MOLECULAR INTERACTION AND LATERAL DOMAIN FORMATION IN MONOLAYERS CONTAINING CHOLESTEROL AND PHOSPHATIDYLCHOLINES WITH ACYL-LINKED OR ALKYL-LINKED C16 CHAINS

The interactions of cholesterol with phosphatidylcholines having acyl- or alkyl-linked C16 chains have been determined in mixed monolayers u sing fluorescence microscopy to visualize lateral domain formation and cholesterol oxidase to probe for the relative strength of sterol-phos pholipid interaction. The phosphatidylcholines of this study included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), almitoyl-2-O-hexad ecyl-sn-glycero-3-phosphocholine (PHPC), -hexadecyl-2-palmitoyl-sn-gly cero-3-phosphocholine (HPPC), and 1,2-O-dihexadecyl-sn-glycero-3-phosp hocholine (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 si milar 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). Howe ver, with both PHPC and DHPC, laterally condensed phases were evident well before the onset of the phase transition as determined fro the fo rce-area isotherms of these lipids. Therefore, the pattern of formatio n of condensed phases was different, depending on the position of the alkyl function. When cholesterol was mixed with these phosphatidylchol ines (at 20, 25, or 33 mol %), cholesterol-rich condensed domains were formed. The domain morphology was similar in cholesterol-DPPC and cho lesterol-PHPC mixed monolayers, whereas cholesterol-HPPC and cholester ol-DHPC mixed monolayers had partly different domain morphologies, wit h more extensive fusion of the cholesterol-rich domains. When choleste rol oxidase was used to probe for the relative strength of intermolecu lar association between cholesterol and each of the phosphatidylcholin es, it was observed that the interaction was loosest with DPPC (highes t rate of cholesterol oxidation catalyzed by cholesterol oxidase), whe reas the association was somewhat stronger in PHPC (lower rate of chol esterol oxidation) and much stronger in HPPC and DHPC mixed monolayers (no detectable oxidation). In conclusion, the presence of an alkyl fu nction at position 1 or 2 (or both) of a phosphatidylcholine molecule markedly changed its properties in monolayer membranes.