Computational Chemistry and Molecular Modeling
Atomic, Molecular and Optical Physics
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The standard electrode potential for the quinone (Q)-hydroquinone (QH2) couple in aqueous acidic media has been explicitly calculated. Molecular geometries of Q and QH2 have been optimized. Protonation of Q, i.e., the formation of QH+ and QH22+, have been considered. Molecular geometries of these species have been thoroughly optimized. The energy of complexation of these molecules with water have been calculated by optimizing the structures of the hydrated complexes Q · 6H2O, QH2 · 6H2O, QH+ · 6H2O. and QH22+ · 6H2O. The ion-solvent interaction energy of QH+ · 6H2O, in turn, has been calculated by considering the complex QH+ · 6H2O… 2H2O, where the two extra water molecules approach the charge center of the complex QH+ · 6H2O vertically from top and bottom of the quinonoid ring. The standard reduction potential calculated by the CNDO method, 0.8548 V, is somewhat larger than the experimental potential, 0.6998 V, at 25°C. But the INDO value, 0.7085 V, is in excellent agreement with the observed potential. The electrode potential for the plastoquinone (PQ)-plastohydroquinone (PQH2) couple present in the aqueous pool in chloroplast has been calculated by the INDO method. The basic geometries of PQ, PQH+, and PQH2/sb have been synthesized by adopting the optimized geometries of Q, QH+, and QH2 and considering methyl substituents as well as an isoprenoid side chain containing up to 3 isoprene units with possible geometrical isomerism. The hydrated species PQ · 6H2O, PQH+ · 6H2O, and PQH2 · 6H2O are unstable compared to the isolated species PQ, PQH+, and PQH2, respectively. In fact, we have found that the hydration of PQH+ and PQH2 is much less extensive, and stability arises only when the hydroxyl groups in these two molecules are hydrogen-bonded to water molecules. But PQH+ is also stabilized through the association of two more water molecules in the vertical direction. For this reason, we have calculated the reduction potential of the PQ/PQH2 system from the energies of the isolated 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°C, which is in good agreement with the reduction potential 0.11 V observed for plastoquinone in the aqueous pool in chloroplast. © 1994 John Wiley & Sons, Inc.
Type of Medium: