J. Wang^a, D. Gosztola^b, SV. Ruffle^a, C. Hemann^c, M. Seibert^d, MR. Wasielewski^e, CR. Hille^c ,TL. Gustafson^f, RT. Sayre^a
Departments of ^aPlant Biology, ^cMedical Biochemistry, ^fChemistry, Ohio State University, OH 43210. ^bArgonne National Laboratory, Argonne, IL 60439, ^dNREL, Golden, CO 80401. ^eDept. Chemistry, Northwestern University, Evanston, IL 60208

The photosystem II reaction center complex contains an additional pair of peripheral Chls that are not present in the structurally similar quinone-type reaction center of non-oxygenic photosynthetic bacterial. These peripheral Chls apparently participate in energy transfer from the proximal antennae (CP43 and CP47) to the PS II core pigments. Biochemical and spectroscopic studies have been conducted on mutants of the symmetry related histidine residues (D1-H118 and D2-H117) previously shown to coordinate the peripheral accessory Chls. We observed that conservative mutations of the D2-H117 residue caused decreased Chl fluorescence quenching efficiency under conditions that lead to photoaccumulation of Chl_Z⁺. In contrast, a symmetry related mutation at D1-H118 had little effect on the kinetics of Chl fluorescence quenching. This offers plausible argument that Chl_Z, which participates in a low quantum-yield electron transfer cycle around PS II, is coordinated by the D2-H117 residue. This conclusion is further supported by recent structural evidence that indicates that the electron donor to Chl_Z, cytochrome b559, is located next to the D2 protein. To explore possible perturbations of primary processes in the RCs, a protocol was established to isolate PS II RC complexes from WT and both D1 and D2 peripheral Chl mutants. We observed a decrease in the Chl optical activity in D2-H117N RCs as shown by a reduced CD doublet in the red spectral region. This loss in optical activity reflects the fact that the D2-H117N mutation has induced changes in the mutual orientation between the Chl coordinated at this site and P680. Transient absorption measurements also indicated that the coupling between the peripheral accessory Chl and P680 was altered in the D2-H117 mutants. Pumping D2-H117N RCs on the blue side of the Q_y region generated a new ultrafast (~400 fs) component of Pheo Q_x band bleach growth kinetics not seen in the WT and D1-H118Q mutant. The origin of this component may be related to (a) delayed fast energy equilibration between the core pigments of this mutant, or (b) a new charge separation route between Chl_Z and Pheo_inactive without the involvement of *P680.