The behavior of sterols on silica surfaces and at other interfaces

The specific manner in which a molecule is oriented at an interface determines the reactions in which the molecule can participate. Sterols, being highly anisotropic molecules, present 舠ends舡 and 舠sides舡 to surfaces which differ greatly in their reactivity and interaction with the surface. Seen from the 3ॆ‐hydroxy end, as at an air‐water interface, sterols are largely indistinguishable from one another in their behavior. On the other hand, at an organized interface, such as that of an absorbent, a great many differences in structure can be demonstrated. These include the distinction between nuclear and side‐chain double bonds, the number and location of nuclear double bonds, the substituents in the side chain and the number of methyl groups in the molecule. We have been interested in the degree of specificity which such interaction can display because of the obvious parallel to sterol‐protein interactions of a structural or enzymatic nature. Through the synthesis of a variety of sterol structures possessing the desired arrangement of double bonds and numbers of methyl groups, we have established a reproducible correlation between adsorptive interaction and double bonds in the ख5, ख7, ख8, ख5,7 and ख7, 9 positions, as well as for the saturated stanol. These correlations have been extended for the 4ॅ, 4ॆ, 4,4 dimethyl and 4,4,14ॅ trimethyl series as well. The elucidation of the structure of macdougallin as a 14ॅ methyl sterol in which the 4,4, gem dimethyl grouping was absent prompted a further study of the 14ॅ methyl sterols, of which the ख7, ख8, ख7,9 and unsaturated varieties have been prepared and tested. The presence of a 14ॅ methyl group in the absence of other methyl groups confers a unique behavior on the molecule in that this series does not fit into the adsorptive scheme for the other sterols. Instead, the interaction of the double bond with the surface is almost completely independent of its position, suggesting that the molecule is 舠perched舡 on the 14ॅ methyl group when absorbed in the surface. This appears to be a highly significant feature in the orientation of the molecule, since the removal of this methyl group is usually regarded as an essential first step in the conversion of lanosterol to cholesterol.