Molecular packing in steroid-lecithin monolayers, part I: Pure films of cholesterol, 3-doxyl-cholestane, 3-doxyl-17-hydroxyl-androstane, tetradecanoic acid and dipalmitoyl-phosphatidylcholine

Abstract

A detailed study has been carried out 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°C temperature range. The resemblence between pure films of cholesterol and the β-epimer of 3-DC is quite striking. Where molecular packing and hydrophobic interactions are concerned, the two molecules behave in a similar way. Where surface potentials or dipole moments are concerned the differences are consistent with the differing polar groups. In contrast, 3-DA differs substantially in its film behavior from that of either cholesterol or 3-DC. The differences can be ascribed both to the bipolar nature of 3-DA and its lack of an alkane chain at the C 17 steroid position. Based on these differences we ascribe the bulk of cholesterol's small coefficient of thermal expansion to its possessing an alkane chain.