Diffusivity Study of Dihydroxy-Trihydroxy Bile Salt Systems

Previously, structural units inferred from X-ray diffraction analysis of fibers, crystals, and concentrated aqueous solutions were identified as models of dihydroxy and trihydroxy bile salt micellar aggregates. Dimers and their multiples (mainly octamers and hexadecamers) or structures derived from 7/1 helices (formed by trimers) were proposed for trihydroxy salts sodium glycocholate (NaGC) and taurocholate (NaTC) or for dihydroxy salts sodium glycodeoxycholate (NaGDC) and taurodeoxycholate (NaTDC), respectively, on the basis of strong experimental evidences. However, the radii of the cylindrical structures derived from 7/1 helices were unknown. This paper deals with small-angle X-ray scattering (SAXS) measurements that have permitted to estimate the cylindrical aggregate radius of NaGDC (about 22 Å), whereas that of NaTDC is very probably about 3 Å longer. SAXS and quasi-elastic light-scattering (QELS) data have shown a linear increase of the apparent mean hydrodynamic radius (within the approximate range 40−70 Å) with the height of NaGDC and NaTDC cylindrical aggregates, suggesting that the main interparticle correlation effects, ascribed to charge, van der Waals, and volume excluded interactions, are small or compensate each other. Moreover, aqueous solutions containing one dihydroxy and one trihydroxy bile salt (NaGDC and NaGC, NaTDC and NaTC, NaGDC and NaTC, NaTDC and NaGC) and 0.8 M NaCl have been studied. QELS measurements give information on the growing processes of NaGDC and NaTDC aggregates as a function of NaGC and NaTC concentration. Experimental data and calculations performed using the above-mentioned models support that 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 smaller size aggregates. Reasonably, this phenomenon is due to the very different and not interchangeable structures of dihydroxy and trihydroxy bile salt aggregates. It seems also that the aggregate growth is a little more inhibited when the trihydroxy molecule polar head is different from that of the aggregate molecules. Trihydroxy salt monomers and oligomers compete with dihydroxy salt trimers in the growth process and interact with their cylindrical aggregates especially by means of polar forces.