Cacace Fulvio, de Petris Giulia, Edoardo Giglio, Punzo Francesco, Troiani Anna
Dipartmento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università di Roma La Sapienza, Italy.
Chemistry. 2002 Apr 15;8(8):1925-33. doi: 10.1002/1521-3765(20020415)8:8<1925::AID-CHEM1925>3.0.CO;2-X.
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).
此前通过X射线衍射分析已在胆汁盐的晶体和纤维中鉴定出螺旋和有序结构，并分别提出其作为由二羟基盐和三羟基盐的三聚体或二聚体单元形成的胶束聚集体的模型。这些模型得到了用不同实验技术对胶束胆汁盐溶液进行研究的结果的支持。现在，该研究已扩展到气相，通过利用电喷雾电离质谱法（ESIMS）来研究通过非共价相互作用稳定的聚集体的形成和组成，这些相互作用包括极性（离子-离子、离子-偶极、偶极-偶极、氢键等）和非极性（范德华力和排斥力）相互作用。在不同实验条件下记录的甘氨脱氧胆酸钠（NaGDC）、牛磺脱氧胆酸钠（NaTDC）、甘氨胆酸钠（NaGC）和牛磺胆酸钠（NaTC）水溶液的正、负离子ESIMS光谱首先表明，与胶束溶液中存在的聚集体类似的聚集体在气相中也存在。此外，与凝聚相模型一致，正离子光谱表明，在二羟基盐（NaGDC和NaTDC）的聚集体中，三聚体是最稳定的低聚物，而在三羟基盐（NaGC和NaTC）的聚集体中，二聚体是最稳定的。此外，大多数气态低聚物中组成甘氨胆酸盐单元的结合能超过相应牛磺胆酸盐单元的结合能。对NaGC和NaTC晶体以及NaGDC和NaTDC纤维进行的蒸气压测量证实了ESIMS的证据，测量结果表明，甘氨胆酸盐的蒸发焓比牛磺胆酸盐的蒸发焓高出约50 kJ mol(-1)。
