Evaluation of Binding Selectivities of Caged Crown Ligands toward Heavy Metals by Electrospray Ionization/Quadrupole Ion Trap Mass Spectrometry

Abstract

Electrospray ionization (ESI)/quadrupole ion trap mass spectrometry is used to evaluate the heavy metal binding selectivities of five caged crown ethers and two polyether reference compounds in methanol solution. The binding preferences for Hg2+, Pb2+, Cd2+, and Cu2+ were analyzed by comparison of ESI mass spectral intensities with the aim of developing this method for the rapid screening of binding selectivities of new synthetic ligands. The cage compounds preferentially bind Hg2+, except for the cage cryptand derivative, which favors Pb2+. The preference for Hg2+ stems from the favorable positioning of the nitrogen or sulfur atoms for linear coordination of Hg2+, whereas the cryptand derivative favors Pb2+ because of its larger cavity size. The counterions of the metal salts influence the type of complexes observed in the ESI mass spectra because the strengths of the metal-anion bonds affect retention of the anion in the complexes.

Molecular recognition and supramolecular chemistry in the gas phase

“…A large variety of crowns have been studied, from the simplest representatives, including their aza-and thia-analogs (Takahashi et al, 1992;Colton et al, 1995a), to bis- (Beer, 1985) and triscrowned structures (Wang et al, 1995a;Wang & Gokel, 1996;also, see: Wang et al, 1995b), cleft-like (Blair et al, 1998a), caged (Blair et al, 2000;Reyzer et al, 2001), and acetal crown ethers (Oshima, 1998). Also, the binding ability to alkaline earth Shen et al, 2000) and heavy metal ions was examined (Blades et al, 1990;Colton et al, 1995a;Blair et al, 2000). Finally, crown ethers have been employed for the gas-phase stripping of excess alkali cations from biomolecules (Pope et al, 1998).…”

Poly(ethylene glycol) doubly and singly cationized by different alkali metal ions: Relative cation affinities and cation-dependent resolution in a quadrupole ion trap mass spectrometer

“…E xtensive solution and gas phase investigations of alkali metal ion cationization of small podands, which are linear or ring structures with repeating (-Carbon-Carbon-Heteroatom-) units, have lead to improved understandings of host-guest complexation [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Factors involved in the solution phase molecular recognition are solvation enthalpy and entropy for both species and the number of atoms from the ligand that are involved with the binding of the metal ion, plus any conformational change of the ligand between its unbound and bound forms [21].…”

Metal-complex formation in continuous-flow ligand-exchange reactors studied by electrospray mass spectrometry

“…Using MS as a detection technique offers the advantage of being, in general, more sensitive than NMR and X-ray diffraction and opens up the possibility to screen complex samples. Moreover, MS enables simultaneous monitoring the response of ligand-exchange reactions and additionally obtaining chemical information about the specific compound [6,7].Today, MS is often used in studying host-guest complexes [8 -10], structural elucidation by collisioninduced dissociation (CID) MS [11,12], and studying relative binding constants of ligands to metal-ions [13,14]. Additionally, due to the liquid-based ionization in the ESI process, it is suitable for studying liquid-phase reactions, e.g., ligand-exchange reactions [6,7], which enables the study of complex systems.…”

“…Today, MS is often used in studying host-guest complexes [8 -10], structural elucidation by collisioninduced dissociation (CID) MS [11,12], and studying relative binding constants of ligands to metal-ions [13,14]. Additionally, due to the liquid-based ionization in the ESI process, it is suitable for studying liquid-phase reactions, e.g., ligand-exchange reactions [6,7], which enables the study of complex systems.…”

“…FAB-MS and ESI-MS have been used in the past to evaluate binding selectivities of various compounds, e.g., caged crown ethers to metal-ions [16]. An excellent correlation between obtained ESI mass spectral data and expected binding selectivities has been reported in several papers [13,17,18], although several parameters should be kept in mind. For a good correlation between complex formation in solution and what is observed in the mass spectrum [14], particularly the competing effect of the solvent, the conditions and ionization efficiencies of the different complexes, and molecules have to be taken into account.…”