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Sodium glycocholate (NaGC) and taurocholate (NaTC) have been studied by means of X-ray and circular dichroism (CD) measurements, using bilirubin-lXa (BR) as probe molecule, together with potential-energy calculations. Helical models for the micellar aggregates of NaGC and NaTC were inferred from crystal structures solved by X-ray analysis. Since it is known that chiral molecules, micellar aggregates and macromolecules select preferentially or exclusively one of the two enantiomeric conformers of BR, CD spectra of BR in submicellar and micellar aqueous solutions of NaGC and NaTC were recorded as a function of pH and BR concentration in order to verify these helical models and the enantioselective ability of the bile salt monomers and micellar aggregates. Potential-energy calculations supported the CD experimental results and provided reasonable bile salt-BR interaction models. The behaviour of NaGC and NaTC is compared with that of sodium deoxycholate (NaDC), previously studied. The CD spectra of the bile salt-BR systems seem to allow characterisation of the typical structure of the bile salt micellar aggregates.

supported on alumina. The appearance of all of the products and the disappearance of one of the reactants (OH-) can be simultaneously followed by FTIR.RhC13 has been shown to react with surface O H groups on alumina at 180 "C. The product of this reaction is believed to be Rh3+ bonded to a surface oxygen on alumina. This species is an intermediate in the room-temperature oxidation of CO listed above.Removal of the surface O H groups by reaction with RhC1, dramatically suppresses the reaction of COz with alumina.Acknowledgment. This work has been funded in part by the Office of Naval Research, and the University of Richmond Faculty Research Committee. We thank Dr. John Yates for his helpful comments and for his assistance in designing the experimental setup and Hampton Rexrode for his work on the apparatus construction. Previously a helical model was satisfactorily verified for sodium (NaDC) and rubidium (RbDC) deoxycholate micelles in aqueous solutions by means of SAXS, EXAFS, NMR, ESR, and CD measurements. Here we report the beginning of an analogous study carried out on sodium glycodeoxycholate (NaGDC) and taurodeoxycholate (NaTDC) following the strategy applied to NaDC and RbDC. The crystal structure of NaGDC sesquihydrate, solved by X-ray analysis, provides again a helical model that is compared with those of NaDC, RbDC, and NaTDC. Since it was previously observed that bilirubin-IXa (BR) exhibits a bisignate CD Cotton effect in NaDC aqueous solutions, and it was suggested that the chiral micelles of NaDC interact preferentially or exclusively with one of the two enantiomeric conformers of BR, we have recorded CD spectra of BR in aqueous micellar solutions of the bile salts in order to check the helical models. The spectra show in all cases two large and proximate bands of opposite sign between 400 and 500 nm, which support both our chiral models and the selection of the BR left-handed enantiomer. Moreover, we have accomplished van der Waals energy calculations for the system formed by a NaGDC helix and the left-or right-handed BR molecule to test if the interaction energy is lower for one of the two BR enantiomeric conformers. The results of the calculations seem to indicate that the NaGDC helix binds preferentially the BR molecule with left-handed chirality. Interaction models are proposed. IntroductionStudies carried out on sodium and rubidium deoxycholate (NaDC and RbDC, respectively) following a strategy previously reported' showed that a helical model, observed in the crystal structures of NaDC and RbDC,2v3 describes very satisfactorily the behavior of their micellar aggregates in aqueous solutions.'*e7 The helical structure of the NaDC and RbDC micelles was verified unambiguously by nuclear magnetic resonan~e,'.~ circular dichroism! electron spin resonance! small-angle X-ray scattering,6 and extended X-ray absorption fine structure7 measurements.Subsequently, the investigation of the structure of the sodium taurodeoxycholate (NaTDC) micellar aggregates was undert a k e~? +~ and other helical models were...

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Quasi-elastic light-scattering data of sodium chenodeoxycholate (NaCDC), sodium deoxycholate (NaDC), and sodium taurodeoxycholate (NaTDC) show that the micellar size increases upon increasing ionic strength (NaCDC, NaDC, and NaTDC) and upon decreasing temperature and pH (NaCDC and NaDC). Their mean hydrodynamic radii (Rh) are in the following order: NaDC > NaTDC > NaCDC. The difference between the Rh values of NaDC and NaCDC is negligible without NaCl and increases upon increasing the NaCl concentration. The average intensities scattered by 0.01-0.10 M aqueous solutions are in the sequence NaTDC > NaDC > NaCDC. Thus, the growth of NaCDC is less than those of NaDC and NaTDC. The plots of the average intensity vs the square root of the mole fraction show that the experimental data are fitted by one straight line for NaCDC and two straight lines for NaDC and NaTDC, suggesting that the NaCDC or NaDC and NaTDC micellar aggregates can be represented by one or two structures, respectively. Since the R-h and the average intensity values of NaCDC and NaDC with added NaCl decrease by increasing the temperature within the range 15-85 degrees C, their micellar aggregates are stabilized mainly by polar interactions as well as the helical micellar aggregates previously proposed for NaDC and NaTDC. The decrease of pH causes a slight increase of the R-h values of NaDC and NaCDC up to a certain pH value, where a sudden increase is observed. Circular dichroism spectra of NaDC and NaCDC aqueous solutions above and below the critical micellar concentration indicate that the two salts give rise to different types of aggregation. Those of the interaction complexes between bilirubin-IXa and NaCDC or NaDC or NaTDC suggest that the micellar structures are chiral and helical. The enantioselective abilities of NaDC and NaTDC are somewhat similar, but they are different from that of NaCDC. The X-ray fiber patterns of NaCDC, NaDC, and NaTDC can be interpreted by means of different helices and support the quasi-elastic light-scattering and circular dichroism results