Probing Cyclodextrin-Guest Associations Using Electrospray Mass Spectrometry

Mass spectrometric investigation of noncovalent interactions has been made possible with the advent of electrospray ionization technique. Early electrospray studies of these interactions have typically examined biopolymers such as proteins and nucleic acids. Noncovalent interactions also exist between host cyclodextrins and various guest molecules. In this work, we use electrospray mass spectrometry to examine noncovalent host-guest associations between cyclodextrins and several organic molecules with the goal of identifying the types of interactions that predominate in the inclusion complexes formed. The importance of "mild" electrospray conditions and sample preparation procedures are discussed in reference to these studies. Ortho-, meta-, and para-nitrophenol are used as guest molecules with α, β and γ cyclodextrin hosts to study the effects of host size, guest substituent position, and host:-guest concentration ratio on complex formation. Relative binding affinities are determined for meta-and para-nitrophenol and agree with other solution phase studies. We also present the first report of complexes between the very hydrophobic polycyclic aromatic hydrocarbon pyrene and β-cyclodextrin as observed under "mild" electrospray conditions.