Allosteric effects in polynuclear triple-stranded ferric complexes

The synthesis and iron(III) coordination properties of three tripodal ligands (L-1, L-2, and L-3) possessing hydroxamate coordination cavities are examined by various methods (ESMS, UV-vis, CD). The ligands rely on a trisamine as anchor, which is extended by an alternating sequence of variable spacers and hydroxamates as ion binding groups. This modular strategy of design is adopted for the compounds' preparation and enables modifications of each structural element independently. The coordination properties of these iron binding molecules and particularly the presence of allosteric effects are examined by classical spectrophotometric titrations in combination with electrospray mass spectrometric measurements (ESMS). A good match between these two methods is observed, as both indicate the formation of three species in thermodynamic equilibrium: mononuclear, binuclear, and trinuclear ferric complexes. The respective stability constants are determined at p[H] = 6.5 +/- 0.1 in methanol, and the corresponding distribution curves clearly illustrate the variations from ligand to ligand. These findings demonstrate that subtle structural changes have a pronounced effect on these compounds' coordination properties. Moreover, among the binders studied representatives of opposite cooperative behavior is identified. The observed dependence of the ligands' coordination properties on their structural features are rationalized.