Dication induced stabilization of gas-phase ternary beta-cyclodextrin inclusion complexes observed by electrospray mass spectrometry.

Electrospray mass spectrometry (ES-MS) is an important tool for characterization of non-covalent binding in the gas phase. In this study, iron (II) has been introduced as a dication to enhance the detection of cyclodextrin (CD) plus aromatic compound complexes in ES-MS. Evidence that a novel ternary complex comprised of one beta-CD, one iron (II) and one toluene exists as an inclusion complex has been compiled via ES-MS and ES-MS/MS experiments as well as by a computational approach. This evidence strongly suggests that iron (II) serves to modify the conformation of the beta-CD ring, and that toluene inclusion is stabilized by dication interaction with the toluene pi-system and by crimping of the beta-CD ring leading to stronger van der Waals interactions with toluene. Mg(II), another dication of similar radius, showed similar behavior, while added group one cations (H(+) and Na(+)) were ineffective at producing observable ions representative of the complex. The ternary beta-CD complex with iron (II) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) has also been examined. ES-MS and ES-MS/MS experiments suggest that it is the polar portion of 2,4,5-T (i.e., the carboxylic acid moiety) that is favored for inclusion in the beta-CD cavity, rather than the non-polar aromatic part.