CARBOXYLIC-ACID OR PRIMARY AMINE TITRATION AT THE LIPID-WATER INTERFACE - ON THE ROLE OF ELECTRIC CHARGES AND PHOSPHOLIPID ACYL CHAIN COMPOSITION - A SPIN LABELING EXPERIMENT

The dissociation equilibrium pH of a stearic acid spin probe and of the primary amine group of cysteamine was evaluated in the phospholipidic matrix of model membranes in gel phase (Lβ) and in liquid-crystalline phase (Lα). This study shows that the apparent pKa or pKb values depend on: (i) the thermodynamic phase of the lipidic bilayers; (ii) the nature of the lipidic components including either the polar head region (choline, serine moieties or exogenous electric charge-carrying cholesteryl esters) or the hydrophobic core (different phospholipid acyl chain length); (iii) the nature of the ionizable group, ΔpKbilayer - pKwater) of carboxylic acid or primary amine groups being opposite respectively (ΔpKa = + 2.5 for stearic acid and ΔpKb = - 4.9 for cysteamine, in DPPC in fluid phase). An interpretation of this pK shifting is given by an interaction model of the ionizable molecule with the phospholipid bilayer, showing that ΔpK can be modulated by two parameters: the partition coefficient ratio of both the non-ionized and the ionized forms (KH/K) of the interacting molecule and the surface charge density (Ψ1) at the lipid/water interface:. © 1990.