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 fibre s have been successfully used to represent bile salt micellar aggregates in aqueous solutions and electromotive force measurements have shown that pro tonated micellar species are present below some critical values of pH. This paper deals with the crystal structures of 3 alpha,12 alpha-dihydroxy-5 be ta-cholanoylglycine (HGDC), 3 alpha,12 alpha-dihydroxy-5 beta-cholanoyltaur ine (HTDC) and 3 alpha,7 beta-dihydroxy-5 beta-cholanoyltaurine (HTUDC), wh ich were solved to obtain models of protonated micellar aggregates. The mod els are compared with those found in crystals and fibres of sodium and rubi dium salts of HGDC and HTDC (NaGDC, NaTDC, RbGDC, RbTDC) in order to verify whether the acid structures match with the salt structures. The HGDC packi ng resembles that of a NaTDC crystal and is stabilized mainly by hydrogen b onds as well as by dipole-dipole interactions between acetone molecules and carboxylic groups. Three different 3(1) helices are identified. One of the se can be easily transformed into the 7/1 helix which satisfactorily descri bes the NaGDC, NaTDC, RbGDC and RbTDC micellar aggregates. The HTDC and HTU DC crystal structures are practically the same. Strong hydrogen bonds betwe en H3O+ (hydronium ion) and three oxygen atoms of the anions show O ... O d istances 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 electromoti ve force data indicates that the glycodeoxycholate and taurodeoxycholate mi cellar aggregates' proton affinities increase as their sizes increase and t hat those of the bigger aggregates seem to converge, even though the proton affinity of COO- is greater than that of SO3-. These findings strongly sug gest that micellization induces H3O+ binding. HTDC and HTUDC form micellar aggregates which increase their apparent hydrodynamic radius by adding HCl.