Thermodynamic investigation of the binary and ternary complexes involving 1-aminocyclopropane carboxylic acid with reference to plant hormone

Complex formation equilibria of 1-aminocyclopropane carboxylic acid (ACC) and 3,3-bis(1-methylimidazol-2-yl) propionic acid (BIMP) with metal ions Cu²⁺, Ni²⁺, Co²⁺, Zn²⁺, Mn²⁺ and Fe²⁺ were investigated. ACC forms 1:1 and 1:2 complexes in addition to the hydrolysed form of the 1:1 complex, except in the case of Mn²⁺ and Fe²⁺, where the hydrolysed complex is not formed. BIMP forms 1:1 and 1:2 complexes in addition to the hydrolsed form of the 1:1 complex in the case of Mn²⁺ and Cu²⁺, however the hydrolysed complex is not detected for Ni²⁺, Co²⁺, Zn²⁺ and Fe²⁺. The concentration distribution diagrams of the complexes were determined. The Fe²⁺-complex with BIMP is exothermic and the thermodynamic parameters were calculated. The effect of organic solvent on the acid dissociation constants of 1-aminocyclopropane carboxylic acid (ACC) and 3,3-bis(1-methylimidazol-2-yl) propionic acid (BIMP) and the formation constants of Fe²⁺ complexes were investigated. Fe²⁺ forms a mixed-ligand complex with ACC and BIMP with stoichiometric coefficients 1:1:1. The formation constant was determined. The ternary complex is enhanced by back donation from the negatively charged 1-aminocyclopropane carboxylate to the π-system of BIMP. From the concentration distribution diagram, the ternary complex prevails in the physiological pH range.