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

Flash-induced absorption changes in the Soret region arising from the [PP] 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 is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when Tyr is present, an additional signal in the [PP]-minus-[PP] difference spectrum was observed when compared to the first flash when Tyr is not oxidized. The additional feature was "W-shaped" with troughs at 434 nm and 446 nm. This feature was absent when Tyr was reduced, but was present (i) when Tyr 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 and its H bonding partners to be present. We found no evidence of involvement of P, Chl, Phe, Phe, Tyr, and the Cytb heme in the W-shaped difference spectrum. However, the use of a mutant of the P axial His ligand, the D2-His197Ala, shows that the P environment seems involved in the formation of "W-shaped" signal.