MOLECULAR PACKING IN STEROID-LECITHIN MONOLAYERS .1. PURE FILMS OF CHOLESTEROL, 3-DOXYL-CHOLESTANE, 3-DOXYL-17-HYDROXYL-ANDROSTANE, TETRADECANOIC ACID AND DIPALMITOYLPHOSPHATIDYLCHOLINE

A detailed study was completed of pure films of cholesterol, 3-doxyl cholestane (3-DC), 3-doxyl-17-hydroxyl androstane (3-DA), tetradecanoic acid and dipalmitoylphosphatidylcholine (DPPC), in the 20-45.degree. C temperature range. The resemblence between pure films of cholesterol and the .beta.-epimer of 3-DC is quite striking. Where molecular packing and hydrophobic interactions are concerned, the 2 molecules behave in a similar way. Where surface potentials or dipole moments are concerned the differences are consistent with the differing polar groups. 3-DA differs substantially in its film behavior from that of either cholesterol or 3-DC. The differences can be ascribed to the bipolar nature of 3-DA and its lack of an alkane chain at the C17 steroid position. Based on these differences the bulk of cholesterol''s small coefficient of thermal expansion was ascribed to its possessing an alkane chain. [This may also relate to cholesterol in membrane systems.].