Electrospray ionization mass spectrometry of cyclodextrin complexes with amino acids in incubated solutions and in eluates of gel permeation chromatography

1:1 complexes of beta-cyclodextrin (CD) with three amino acids (Gly, Phe an d Trp) have been detected as ions in the gas phase using infusion positive and negative ion electrospray ionization mass spectrometry (ESI-MS). In con trast with the positive ion ESI mass spectra of simple aqueous solutions, t he aggregates and adducts usually formed in the ESI process did not appear in the positive ion ESI spectra of solutions buffered with ammonium acetate (NH4Ac), even at higher analyte concentrations, These studies suggest that addition of buffer and/or use of a low analyte concentration should be use d to overcome formation of aggregates and metal ion adducts in such mass sp ectrometry studies. Also, the deprotonated complexes are dissociated by col lision induced dissociation (CID) to form an abundant product ion, the depr otonated CD, requiring transfer of a proton to the amino acid carboxyl grou p, To understand formation of complexes in the gas phase, gel permeation ch romatography (GPC) was used to separate free amino acids (AAs) from complex es in an incubated solution. The ESI mass spectra of the GPC fractions show the presence of 1:1 complexes of both CD-aromatic amino acids and CD-aliph atic amino acids. Compared with CD-aliphatic amino acid complexes, CD-aroma tic amino acid complexes appear to be destabilized in the gas phase, possib ly because the hydrophobic interaction which binds the aromatic group of am ino acids in the CD cavity in solution may become repulsive when solvent ev aporates from the droplets during the electrospray process, whereas those c omplex ions formed as proton bound dimers are stabilized by electrostatic f orces, the major binding force for such complexes in the gas phase. In addi tion, the GPC technique coupled with off-line ESI-MS can rapidly separate C D complexes by size, and provides some information on the character of the complexes in solution. (C) 1998 John Wiley & Sons, Ltd.