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).