Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [P D1 P D2 ] .

Flash-induced absorption changes in the Soret region arising from the [P D1 P D2 ] + state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, Tyr D is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when Tyr D ● is present, an additional signal in the [P D1 P D2 ] + -minus-[P D1 P D2 ] difference spectrum was observed when compared to the first flash when Tyr D is not oxidized. The additional feature was "W-shaped" with troughs at 434 nm and 446 nm. This feature was absent when Tyr D was reduced, but was present (i) when Tyr D was physically absent (and replaced by phenylalanine) or (ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434 nm and 446 nm, seemed to require the native structural environment around the reduced Tyr D and its H bonding partners to be present. We found no evidence of involvement of P D1 , Chl D1 , Phe D1 , Phe D2 , Tyr Z , and the Cytb 559 heme in the W-shaped difference spectrum. However, the use of a mutant of the P D2 axial His ligand, the D2-His197Ala, shows that the P D2 environment seems involved in the formation of "W-shaped" signal.