Surface areas of 1-palmitoyl phosphatidylcholines and their interactions with cholesterol.

1-Palmitoyl phosphatidylcholines (1-palmitoyl PCs), in which the 2-position was occupied respectively by C10:0, C12:0, C14:0, C14:1, n-7, C16:0, C16:1, n-7, C18:0, C18:1(t), n-9, C18:1, n-9, C18:2, n-6, C18:3, N-3, C18:3, n-6, C18:3(5t,9,12), C22:0, C22:1, n-9, C22:2, n-6, C22:3, n-3, C22:4, n-6, C22:5, n-6 or C22:6, n-3 fatty acids, were studied as monolayer films at the air/water interface. Results for molecular area indicated that the areas of the PC (phosphatidylcholine) did not continuously decrease as the length of one chain increased. For series of saturated, monoenoic and dienoic 1-palmitoyl PCs the smallest molecular area was occupied by the PC containing a 20-carbon acid at the 2-position. In the 18-carbon series, introduction of the first and third cis double bonds caused a large increase in molecular area, but in the 22-carbon series the first and second cis double bonds produced large increases in molecular area. Molecules containing three or more cis double bonds varied little in molecular area, regardless of chain length (18-22 carbon atoms). The influence of a trans double bond was intermediate between that of a saturated and a cis double bond. The 18- and 22-carbon series of PCs were studied in mixed monolayers with cholesterol and desmosterol. Condensation of molecular areas occurred in all sterol PC mixed films, and similar results were obtained with cholesterol and desmosterol. Condensation of PC containing a cis or trans double bond within 10 carbon atoms of the carboxy group initially increased with increasing surface pressure. Condensation of other PCs decreased as surface pressure increased. All cis- or trans-unsaturated PCs condensed maximally in mixtures of approximately equimolar ratios with sterols, but saturated PCs condensed to the greatest extent in mixtures that contained about 30 mol% sterol.