Electrospray mass spectrometry and supramolecular complexes: quantifying the metal ion binding properties of cholic acid derivatives

An electrospray mass spectrometry protocol has been developed for the rapid screening of metal ion binding affinities, and it has been applied to the study of the alkali metal ion binding characteristics of neutral monomeric and macrocyclic steroid derivatives. It is shown that experiments involving competition between a range of metal ions and a single host can yield useful information on relative binding affinities; these results correlate well with synthetic templating results in solution. The reverse experiment, in which several hosts compete for the same ion, does not give reliable results (a) because unbound hosts remain uncharged and are not detected in the spectrometer and (b) because different hosts display radically different inherent detectabilities.