Proximity of acetate, manganese, and exchangeable deuterons to tyrosine Y-Z(center dot) in acetate-inhibited photosystem II membranes: Implications for the direct involvement of Y-Z(center dot) in water-splitting

The environment of the photosystem II Y-Z(.) radical, trapped in the ''split-signal'' form, is examined in acetate-treated PSII membranes using pulsed EPR methods. The split-signal line shape is simulated with dipolar and exchange couplings to the Mn cluster of 1260 and -28 MHz, respectively. The 1260-MHz dipolar coupling corresponds to a Mn-Y-Z(.) distance of 3.5 Angstrom in the point dipole limit. A 0.117-MHz dipolar coupling is observed between nonexchangeable deuterons of methyl-deuterated acetate and Yz. This interaction is modeled with a 3.1-Angstrom distance between an acetate methyl group deuteron and the phenoxy oxygen of Y-Z(.). Since acetate inhibition is competitive with Cl-, this result strongly suggests a close proximity between Y-Z(.) and the Cl- cofactor binding site. Analysis of pulsed ENDOR and ESEEM experiments investigating the proximity of deuterons exchanged into the vicinity of Y-Z(.) after incubation in (H2O)-H-2-enriched buffer demonstrates that Yi trapped in the split-signal form participates in two hydrogen-bonding interactions, in contrast to Y-D(.), which forms a single hydrogen bond. This result is inconsistent with a simple electron transfer role for Y-Z(.) and provides direct experimental evidence for a role for Y-Z(.) in proton or hydrogen atom transfer.