Lateral domain formation in cholesterol/phospholipid monolayers as affected by the sterol side chain conformation

The interaction of side-chain variable cholesterol analogues with dipalmitoylphosphatidylcholine (DPPC) or N-palmitoylsphingomyelin (N-PSPM) has been examined in monolayer membranes at the air/water interface. The sterols had either unbranched (n-series) or single methyl-branched (iso-series) side chains, with the length varying between 3 and 10 carbons (C3-C10). The efficacy of interaction between the sterols and the phospholipids was evaluated based on the ability of the sterols to form condensed sterol/phospholipid domains in the phospholipid monolayers. Domain formation was detected with monolayer fluorescence microscopy using NBD-cholesterol as the fluorescent probe. In general, a side chain length of at least 5 carbons was necessary for the unbranched sterols to form visible sterol/phospholipid domains in DPPC or N-PSPM mixed monolayers. With the iso-analogues, a side chain of at least 6 carbons was needed for sterol/phospholipid domains to form. The macroscopic domains were stable up to a certain surface pressure (ranging from 1 to 12 mN/m). At this onset phase transformation pressure, the domain line boundary dissipated, and the monolayer entered into an apparent one phase state (no clearly visible lateral domains). However, with some DPPC monolayers containing short chain sterols (n-C3, n-C4,n-C5, and i-C5), a new condensed phase appeared to form (at 20 mol%) when the monolayer was compressed beyond the phase transformation pressure. These precipitates formed at surface pressures between 6-8.3 mN/m, were clearly observable up to at least 30 mN/m. When the monolayers containing these four sterols were allowed to expand, the condensed precipitates dissolved at the same pressure at which they were formed during monolayer compression. No condensed precipitates were observed with these sterols in corresponding N-PSPM monolayers. Taken together, the results of this study emphasize the importance of the length and conformation of the cholesterol side chain in determining the efficacy of sterol/phospholipid interaction in model membranes. The major difference between DPPC and N-PSPM monolayers at different sterol compositions was mainly the lateral distribution and the size of the domains as well as the onset phase transformation pressure intervals.