| Abstract
| - Thin film voltammetry was used to obtain direct, reversible, electron-transfer peaks betweenelectrodes and the spinach photosystem II (PS II) reaction center in lipid films for the first time. Threewell-defined pairs of reduction−oxidation peaks were found using cyclic and square wave voltammetry at4 °C at pH 7.5, reflecting direct, reversible electron transfer involving cofactors of PS II. These peaks wereassigned to the oxygen-evolving complex (OEC) tetramanganese cluster (Em = 0.2 V vs NHE), quinones(Em = −0.29 V), and pheophytin (Em = −0.72 V). PS II that was depleted of the OEC did not give the peakat 0.2 V. Observed Em values, especially for the OEC, may be influenced by protein−lipid interactions andelectrode double-layer effects. Voltammetry at pH 6 and at pH 7.5 with a time window of >100 ms revealedthat the manganese cluster oxidation is gated by slow deprotonation of a reduced form. Additional rapidprotonation/deprotonation steps are also involved in the electrochemical reduction−oxidation pathways.
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