Mechanism of Mixed-Valence Fe 2.5+ ···Fe 2.5+ Formation in Fe 4 S 4 Clusters in the Ferredoxin Binding Motif.

Most low-potential Fe 4 S 4 clusters exist in the conserved binding sequence CxxCxxC (C n C n +3 C n +6 ). Fe(II) and Fe(III) at the first (C n ) and third (C n +6 ) cysteine ligand sites form a mixed-valence Fe 2.5+ ···Fe 2.5+ pair in the reduced Fe(II) 3 Fe(III) cluster. Here, we investigate the mechanism of how the conserved protein environment induces mixed-valence pair formation in the Fe 4 S 4 clusters, F X , F A , and F B in photosystem I, using a quantum mechanical/molecular mechanical approach. Exchange coupling between Fe sites is predominantly determined by the shape of the Fe 4 S 4 cluster, which is stabilized by the preorganized protein electrostatic environment. The backbone NH and CO groups in the conserved CxxCxxC and adjacent helix regions orient along the Fe C n ···Fe C( n +6) axis, generating an electric field and stabilizing the Fe C n (II)Fe C( n +6) (III) state in F A and F B . The overlap of the d orbitals via -S- (superexchange) is observed for the single Fe C n (II)···Fe C( n +6) (III) pair, leading to the formation of the mixed-valence Fe 2.5+ ···Fe 2.5+ pair. In contrast, several superexchange Fe(II)···Fe(III) pairs are observed in F X due to the highly symmetric pair of the CDGPGRGGTC sequences. This is likely the origin of F X serving as an electron acceptor in the two electron transfer branches.