Analysis of pi charge distribution in substituted anthraquinones to assess affinity for the QB binding site

In the accompanying paper (Biochim. Biophys. Acta (1990) 1020, 163-168), we have determined the degree of competition between substituted 9,10-anthraquinones for the QB binding niche through measurements of the additivity of quinone-quenching effects on chlorophyll fluorescence. Quinones inhibit QB function by competitively displacing QB through hydrogen-bond formation with the QB binding protein. The sign of the net pi-charge density on atoms adjacent to the carbonyl moieties is believed to determine the particular hydrogen-bond(s) that result(s). In this study we report CNDO molecular orbital calculations of pi electronic charge distribution in substituted 9,10-anthraquinones to explore the relationship of inhibitor activity and competition to sign of net pi-charge density. We find that the substitution patterns of 9,10-anthraquinones alter the signs of the net pi-charge densities on the carbon atoms adjacent to the carbonyl moieties and thus determine the binding properties of the anthraquinones in the QB niche. While most experimentally studied 9,10-anthraquinones use both carbonyl oxygens to hydrogen bond to the histidine-215 and serine-264 regions of the D-1 QB binding protein, some quinones appear to hydrogen-bond to only one site. Thus, 9,10-anthraquinones constitute a class of QB inhibitors that function as either members of the histidine or serine family of QB inhibitors or as simultaneous representatives of both inhibitor groups.