Electron paramagnetic resonance of the interaction of semiquinone, Na+ ion pairs with phosphatidylcholine in ethereal solvents

Abstract

The solubilization of sodium salts of 1,4-benzosemiquinone (BQ^˙–), 1,4-naphthosemiquinone (NQ^˙–), 2,3,5,6-tetramethylbenzosemiquinone (DQ^˙–), 2-methyl-1,4-naphthosemiquinone (MNQ^˙–), 2,6-di-tert-butyl-1,4-benzosemiquinone (DTBQ^˙–) and 9,10-anthrasemiquinone (AQ^˙–) in 1,2-dimethoxyethane (DME) and diethyl ether (DEE), in the presence and absence of egg yolk phosphatidylcholine (PC), under very dry conditions, has been studied utilizing electron paramagnetic resonance (EPR) spectroscopy. EPR spectra corresponding to both the ion pair and the ‘free’ ion species were observed simultaneously for NQ^˙–, DQ^˙– and MNQ^˙– in the presence of small quantities of PC in DME. Weighted-average EPR spectra were observed for DTBQ^˙– and AQ^˙– in DME, evidenced by the monotonic variation of their corresponding g values or proton coupling constants as a function of added PC. Binding constants of the sodium semiquinone salts to PC were determined for both DME and DEE solutions. Binding constants of the solvated sodium–semiquinone ion pairs of PC were also determined for DME solutions. The roles of the solvent and PC in solvating the sodium–semiquinone ion pairs are strongly interrelated. Addition of methyl or tert-butyl groups and extension of the π cloud to both the BQ^˙– and NQ^˙– structures decreases the solvated sodium–semiquinone binding to PC in DME implying that solubilization occurs preferentially at the charged sites of the PC molecules.