Synthesis and iron(III) complexing ability of CacCAM, a new analog of enterobactin possessing a free carboxylic anchor arm. Comparative studies with TRENCAM

Abstract

A new tricatecholate, CacCAM, has been synthesized by attachment of three catecholamide subunits to a CO₂H-functionalized triamine backbone. The solution coordination chemistry of the ligand and its iron(III) complex have been investigated by potentiometric, spectroscopic and kinetic methods. The results are compared to those obtained with TRENCAM, which possesses the same catecholamide subunits, but a TREN [tris(2-aminoethyl)-amine] backbone. The stability constant (log β₁₁₀=42.9, pFe=27.5) is close to that of TRENCAM (log β₁₁₀=43.6, pFe=27.8), showing that a change of backbone does not significantly alter the iron chelating ability of the ligand. Kinetics of iron exchange between Fe(III)–EDTA and CacCAM or TRENCAM involves similar rate constants (16±1 and 14±2 M⁻¹ s⁻¹, respectively). EPR data show that the iron(III) does not have the same distorted rhombic geometry in the two complexes. Fe(III)–CacCAM has been tested as a source of iron in nutritional experiments with Arabidopsis thaliana plant cells: its efficiency is good, comparable to that of Fe(III)–EDTA, the most widely used iron complex for plant nutrition.