Molecular interaction and lateral domain formation in monolayers containing cholesterol and phosphatidylcholines with acyl- or alkyl-linked C16 chains

The interactions of cholesterol with phosphatidylcholines having acyl- or alkyl-linked C16 chains have been determined in mixed monolayers using fluorescence microscopy to visualize lateral domain formation and cholesterol oxidase to probe for the relative strength of sterol-phospholipid interaction. The phosphatidylcholines of this study included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1-palmitoyl-2-O-hexadecyl-sn-glycero-3-phosphocholine (PHPC), 1-O-hexadecyl-2-palmitoyl-sn-glycero-3-phosphocholine (HPPC), and 1,2-O-dihexadecyl-sn-glycero-3-phosphocholine (DHPC). As the two-dimensional crystallization of the liquid-condensed phase was visualized with fluorescence microscopy (using 0.5 mol % NBD-cholesterol as a probe), both DPPC and HPPC displayed a similar nucleation and growth of the liquid-condensed phase at the onset pressure of the liquid-expanded to liquid-condensed phase transition (although the size and shapes of the condensed domains differed). However, with both PHPC and DHPC, laterally condensed phases were evident well before the onset of the phase transition as determined fro the force-area isotherms of these lipids. Therefore, the pattern of formation of condensed phases was different, depending on the position of the alkyl function. When cholesterol was mixed with these phosphatidylcholines (at 20, 25, or 33 mol %), cholesterol-rich condensed domains were formed. The domain morphology was similar in cholesterol-DPPC and cholesterol-PHPC mixed monolayers, whereas cholesterol-HPPC and cholesterol-DHPC mixed monolayers had partly different domain morphologies, with more extensive fusion of the cholesterol-rich domains. When cholesterol oxidase was used to probe for the relative strength of intermolecular association between cholesterol and each of the phosphatidylcholines, it was observed that the interaction was loosest with DPPC (highest rate of cholesterol oxidation catalyzed by cholesterol oxidase), whereas the association was somewhat stronger in PHPC (lower rate of cholesterol oxidation) and much stronger in HPPC and DHPC mixed monolayers (no detectable oxidation). In conclusion, the presence of an alkyl function at position 1 or 2 (or both) of a phosphatidylcholine molecule markedly changed its properties in monolayer membranes.