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Bile Salt Aggregates in the Gas Phase: An Electrospray Ionization Mass Spectrometric Study
Author(s) -
Cacace Fulvio,
de Petris Giulia,
Giglio Edoardo,
Punzo Francesco,
Troiani Anna
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020415)8:8<1925::aid-chem1925>3.0.co;2-x
Subject(s) - chemistry , electrospray ionization , hydrogen bond , aqueous solution , phase (matter) , salt (chemistry) , ion , mass spectrometry , counterion , analytical chemistry (journal) , crystallography , molecule , chromatography , organic chemistry
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 .