The effect of a number of long chain compounds on the phase transition temperature, Tm. of several phospholipids was measured at different pH values by differential scanning calorinretry. Only single chain compounds capable of intermolecular hydrogen bonding interactions, such as pahnitic acid, hexadecanol, and hexadecylamine, were able to form 2 : 1 (m/m) complexes with dipalmitoylphosphatidylcholine (DPPC), dihexadecylphosphatidylcholine (DHPCL and dipahnitoylphosphatidylglycerol (DPPG), which melted at a temperature 20-27°C higher than the pure lipid. The Tm-values of these complexes was similar to those of pure dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidic acid (DPPA) at neutral pH. These complexes formed only at pH values where both hydrogen bond donating and accepting groups were present. The hydrogen bonding compounds could be incorporated into DPPE and DPPA also but caused only a small increase in their Tin-values. Non-hydrogen bonding single chain C-16 compounds, such as methyl pahnitate, hexadecane, and hexadecyl glycerol, had a smaller effect on tire Tin-values of DPPC, DHPC, and DPPG, increasing them by only 4 12°C. l:urthermore these compounds decreased the Tin-values of DPPEandDPPA.
These results suggested that the large increase in T m produced in DPPC, DHPC, and DPPG by tire hydrogen bonding compounds and the large Tin-values of pure DPPE and DPPA are a result of intermolecular hydrogen bonding interactions involving tire lipid phosphate and not of differences in size or charge of the lipid head groups. However, the smaller increase produced by the non-hydrogen bonding compounds may be the result of a reduction of the surface charge density of thc bilayer. Consideration of hydrogen bonding interactions as well as head group size and charge characteristics helps to understand the behavior of these lipids and their rc~le in biological membranes.