CATION SELECTIVITY OF NATURAL AND SYNTHETIC IONOPHORES PROBED WITH LASER-INDUCED LIQUID BEAM MASS-SPECTROMETRY

The novel laser desorption method laser-induced liquid beam ionization /desorption (LILBID) is applied to the mass spectrometric examination of selective ion binding by natural and synthetic ionophores in methan ol solutions. The ions are desorbed from a liquid jet with an IR laser pulse and then extracted perpendicularly into a reflection time-of-fl ight (RE-TOF) analyzer. LILBID studies on the natural ion carriers val inomycin and monensin A are presented, as well as those on the synthet ic crown ethers 18-crown-6, diaza-18-crown-6, and benzo-15- crown-5. N o fragment ions are detected, and the measured ion selectivity is in g ood qualitative agreement with published stability constants of the co mplexes. The observed specific recognition of silver ions by diaza-18- crown-6 can be rationalized by the principle of hard and soft acids an d bases, which predicts stable complexes when the polarizabilities of Lewis acid and base are similar. Weak, noncovalent interactions like t hose in the sandwich complex between two benzo-15-crown-5 molecules wi th one potassium ion are detected with LILBID. Their preservation duri ng the process of ion desorption depends on the laser intensity. A com parison with spectra obtained by using electrospray ionization (ESI) a nd matrix assisted laser desorption/ionization (MALDI) shows that LILB ID can potentially become a sensitive tool for the screening of weak b ut specific molecular interactions.