Monolayers of some naturally occurring polycyclic compounds

Abstract

A detailed study of the monolayer characteristics of some films of amphipathic sterol or sterol-like, rigid polycyclic molecules, has been carried out. These include cholesterol, cholesteryl butyrate and caproate, lanosterol, α-amyrin, β-estradiol diacetate, and abietic acid. Particular attention has been paid to extending the findings of Adam et al. (1, 2) concerning those factors which govern whether the isotherm will be condensed or expanded, and to determining the ability of cholesterol to condense such expanded isotherms. The results confirm that when the hydrophilic head group is suitably located on the molecule so as to allow close packing, a condensed isotherm always results. Expanded isotherms arise when the immersion requirements of the head group or groups bring about a tilting of the molecule away from a vertical position. In the extreme case, when two hydrophilic groups are located at opposite ends of the molecule a horizontal orientation is found at low surface pressures. In mixed films, cholesterol is capable of condensing expanded isotherms of rigid polycyclic materials when the head group is located in the 3-position of a sterol structure. The ability of cholesterol to erect a horizontally oriented estradiol diacetate molecule indicates that molecular interactions play a significant role in such condensation effects.