Dissociation constants of lipid ionizable groups - I. Corrected values for two anionic lipids

Literary data on the dissociation constants of lipid ionizable groups were revised in the light of our recent finding that the standard approach used in the determination of pK values from experimental titration curves needs corrections for cation adsorption. Such corrections were made for both the apparent and the intrinsic pK of two anionic lipids, phosphatidylserine and phosphatidylglycerol. Analysis of the corrected pK values shows that (1) the true apparent pK (pK(app)) is in most cases greater than the corresponding uncorrected value and (2) the intrinsic pK (pK(i)) may suffer changes with both the membrane composition and the composition of the membrane-bathing solution. The latter observation is consistent with the results of analysis of the terms used for the definition of dissociation constants: it is found that the term "intrinsic" is not correct when applied to the pK value of an ionizable group in a lipid membrane; more adequate is the term "surface" (or "interfacial"). It is shown that the surface pK (pK(s)=pK(i)) may in principle depend on the membrane composition as well as on the composition of the membrane-bathing solution because of the influence of membrane hydration and/or intra- or intermolecular interactions of the ionizable groups. (C) 1998 Elsevier Science B.V.