Surface pressure-surface area characteristics of a series of autoxidation products of cholesterol.

The pi-A characteristics of seven known oxidation products of cholesterol were determined. In all cases, the oxidation products yielded films which were more expanded than the film of cholesterol. Shifts in the position of the functional polar groups or the double bond within the sterol molecule results in marked changes in the pi-A curves. Furthermore, replacement of the 3-hydroxy group by a keto group results in a significant decrease in the collapse pressure of the films. Mixed films of each of the oxidation products with dipalmitoyl glycerylphosphorylcholine, egg lecithin, and cholesterol showed marked condensation effects. However, the data suggest that if air oxidation of cholesterol did occur at a biological membrane containing cholesterol and phospholipids, the permeability characteristics and other properties of the membrane might not be altered significantly.