Direct electrochemistry of the photosynthetic blue copper protein plastocyanin. Electrostatic promotion of rapid charge transfer at an edge-oriented pyrolytic graphite electrode

Abstract

Direct (unmediated) electrochemistry of the photosynthetic "blue" copper protein plastocyanin may be readily achieved at an edge-oriented pyrolytic graphite electrode over the pH range 4-8. Promotion of rapid charge transfer by Mg2+ ions (<5 mM) or by mild acidification (pH 4-6) of the medium closely parallels in vivo regulatory characteristics for plastocyanin at the thylakoid membrane. The edge face of pyrolytic graphite, subjected to standard polishing procedures in air, contains a variety of hydrophilic C-O functional groups which are expected to interact favorably with plastocyanin once electrostatic repulsion has been overcome. Optimal heterogeneous rate constants at 3°C range between 10 ± 5 × 10-3 cm s-1 at pH 4 and 2 ± 1 × 10-3 cm s-1 at pH 8. Apparent E1/2 values at decreasing low pH increase by ca. 55 mV per pH unit, with pK ∼ 5.5. This may be identified with the protonation of active-site histidine-87 in accordance with established crystallographic and kinetic evidence. Plastocyanin behaves as an effectively symmetrical redox system (ipa ∼ ipc) throughout the pH range 4-8. At scan rates of up to 500 mV s-1 at pH 4, an effective equilibrium between redox-active and redox-inactive (protonated) forms of reduced plastocyanin is maintained at the electrode. Accordingly, f1/2 for deprotonation of H+-CuI plastocyanin is estimated to be <1 ms. © 1985 American Chemical Society.