QELS and X-ray study of two dihydroxy bile salt aqueous solutions

Abstract

Previously, aqueous solutions containing one dihydroxy and one trihydroxy bile salt together with 0.8 M NaCl were studied by means of small-angle X-ray scattering (SAXS) and quasi-elastic light-scattering (QELS) techniques. It was observed that even a small amount of the bile salt forming smaller size aggregates (trihydroxy salt) inhibits the growth of the bile salt forming bigger size aggregates (dihydroxy salt), giving rise to an apparent mean hydrodynamic radius ( R h) value lower than that of the dihydroxy salt alone. This phenomenon occurred when sodium glycocholate and taurocholate were added to sodium glycodeoxycholate (NaGDC) and taurodeoxycholate (NaTDC) cylindrical aggregates, formed by trimers. In this paper observed SAXS data of a NaGDC aqueous electrolyte solution are reported and compared with the calculated ones by using the cylindrical aggregate model. The satisfactory agreement strongly supports the validity of the model. Moreover, aqueous solutions containing two dihydroxy salts (NaGDC and NaTDC), together with 0.8 M NaCl to minimize charge interactions, are investigated by QELS measurements that give information on the growth processes of NaGDC and NaTDC aggregates as a function of their relative concentration. The R h-value of a fixed amount of bile salt increases by adding the other bile salt, in agreement with the presence of two-component aggregates formed by random or block association of NaGDC and NaTDC trimers and in disagreement with the hypotheses of the growth inhibition and the formation of two one-component aggregates. X-ray and atomic emission spectroscopy measurements of fibers drawn from NaGDC + NaTDC solutions show that the fiber composition reflects with good approximation that of the solution. Under the same bile salt and NaCl concentrations and at the same temperature, the R h-values at high ionic strength of one-bile-salt solution follow the order NaGDC > NaTDC. Our results show that the R h-value of a 100 mM two-bile-salt solution (NaGDC + NaTDC) is intermediate between those of 100 mM NaGDC and NaTDC pure solutions. Moreover, R h-values of solutions with constant NaGDC + NaTDC total concentration (100 mM) versus NaGDC or NaTDC concentration are fitted by straight lines, thus indicating that the aggregate size linearly depends on NaGDC and NaTDC molar fractions. All these results indicate that NaGDC and NaTDC trimers seem to be interchangeable to some extent in the two-component aggregates that very probably are formed by means of the block association process.