Chemoselective phospho-anion binding studies
An investigation has been carried out, studying the interaction of various bioactive anions, particularly phosphorylated species, with well-defined chiral lanthanide complexes. In each ternary anion adduct, proton NMR analysis revealed a distinctive pattern for the most high frequency shifted resonances. Furthermore, the mean chemical shift of these four resonances can be used to identify the anion adducts present in solution and indicate the binding mode exhibited towards the lanthanide(III) centre. Similar differentiation of anion binding modes is also available from luminescence spectroscopy, via observation of the splitting pattern for the (^7)F(_1) ← (^5)D(_0)transition in europium complexes. Thulium and ytterbium complexes display the greatest affinity towards anions. With 0֊phosphorylated amino acids and peptides, amino acid or N-terminal chelation is a competitive binding mode. In contrast, the corresponding europium complexes display a significant preference towards the phosphate binding mode and such complexes may be considered to function as selective receptors for phosphorylated peptides or proteins. For all the lanthanides, complexes involving an N-methylated ligand show an increase in competition from the chelating binding modes. This effect is most significant for the smaller lanthanide(III) ions. Parallel investigations into the suitability of 1-azaxanthone derivatives as long wavelength sensitisers for europium(III) and terbium(III) emission were performed. The introduction of electron donating groups into the 1-azaxanthone compound led to an increase in the absorption wavelength maximum. However, perturbation of the excited states was accompanied by competitive fluorescence emission and in certain cases quenching of the excited states via intramolecular hydrogen bonding. Formation of the TV-oxide derivative of 1-azaxanthone also led to a significant red shift in the absorption maximum. The related 1 ֊aza-10-thioxanthone compound displayed the greatest potential as a lanthanide(III) sensitiser, and its incorporation into a phosphate selective europium receptor was investigated with the aim of achieving optical detection of low concentration of anions.