Role of membrane charge and semiquinone structure on naphthosemiquinone derivatives and 1,4-benzosemiquinone disproportionation and membrane-buffer distribution coefficients

Semiquinone membrane/buffer partition coefficients have been determined for 1,2-naphthosemiquinone (ONQ(.-)), 1,4-naphthosemiquinone (NQ(.-)) and two of its hydroxylated derivatives, 5,8-dihydroxy1,4-naphthosemiquinone (NZQ(. -)) and 5-hydroxy1,4-naphthosemiquinone (JQ(.-)) as a function of membrane charge in multilamellar vesicles of phosphatidylcholine (PC) and equimolar mixtures of this lipid and phosphatidic acid (PC:PA) and cetyltrimethylammo nium bromide (PC:CTAB) at physiological pH with the exception of values cor responding to PC:PA mixtures which were obtained at pH 9. These coefficient s follow the order PC:PA<PC<PC:CTAB in agreement with the negative charge o f the semiquinones. The disproportionation equilibria of the naphthosemiqui none derivatives are shifted to the semiquinone in the presence of neutral and positive membranes, being more pronounced in the latter. However, very low partition coefficients as well as small shifts in the semiquinone dispr oportionation equilibrium were observed for ONQ(.-) as compared to the othe r semiquinones. No partition of 1,4-benzosemiquinone (BQ(.-)) into the lipi d phase was detected for either charged or neutral lipid membranes. The pre sence of lipid membranes decreases the BQ(.-) equilibrium concentration in the presence of all the types of membranes considered here.