Allosteric Effects in Polynuclear Triple-Stranded Ferric Complexes

Abstract

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.

A Simple Thermodynamic Model for Quantitatively Addressing Cooperativity in Multicomponent Self‐Assembly Processes—Part 1: Theoretical Concepts and Application to Monometallic Coordination Complexes and Bimetallic Helicates Possessing Identical Binding Sites

“…This result is reminiscent of the reported cooperative complexation of Fe III to anionic preorganized polyhydroxamate receptors displaying strong peripheral NH···OC bonds. [21] The recognition of such deviations suggests that programming positive cooperativity in multimetallic assemblies requires a judicious design of secondary interactions, in order to overcome unavoidable intermetallic repulsions. It is worth noting here that the original site-binding model considers only DE MM , and that the global positive cooperativity evidenced for the formation of [Eu 2 (L3) 3 ] from its components, escapes detection with this rough approach.…”

Lanthanide Homobimetallic Triple‐Stranded Helicates: Insight into the Self‐Assembly Mechanism

“…Therefore, the binding of a second Eu III cation does not involve any structural rearrangement, but is mainly governed by the de-solvation of the entering cation and by intramolecular electrostatic repulsions. The latter induce a sizeable negative cooperative effect [27,34] as demonstrated by the ratio K Eu 2 L /K EuL ഠ 10…”

Self-Assembly of Tricuprous Double Helicates: Thermodynamics, Kinetics, and Mechanism

“…Thermodynamic cooperativity was studied on double-stranded Cu I [36] and Ag I [37] trinuclear helicates. More recently, the stability constants of anchored triple ferric helicates in MeOH have been determined [38]. The formation mechanism of such structures has been investigated [39] [40], but further information on the parameters involved in the complexation process are required.…”