Monolayers of sterols and phosphatidylcholines containing a 20-carbon chain.

Pressure-area curves of monolayer films were measured for phosphatidylcholines (PC) in which the 1-position was occupied by palmitic acid and the 2-positions were occupied respectively by: 20 : 0,20 : 1n9, 20 : 2n6, 20 : 3n3, 20 :3n6, 20 :3n9, 20 :4n6 or 20 : 5n3 fatty acids. The interactions of these PC with cholesterol or desmosterol were studied. Fully saturated PC (16 : 0--20 : 0) displayed a relatively small molecular area. The presence of one double bond greatly increased the molecular area, but a second double bond resulted in only a small additional increase in area. A third double bond caused a further large expansion in area, but the presence of a fourth or fifth double bond had little additional effect. Condensation of molecular area was observed with all sterol/PC mixed films. Approximately equimolar mixtures of sterols and unsaturated PC condensed maximally, but 16 : 0--20 : 0 PC condensed most in mixtures containing 20--30 mol% of either sterol. The extent of condensation varied with surface pressure. The pressure at which maximum condensation occurred depended upon the structure of the PC and was always 20 dyn/cm or lower. The pressure at which maximum condensation with cholesterol occurred increased with increasing unsaturation of the PC.