Condensed complexes and the calorimetry of cholesterol-phospholipid bilayers

A recent thermodynamic model describes a reversible reaction between choles terol (C) and phospholipid (P) to form a condensed complex CnqPnp, Here q a nd p are relatively prime integers used to define the stoichiometric compos ition, and n is a measure of cooperativity. The present study applies this model to the scanning calorimetry of binary mixtures of cholesterol and sat urated phosphatidylcholines, especially work by McElhaney and collaborators . These mixtures generally show two heat capacity peaks, a sharp peak and a broad peak. The sharp heat absorption is largely due to the chain melting transition of pure phospholipid. In the present work the broad heat absorpt ion is attributed to the thermal dissociation of complexes. The best fits o f the model to the data require the complex formation to be highly cooperat ive, with cooperativity n = 12. Detailed comparisons are made between model calculations and calorimetric data. A number of unusual features of the da ta arise naturally in the model. The principal discrepancy between the calc ulations and experimental results is a spurious calculated heat absorption peak. This discrepancy is related to the reported relative magnitudes of th e integrated broad and sharp heat absorption curves.