Morphology of bile salt micelles as studied by computer simulation methods

The relative arrangement of the neighboring bile ions and the shape of the hydrophobic and hydrogen-bonded primary micelles as well of the large secondary micelles formed by these ions are analyzed in detail on the basis of molecular dynamics computer simulations of 30 and 300 mM sodium cholate and sodium deoxycholate solutions. In the lower concentration considered, the systems only contain primary micelles, whereas in both of the 300 MM systems secondary micelles are also present. The simulations performed were long enough that the systems reached thermodynamic equilibrium. It is found that the neighboring cholate ions prefer alignments in which their quasi-planar tetracyclic ring systems are parallel with each other, whereas for deoxycholate an opening of the angle between these planes is observed. The shape of the micelles is characterized by the ratio of their three principal moments of inertia. The primary deoxycholate micelles are found to be rather spherical, whereas in the case of cholate somewhat flattened, disklike or oblate shaped ellipsoidal primary micelles are found, irrespective of whether these micelles are kept together by hydrogen bonds or are of hydrophobic origin. Finally, the secondary micelles are found to exhibit a large variety of shapes, ranging from flattened oblates to rodlike objects through various different irregular shapes, characterized by markedly different values of the three principal moments of inertia. The observed preferences of the relative arrangement of the neighboring ions and of the aggregate shapes as well as the differences observed in the behavior of the two bile ions studied in these respects are traced back to the molecular structure of these ions.