Comparative kinetic and energetic modelling of phyllosemiquinone oxidation in Photosystem I

Abstract

The oxidation kinetics of phyllo(semi)quinone (PhQ), which acts as an electron transfer (ET) intermediate in the Photosystem I reaction centre, are described by a minimum of two exponential phases, characterised by lifetimes in the 10–30 ns and 150–300 ns ranges. The fastest phase is considered to be dominated by the oxidation of the PhQ molecule coordinated by the PsaB reaction centre subunit (PhQ_B), and the slowest phase is dominated by the oxidation of the PsaA coordinated PhQ (PhQ_A). Testing different energetic schemes within a unified theory-based kinetic modelling approach provides reliable limit-values for some of the physical–chemical parameters controlling these ET reactions: (i) the value of ΔG⁰ associated with PhQ_A oxidation is smaller than ∼+30 meV; (ii) the value of the total reorganisation energy (λ_t) likely exceeds 0.7 eV; (iii) different mean nuclear modes are coupled to PhQ_B and PhQ_A oxidation, the former being larger, and both being ≥100 cm⁻¹.