Dimorphism in bile salt/lecithin mixed micelles.

We investigated the dependence of certain properties of bile salt/lecithin mixed micelles on the molar ratio of the two components. UV absorption suggested that the micellar system is fundamentally altered on passing through a bile salt:lecithin molar ratio of 1.8:1. Differential scanning calorimetry confirmed this transition ratio; in addition, it indicated a bilayer arrangement of lecithin, in support of the mixed-disk model of micellar structure, but only for micelles having a molar ratio less than 1.8:1. For micelles having larger ratios, which are those of physiological interest, calorimetry was inconsistent with the mixed-disk model. These observations support the X-ray structure analysis of Müller.