Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [P$_{D1}$ P$_{D2}$]$^+$
Abstract
Flash-induced absorption changes in the Soret region arising from the [P$_{D1}$ P$_{D2}$]$^+$ 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, Tyr D is reduced before the first flash but oxidised before subsequent flashes. When TyrD$^•$ 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. 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 when Tyr$_D$ 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 Tyr$_D$ and its H bonding partners to be present. A range of PSII variants were surveyed, and we found no evidence of involvement of P$_{D1}$ , Chl$_{D1}$ , Phe$_{D1}$ , Phe$_{D2}$ , TyrZ, and the Cyt$b_{559}$ heme in difference spectrum. Direct data ruling out the participation of P$_{D2}$ is lacking. It seems possible that the specific H-bonding environment of around reduced Tyr$_D$ allows a more homogenous electrostatic environment for [P$_{D1}$$P_{D2}$]$^+$ . A role for P$_{D2}$ in the double-trough Soret signal may be tested using mutants of P$_{D2}$ axial His ligand D2-His197.
Domains
Life Sciences [q-bio]
Origin : Files produced by the author(s)
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