Precision scanning calorimetry of bile salt-phosphatidylcholine micelles

Precision scanning calorimetry has been used to examine the thermal behavior of mixed micelles formed between bile salts and dipalmitoylphosphatidylcholine (DPPC). Complex thermal transitions are observed which change dramatically with the mole ratio of bile salt to DPPC, dilution, and ionic strength. Comparison of the behavior of sodium taurocholate (TC) mixed micelles with sodium taurodeoxy-cholate (TDC) mixed micelles indicates similarity in the thermal transitions at high dilution or when the actual micellar composition is similar. It was found through equilibrium dialysis that considerably less TC than TDC is incorporated into mixed micelles with DPPC at a given bile salt concentration. Accounting for these concentration differences provides a means for more direct analysis of changes in the thermal transitions with mole ratio and dilution for the two bile salt components. Resolution of the thermal transitions into several component contributions is employed as an aid to interpretation of the differential scanning calorimetry curves. The curve resolutions lead to estimates of van't Hoff and calorimetric enthalpies of the individual contributions. The results of the curve resolutions, along with the behavior of the total enthalpies of the transitions, are consistent with a transformation in micellar structure occurring when the actual micellar composition is a mole ratio of bile salt to DPPC of about 1 to 1. The transformation region is near that found from X-ray evidence and is thought to correspond to a change from disk-shaped to spherical micelles.