Systematic study of phospholipids linked to a steroid derivative, spread into a monolayer at the air/water interface.

Isothermal pressure-area curves of different phospholipids linked to a cortisol derivative, spread into monolayers at the air/water interface are studied. It is shown that derivatives containing saturated lipid chains and those with unsaturated chains present quite different behaviours. With saturated derivatives, the main phase transition plateau and the stability of the fluid phase are very sensitive to the length of the lipid chains, the presence of a spacer between the lipid and the steroid moieties, the temperature and the presence of di- and trivalent cations in the aqueous subphase; the calcium ion shows an especially high effect, compared to the other ions studied. The presence of the steroid on the lipid modifies the specific area of the molecules of unsaturated lipids, which is not the case with saturated lipids, probably due to differences in the lipophilic cohesion.