Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [P_D1P_D2]⁺

Abstract

Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon excitation of Photosystem II (PSII), were obtained using Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is reduced before the first flash but oxidised before subsequent flashes. When TyrD• is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash. The additional feature was W-shaped with troughs at 434 nm and 446 nm. This feature was absent when TyrD was reduced, but was present when TyrD was physically absent (and replaced by phenylalanine) or when its H-bonding histidine (D2-His190) was physically absent (replaced by a Leucine),. Thus, the simple difference spectrum without the double trough feature at 434 nm and 446 nm, required the native structural environment around the reduced TyrD and its H bonding partners to be present. A range of PSII variants were surveyed, and we found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in difference spectrum. Direct data ruling out the participation of PD2 is lacking. It seems possible that the specific H-bonding environment of around reduced TyrD allows a more homogenous electrostatic environment for [PD1PD2]+. A role for PD2 in the double-trough Soret signal may be tested using mutants of PD2 axial His ligand D2-His197.