Edoardo Giglio

Some Thermodynamic Properties of C76and C84

The vapor pressures of C76 were measured over the temperature range 834−1069 K by the torsion−effusion method. The results are well represented by the following linear equation:  log(p/kPa) = (8.23...

From crystal to micelle: A new approach to the micellar structure

The structure of sodium and rubidium deoxycholate micellar aggregates in aqueous solutions was found to be helical and to be stabilized mainly by polar interactions. Astonishingly, the lateral surface of the helix is covered by nonpolar groups and the interior part is filled with cations surrounded by water molecules, as in the case of an inverted micelle. This helical model was inferred from the crystal structures of sodium and rubidium deoxycholates and proved by spectroscopic and diffractometric experimental data. The strategy of the approach to the determination of the micellar structure and the comparison with another model, previously proposed for the bile salt micelles, are reported. On the basis of some results obtained for sodium tauro- and glyco-deoxycholates, micellar models are suggested which could account for the biological function of these important conjugated bile salts.

Determination of the molecular packing in the crystal of coumarin by means of potential-energy calculations

Coumarin (o-coumaric acid lactone, C 9 H 6 0 2 ) forms orthorhombic crystals, space group Pca21, Z= 4, a= 15.466 (12), b= 5.676 (6), c= 7.917 (6) A. The structure was solved by potential-energy calculations coupled with the minimum residual analysis. Three-dimensional X-ray data were measured with an offline Siemens automatic single-crystal diffractometer by the co-scan technique. The refinement was carried out by least-squares methods and the final R is 0.048. The carbon and oxygen atoms of coumarin lie in the same plane, the greatest deviation being 0.015 A. The crystal packing is characterized mainly by van der Waals and dipole-dipole interactions. However, the dipole-dipole contribution to the total potential energy does not seem to determine the molecular packing.

Bile salt aggregates in the gas phase: an electrospray ionization mass spectrometric study.

Helical and ordered structures have previously been identified by X-ray diffraction analysis in crystals and fibers of bile salts, and proposed as models of the micellar aggregates formed by trimeric or dimeric units of dihydroxy and trihydroxy salts, respectively. These models were supported by the results of studies of micellar bile salt solutions performed with different experimental techniques. The study has now been extended to the gas phase by utilizing electrospray ionization mass spectrometry (ESIMS) to investigate the formation and the composition of aggregates stabilized by noncovalent interactions, including polar (ion-ion, ion-dipole, dipole-dipole, hydrogen bonding etc.) and apolar (van der Waals and repulsive) interactions. The positive and negative ESIMS spectra of sodium glycodeoxycholate (NaGDC), taurodeoxycholate (NaTDC), glycocholate (NaGC), and taurocholate (NaTC) aqueous solutions, recorded under different experimental conditions, show in the first place that aggregates analogous to those present in micellar solutions do also exist in the gas phase. Furthermore, consistently with the condensed-phase model, the positive-ion spectra show that the trimers are the most stable oligomers among the aggregates of dihydroxy salts (NaGDC and NaTDC) whilst the dimers are the most stable among the aggregates of trihydroxy salts (NaGC and NaTC). Moreover, the binding energy of the constituent glycocholate salt units in most gaseous oligomers exceeds that of the corresponding taurocholate units. The ESIMS evidence has been confirmed by vapor-pressure measurements performed on NaGC and NaTC crystals and NaGDC and NaTDC fibers, the results of which show that the evaporation enthalpy of glycocholate exceeds that of taurocholate by some 50 kJ mol(-1).

The crystal structure of the inclusion compound between cycloveratril, benzene and water

A monoclinic phase of the 1: 0.5 : 1 inclusion compound between cycloveratril, benzene and water crystallizes in space group C2/c, with a = 33.908 (9), b = 9.629 (3), c = 22.748 (5)/k, fl = 134.02 (1) °, Z = 8. The cycloveratril:benzene ratio was confirmed by gas chromatography. The structure was refined to a final R of 0.057. The host molecules have an umbrella shape with their methyl groups approximately coplanar with the benzene rings and are stacked along b. The guest molecules occupy the cavities of cages formed by methyl groups. The water molecules probably form bifurcated hydrogen bonds with the O atoms of cycloveratril. Goldup, Morrison & Smith, 1965). Thus this compound was recognized as 2,3,7,8,12,13-hexamethoxy5,10-dihydro- 15 H-tribenzo [a,d,g] cyclononene (cycloveratril, CVT), for which steric requirements favour a crown conformation for the cyclononatriene ring (Fig. 1).

