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...

Halogen Oxidation of Tetrakis(dithioacetato)diplatinum(II) Complexes, Pt2(CH3CS2)4.Synthesis and Characterization of Pt2(CH3CS2)4X2 (X = Cl, Br, I) and Structural, Electrical, and Optical Properties of Linear-Chain (ju-Iodo)tetrakis(dithioacetato)diplatinum, Pt2(CH3CS2)4I

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The structures of crystals of rubidium glycocholate (monoclinic) and rubidium taurocholate (monoclinic and tetragonal) were solved by X-ray diffraction analysis. These structures, together with those of the sodium salts formerly determined, were used to infer models for the corresponding micellar aggregates in aqueous solutions. The crystal packings of the sodium and rubidium salts were characterized by similar structural units. These were bilayers, 2 1 helices, distorted 21 helices, and units with a two-fold rotation axis. On the other hand, circular dichroism spectra of the sodium and rubidium salts with bilirubin-IXR in aqueous solution indicated the formation of very similar interaction complexes and micellar aggregates. This hypothesis was supported by quasi-elastic light scattering measurements carried out on aqueous micellar solutions as a function of the ionic strength, concentration, and temperature. Low values of the apparent hydrodynamic radius, corresponding to oligomers of small size, were obtained for all the salts. The micellar growth was practically independent of the concentration and temperature, within the range 20-60°C, at low ionic strength, and slightly increased upon increasing the ionic strength. We studied the rubidium salts in place of the sodium ones by extended X-ray absorption fine structure spectroscopy when their structures are similar. The measurements at the Rb + K-edge, previously accomplished on the crystal and the aqueous micellar solution of rubidium glycocholate, were in agreement with the model of the 21 helix and allowed us to discard the bilayer. Circular dichroism spectra, recorded as a function of the ionic strength, pointed out that bilirubin-IXR gives rise to a different enantioselective complexation with dihydroxy or trihydroxy salts. The different structure of the corresponding micellar aggregates was also supported by quasi-elastic light scattering measurements accomplished on dihydroxy and trihydroxy salts. The calculation of the hydrodynamic radius for the 2 1 helix, observed in the monoclinic crystal of sodium taurocholate, showed that there is a possible satisfactory agreement with the quasi-elastic light scattering data.