On the role of hydronium ions in the protonated micellar aggregates of bile salts

In previous work, structural units observed in bile salt crystals and fibres have been successfully used to represent bile salt micellar aggregates in aqueous solutions and electromotive force measurements have shown that protonated micellar species are present below some critical values of pH. This paper deals with the crystal structures of 3 alpha,12 alpha-dihydroxy-5 beta-cholanoylglycine (HGDC), 3 alpha,12 alpha-dihydroxy-5 beta-cholanoyltaurine (HTDC) and 3 alpha,7 beta-dihydroxy-5 beta-cholanoyltaurine (HTUDC), which were solved to obtain models of protonated micellar aggregates. The models are compared with those found in crystals and fibres of sodium and rubidium salts of HGDC and HTDC (NaGDC, NaTDC, RbGDC, RbTDC) in order to verify whether the acid structures match with the salt structures. The HGDC packing resembles that of a NaTDC crystal and is stabilized mainly by hydrogen bonds as well as by dipole-dipole interactions between acetone molecules and carboxylic groups. Three different 3(1) helices are identified. One of these can be easily transformed into the 7/1 helix which satisfactorily describes the NaGDC, NaTDC, RbGDC and RbTDC micellar aggregates. The HTDC and HTUDC crystal structures are practically the same. Strong hydrogen bonds between H3O+ (hydronium ion) and three oxygen atoms of the anions show O ... O distances within the range 2.4-2.6 Angstrom, owing to additional ion-ion and ion-dipole interactions. Very probably, H3O+ replaces Na+ in the micellar aggregates without remarkably changing their structure because the H3O+... O and Na+... O distances are very close. Inspection of previous electromotive force data indicates that the glycodeoxycholate and taurodeoxycholate micellar aggregates' proton affinities increase as their sizes increase and that those of the bigger aggregates seem to converge, even though the proton affinity of COO- is greater than that of SO3-. These findings strongly suggest that micellization induces H3O+ binding. HTDC and HTUDC form micellar aggregates which increase their apparent hydrodynamic radius by adding HCl.