Micellar aggregates of sodium glycocholate and sodium taurocholate and their interaction complexes with bilirubin-IXα. Structural models and crystal structure

Sodium glycocholate (NaGC) and taurocholate (NaTC) have been studied by means of X-ray and circular dichroism (CD) measurements, using bilirubin-IXα(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.

Sodium glycodeoxycholate and glycocholate mixed aggregates in gas and solution phases.

This paper deals with electrospray ionization mass spectrometry (ESIMS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) measurements in order to provide information on the existence, aggregation, composition, and structure of the two-component aggregates of sodium glycocholate (NaGC) and sodium glycodeoxycholate (NaGDC) in the gas and solution phases. Five samples, containing 100% NaGC and 100% NaGDC, and NaGDC/NaGC molar ratios of 3 (75D), 1 (50D), and 1/3 (25D), have been analyzed by ESIMS in positive-ion detection mode starting from 10(-3) and 10(-2) M total bile salt concentration in aqueous solutions. Generally, dimers or trimers prevail in the 100% NaGC or NaGDC samples, respectively, as observed in the preceding one-component ESIMS measurements and in agreement with the proposed micellar aggregate structures in aqueous solution. Moreover, it is observed that the composition of multimers in the samples 75D, 50D, and 25D deviates from the one expected on the basis of a random association of the monomers, the NaGDC contribution generally prevailing on the NaGC one. It happens also under the same percentage condition (50D sample), in agreement with a greater aggregation ability of NaGDC with respect to NaGC. SAXS and DLS data were recorded on six samples containing a NaGC+NaGDC 40 mM total concentration, one bile salt having 40, 32, 24, 16, 8, and 0 mM concentration and the other the complementary one, keeping constant the NaCl concentration (0.6 M). The NaGDC 40 mM sample presents SAXS curves in agreement with a cylindrical shape of the aggregates as shown in a previous paper. For the bile salt mixtures, the progressive decrease of the sizes and change of the aggregate morphology, toward a globular-like geometry, are observed by increasing the NaGC fraction, thus confirming the hypothesis about the ability of trihydroxy salts to inhibit the growth of dihydroxy salt aggregates. Fits on the basis of cylindrical model can be accomplished for all the SAXS spectra, however, when the extracted cylinder parameters are used to estimate theoretical hydrodynamic radii a reasonable agreement is obtained only for the samples at high fraction of NaGDC (NaGDC>or=24 mM).

Crystal structures of bile salts: Sodium taurocholate

Crystals of sodium taurocholate (NaC26H44NO7S · 2.5 H2O) belonging to the triclinic space groupP1 have unit cell parametersa = 12.731 (2),b = 16.104 (2),c = 7.628 (1) ⫗A, α =83.40 (1),β = 101.20 (1), γ = 105.35 (1)°, and two molecules in the asymmetric unit. The refinement, carried out on 4424 observed reflections, gaveR = 0.059 andRw = 0.066. The packing is characterized by bilayers, formed by antiparallel monolayers and with nonpolar outermost surfaces, held together by van der Waals interactions. Inside the bilayers there are channels, lined with polar groups, and filled by sodium ions and water molecules. A structural unit has been identified that could provide a reasonable model for the micellar aggregates of this bile salt.

A possible helical model for sodium glycocholate micellar aggregates

Sodium glycocholate crystallizes in the tetragonal space group14 witha =b = 27.793(4),c = 7.937(1) Å andZ = 8. Refinement based on 2290 observed reflections led to a conventionalR = 0.10. The bile salt molecules are arranged in a helix with 21 symmetry stabilized mainly by polar interactions. Four helices are held together by hydrogen bonds involving water molecules, giving rise to hydrophilic channels, with a small cross section, which can include water molecules. The packing of these tetramers form hydrophobic channels containing some disordered acetone and water molecules. The helices will be checked as a model for the micellar aggregates of this important conjugated bile salt, following the same strategy successfully applied to sodium deoxycholate.

Crystal Structure and van der Waals Energy Study of the 2:1 Inclusion Compound between Deoxycholic Acid and Norbornadiene

The crystals are orthorhombic, space group P2~2t21, with a = 27.125 (8), b = 13.456 (4), c = 14.212 (5) A, Z = 4. The structure has been refined to R = 0.11 and R w = 0.14 for 2451 observed reflections with I > 3o(1). The crystal structure is characterized by an assembly of bilayers which are slightly different from those of other orthorhombic phases studied so far. The section of the cavities in which the norbornadiene is accommodated is almost square, so that molecules or substituents of approximately spherical shape can be occluded. Van der Waals energy calculations allowed the location of the guest molecules which mainly interact with methyl groups.