Surface properties of stigmastanyl phosphorylcholine monolayers spread at the air/water interface

Abstract

Stigmastanyl phosphorylcholine is a synthetic ester that consists of a non-polar stigmastanol group and a phosphatidylcholine residue. This drug forms a stable monolayer of low compressibility (3.3 × 10-3 m/mN) on spreading at the air/water interface. The fact that the limiting area (the area extrapolated to zero pressure) occupied by the monolayer is similar to that of the phospholipid L-α-dipalmitoylphosphatidylcholine suggests that its value, 60 Å2/molecule, is mainly due to the polar group in both compounds; based on this value, the group is assumed to adopt a tilted orientation relative to the interface. This is seemingly confirmed by the results obtained at a variable substrate pH; in fact, the monolayers occupy larger areas on acid or alkaline substrates as a consequence of the ionized state of the polar group, which takes an orientation in between that coplanar with the interface and that normal to the surface. Increasing the subphase temperature or ionic strength results in a more marked expansion of the film through decreased surface packing of the non-polar residue or an orientational change in the polar group (or its solvation state), respectively. © Springer-Verlag 1999.