Differential thermal analysis of dipalmitoylphosphatidylcholine--fatty acid mixtures.

Mixtures of dipalmitoylphosphatidylcholine (DPPC) with palmitic, stearic, and myristic acids and the sodium salts of these acids were analyzed by differential thermal analysis (DTA) over a wide range of lipid compositions, all in excess water. All three fatty acids raise the liquid-crystal phase transition temperature and form sharp-melting complexes, with 1:2 DPPC--fatty acid stoichiometry observed for palmitic and stearic acids and suggested for myristic acid. Phase diagrams of the peritectic type, indicating nonideal mixing, was fitted to the DPPC--palmitic acid and DPPC--stearic acid data. In contrast, DPPC forms nearly ideal mixtures with the putative DPPC--myristic acid complex. At levels of only a few mole percent, both sodium stearate and myristate remove the pretransition and main transition and produce new peaks at approximately 30 and approximately 48 degrees C; the relative areas of the new peaks were unreproducible for the DPPC--myristate system. Sodium palmitate is the least disruptive of any of the sodium soaps or fatty acids; up to 80 mol % palmitate, the transition is lowered 3 degrees C and approximately doubled in width. The pretransition is detectable up to 36 mol %, and the main transition persists up to 88 mol % palmitate. The apparent pK of palmitic acid (12 mol %) in DPPC bilayers was determined to be 10.2 by direct pH measurement of ternary DPPC mixtures with known palmitic acid/sodium palmitate ratios; the intrinsic pK is estimated to be less than or approximately 8.5.