A DENSITY-FUNCTIONAL STUDY OF THE EFFECT OF REDUCTION ON THE GEOMETRYAND ELECTRON-AFFINITY OF HYDROGEN-BONDED 1,4-BENZOQUINONE - IMPLICATIONS FOR QUINONE REDUCTION AND PROTONATION IN PHOTOSYNTHETIC REACTION CENTERS

Hybrid density functional studies using the B3LYP functional show that the two electron reduction of 1,4-benzoquinone (BQ) to the dianion (B Q(2-)) via the semiquinone radical anion (BQ(-)) form is accompanied b y a gradual change in the internal geometry of the quinone from quinon oid to benzenoid form. The hydrogen bonds formed to neighbouring proto n donors are progressively shortened. These changes contribute to the ultimate transfer of the hydrogen bonded protons from the hydrogen bon ded proton donors leading to the formation of the quinol form. The red uced forms are progressively stabilised relative to the oxidised form with increasing hydrogen bonding interactions. Similar changes can be expected to play a crucial role in the reduction and protonation of th e Q(b) site of photosynthetic reaction centres in bacteria and higher plants. (C) 1997 Elsevier Science B.V.