Membrane–buffer partition coefficients of semiquinones using the spin-broadening technique

Abstract

Membrane–buffer partition coefficients have been determined for 1,4-benzosemiquinone (BQ^·–), 1,4-naphthosemiquinone (NQ^·–) and naphthazarin semiquinone (NZQ^·–) in egg-yolk phosphatidylcholine (PC) multilamellar vesicles in sodium cacodylate buffer at pH 7.4, utilizing electron paramagnetic resonance (EPR) spectroscopy. Partition coefficients are in the order BQ^·– < NZQ^·– < NQ^·–, which is the same order followed by the octanol–water partition coefficients of their parent neutral quinones. A method is proposed for the determination of the fraction of semiquinones at the membrane which are located in the hydrophobic inner site. This fraction also follows the same order as the partition coefficients.