Determination of alkali metal ion binding selectivities of calixarenes by matrix-assisted laser desorption ionization and electrospray ionization in a quadrupole ion trap

Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) an d electrospray ionization mass spectrometry (ESI-MS) are used to evaluate t he alkali metal ion binding selectivities of a series of calixarenes. Each calixarene of interest is mixed with one or more alkali metal salts (1:100 ratio of calixarene to metal), either in the: ESI solution or on the MALDI probe surface, and the relative binding selectivities are directly determin ed from the intensities of the calixarene/metal complexes in the mass spect ra. For t-butylcalix[4]arene-tetraacetic acid tetraethyl ester (calixarene 1), complexation of Na+ is favored over complexation of K+, in agreement wi th prior solution results obtained by conventional methods. For the three c alixarenes that do not have t-butyl groups on the upper rims, the calixaren es preferentially bind K+ over Na+, thus demonstrating that size selective complexation can be probed with both the ESI and MALDI methods. Collision-a ctivated dissociation results indicate that the phenyl oxygens, but not nec essarily the ethoxy ethyl oxygens of the lower rims, are the primary bindin g sites for the alkali metal ions. (C) 1999 Elsevier Science B.V.