The intermembrane transfer kinetics and transition-state thermodynamics of pyrenedodecanoylcarnitine (PDC), pyrenedodecanoyl coenzyme A (PDCoA), and pyrenedodecanoic acid (PDA) were measured by observing the time-dependent decay in pyrene excimer fluorescence. Probe molecules transferred more slowly with an increase in vesicle size. Rates of PDC and PDA transfer were increased from a liquid lipid phase when compared to a gel phase, while a saturated lipid phase had variable effects on the transfer kinetics when compared to an unsaturated lipid vesicle. Increasing vesicle surface charge by the introduction of phosphatidylserine (PS) into the vesicle matrix had two distinct effects: (i) a decrease in PDC transfer rates as the PS concentration increased and (ii) an initial increase in transfer rates of the amphiphilic anions PDA and PDCoA, followed by a decrease as the PS content increased. Transfer from natural membranes (cardiac and hepatic reticular and mitochondrial membranes) was markedly decreased (up to 35-fold) when compared to large phospholipid vesicles. These decreases in rates were accompanied by significant increases in the transition-state free energies. Finally, the pyrenedodecanoate esters had critical micelle concentrations similar to the natural long-chain esters, i.e., palmitate. In the presence of acceptor vesicles all probes showed only slight accessibility to quenching by the aqueous quencher nicotinamide.
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