The mechanism of the proton-coupled electron transfer reaction, Q A -Q B -

First and second half‐wave reduction potentials of a series of 1,4‐benzo‐ and 1,4‐naphtho‐quinones related to the naturally occurring ubiquinones, plastoquinones and menaquinones are correlated with substituent effects. Notably, E of 2,3‐dimethoxy‐1,4‐benzoquinone is positive of the values for the 2,5‐ and 2,6‐dimethoxy isomers, and of the value for methoxy‐1,4‐benzoquinone. This phenomenon is attributed to steric inhibition of resonance when two methoxy groups occupy adjacent positions, and the significance of this orientation in the ubiquinone series is highlighted.

Binding free energies of 37 functional replacement quinone cofactors with systematically altered hydrocarbon tail structures have been determined for the QA and QB redox catalytic sites of the reaction center protein isolated from Rhodobacter sphaeroides and solubilized in aqueous and in hexane solutions. The first two and part of the third isoprene units of the 10-unit tail of the native ubiquinone-10 cofactor interact with the protein interior at each site. Contributions of the same tail structures to the binding free energies of quinones at the QA and QB sites are comparable, suggesting that the binding domains share common features. Comparison of the affinities of a homologous series of 10 n-alkyl-substituted ubiquinones resolves the binding forces along the length of the tail binding domain and shows that strong steric constraints oppose accommodation of the tail in its extended conformation. Differences in the contributions of identical tail substituents to ubiquinone- and menaquinone-QA site affinities, and tail-induced changes of up to 5-fold in the rates of QA site-mediated electron-transfer reactions, suggest that the tail adjusts the position of the quinone ring. Substitution of ubiquinone with the native 10-unit isoprene tail does not alter the affinity for the sites as determined in hexane solution. However, one- and two-isoprene-substituted quinones bind more tightly than analogs substituted with saturated-alkyl tail substituents. The sites therefore exhibit binding specificity for the native isoprene tail structure. Calculations indicate that the binding specificity arises primarily from a lower integrated torsion potential energy in the bound isoprene tails. The results suggest that the in vivo tail-protein interaction is designed to deter competitive interference of quinone function by amphiphilic species present in the native membrane.

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