Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2].

Flash-induced absorption changes in the Soret region arising from the [P_D1P_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_D1P_D2]⁺-minus-[P_D1P_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 Cytb₅₅₉ 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.