Bile Salt Aggregation in Aqueous and Non-Aqueous Media
The distribution of four bile salts: sodium cholate (I), sodium deoxycholate (II), sodium chenodeoxycholate (III) and sodium ursodeoxycholate (IV) between aqueous buffer and 1-octanol has been measured as a function of temperature between 25° C and 55° C and as a function of bile salt concentration at concentrations below 0.1 mole/liter in the aqueous phase. The distribution isotherms obtained have been explained on the basis of reversible association in the aqueous phase. The treatment assumes that the bile acid exists as a monomer in the organic phase, which is verified by vapor pressure osmometry. A graphical method has been employed to estimate the association constants in the aqueous phase for the various equilibria encountered. An aggregation number of four for IV and twelve for I, II and III has been estimated. From the results, thermodynamic functions associated with the transfer of each of the bile salts from water to octanol and those related to association processes in the aqueous phase were calculated. These results are consistent with our previous findings that the premicellar association of bile salts occurs by hydrophobic interaction. The value of the thermodynamic parameters for the transfer of bile salts from aqueous buffer to octanol revealed that there is an unfavorable enthalpic and favorable entropic contribution for all four bile salts. However, for IV, which is an epimer of III, both enthalpic and entropic contributions are reduced, compared to III, suggesting that there is a pronounced effect of stereochemical orientation on the hydrophobic interaction.
The distribution of deoxycholate (II) between aqueous buffer and more lipophilic organic phases consisting of 70:30::isooctane:1-octanol (v/v) (system A) or 80:20:: isooctane:chloroform (v/v) (system B) was also studied. The distribution isotherms suggested that II associates strongly in the organic systems A and B unlike in pure 1-octanol. The model was modified to include association of II in the organic phase to describe distribution behavior. The treatment suggests that II exists as a monomer and a dimer in system A with a dimerization constant of 820M −1. A model consisting of monomer-tetramer-hexamer in the organic phase best describes the data for system B. The data support the view that association in the organic phase is due to hydrogen bonding between bile acid molecules. These studies suggest that partition of bile salts from an aqueous to a lipid membrane phase would involve an inversion from hydrophobic “back to back” association in the aqueous phase to hydrogen-bonded association in the lipid phase. In a similar fashion the enterohepatic cycling of bile acids must be accompanied by the inversion of structure, since this process also requires that bile acid partitions from an aqueous environment into a membrane lipid bilayer.
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