Long-chain free fatty acids (FA) play an important role in several physiological and pathological processes such as lipid fusion, alteration of membrane permeability and fluidity, as well as regulation of enzyme and protein activity. FA-facilitated membrane proton transport (flip-flop) and FA-dependent proteinaceous proton transport (as in the case of mitochondrial uncoupling proteins) are governed by the difference between FA's pK and the pH in the immediate membrane vicinity1,2. So far, a quantitative understanding of the process is hampered by the shift in the pK value of carboxylic groups that occurs upon moving the FA from the aqueous solutions into the membrane. Its molecular origin is not understood, conceivably because the reported pKa values for the same FA are scattered between 5 and 10.5. Here we systematically obtained the dependence of apparent pKa values on chain length, number and position of double bonds by measuring the zeta-potential of liposomes reconstituted with FA at different pH. pKa ranged from 6.4 to 7.6. Combining the zeta potential measurements with molecular dynamic simulations, we found that pK is determined by the interplay between the energetic costs for burying a charged moiety into the bilayer and the hydrophobic interactions between the free and lipid-bound fatty acid chains.1. Pohl, E. E., Voltchenko, A. M. & Rupprecht, A. Flip-flop of hydroxy fatty acids across the membrane as monitored by proton-sensitive microelectrodes. Biochim.Biophys.Acta 1778, 1292-1297 (2008).2. Rupprecht, A. et al. Role of the transmembrane potential in the membrane proton leak. Biophys.J. 98, 1503-1511 (2010).
