostridial ferredoxin. When the unbound [4Fe-4Se] PsaC was chemically reduced, a sharp derivative resonance 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 highly 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. When the [4Fe-4Se] PsaC was rebound onto P700-F(x) cores, the spin population derived from the S = 7/2 and 3/2 spin states was negligible. Illumination 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, diagnostic of F(A), and the other with g values of 2.067, 1.941 and 1.878, diagnostic 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, characteristic 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 protein-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 C51D(PsaC).