Mass spectrometric investigations of alpha- and beta-cyclodextrin complexes with ortho-, meta- and para-coumaric acids by negative mode electrospray ionization.

The mass spectrometric characterization of aqueous solutions of alpha- and beta-cyclodextrins (CDs) and o-, m- and p-coumaric acids (CAs) by negative ion electrospray ionization (ESI) indicates that the CD+CA ions were sourced from the inclusion complex present in solution and from the anion attached to CD molecules formed in the spray processes. The anion adducts formed in the spray process contribute significantly to the signal intensity of an ionized inclusion complex thus overestimating the calculated stability constant (K) of solution-phase complexes by one to two orders of magnitude. The relative intensities of anion adducts in mass spectra depend on the concentration ratio of the anion and the CD in spray droplets, while the relative intensity of the ionized inclusion complex depends on CD and CA concentrations in solutions and the value of K. Ion Mobility Spectrometry Mass Spectrometry [IMS-MS] measurements show that the collision cross-section (Omega) values of the CD+CA or (CD)(2+)CA and CD+CA (2-) complex ions are 5-6% larger than or equal to CD(-) or CD (2-), respectively. Therefore, in the gas phase the anion adducts [CD+CA(-)] on cyclodextrin molecules possess the same conformations as the ionized inclusion complexes CD+CA.