Condensed complexes of cholesterol and phospholipids

Mixtures of dihydrocholesterol and phospholipids form immiscible liquids in monolayer membranes at the air-water interface under specified conditions of temperature and 2-dimensional pressure. in recent work it has been disco vered that a number of these mixtures exhibit two upper miscibility critica l points. Pairs of upper critical points can be accounted for by a theoreti cal model that implies the cooperative formation of molecular complexes of dihydrocholesterol and phospholipid molecules. These complexes are calculat ed to be present in the membranes both above and below the critical points. Below the critical points the complexes form a separate phase, whereas abo ve the critical points the complexes are completely miscible with the other lipid components. The cooperativity of complex formation prompts the use o f the terminology condensed complex.