SEMIEMPIRICAL QUANTUM-CHEMICAL TREATMENT OF THE STANDARD REDUCTION POTENTIALS OF QUINONE AND PLASTOQUINONE IN WATER

The standard electrode potential for the quinone (Q) - hydroquinone (Q H-2) couple in aqueous acidic media has been explicitly calculated. Mo lecular geometries of Q and QH-2 have been optimized. Protonation of Q , i.e., the formation of QH+ and QH-2(2+), have been considered. Molec ular geometries of these species have been thoroughly optimized. The e nergy of complexation of these molecules with water have been calculat ed by optimizing the structures of the hydrated complexes Q . 6H2O, QH -2 . 6H2O, QH+ . 6H2O, and QH-2(2+) . 6H2O. The ion-solvent interactio n energy of QH+ . 6H2O, in turn, has been calculated by considering th e complex QH+ . 6H2O ... 2H2O, Where the two extra water molecules app roach the charge center of die complex QH+ . 6H2O vertically from top and bottom of the quinonoid ring. The standard reduction potential cal culated by the CNDO method, 0.8548 V, is somewhat larger than die expe rimental potential, 0.6998 V, at 25-degrees-C. But the INDO value, 0.7 085 V, is in excellent agreement with the observed potential. The elec trode potential for die plastoquinone (PQ)-plastohydroquinone (PQH2) c ouple present in the aqueous pool in chloroplast has been calculated b y the INDC method. The basic geometries of PQ, PQH+, and PQH2 have bee n synthesized by adopting the optimized geometries of Q, QH+, and QH-2 and considering methyl substituents as well as an isoprenoid side cha in containing up to 3 isoprene units with possible geometrical isomeri sm. The hydrated species PQ . 6H2O, PQH+ . 6H2O, and PQH2 . 6H2O are u nstable compared to the isolated species PQ, PQH+, and PQH2, respectiv ely. In fact, we have found that the hydration of PQH+ and PQH2 is muc h less extensive, and stability arises only when the hydroxyl groups i n these two molecules are hydrogen-bonded to water molecules. But PQH is also stabitized through the association of two more water molecule s in the vertical direction. For this reason, we have calculated the r eduction potential of the PQ/PQH2 system from the energies of the isol ated molecules PQH2 and the hydrated species PQH+ . 2H2O. The computed standard reduction potential is 0.2785 V and it yields a potential of 0.07V at pH 7 at 25-degrees-C, which is in good agreement with the re duction potential 0.11 V observed for plastoquinone in the aqueous poo l in chloroplast. (c) 1994 John Wiley & Sons, Inc.