Phytosiderophores structures and properties of mugineic acids and their metal complexes

Abstract

In graminaceous plants such as barley, oats, and wheat, novel iron-chelating amino acids are secreted from the roots. A typical example is mugineic acid. A phytosiderophore, mugineic acid significantly stimulates iron-uptake and chlorophyll synthesis in rice plants. Most microbial siderophores have hydroxamate or phenolate groups as Fe(III)-coordination donors, while phytosiderophores consist of carboxyl, amine, and hydroxyl groups as the ligand functional groups. The mugineic acid-Fe(III) complex and its structurally analogous Co(III) complex have been characterized by some spectroscopic and X-ray diffraction methods. The coordination of mugineic acid to Co(III) and Fe(III) ions involves the azetidine nitrogen, secondary amine nitrogen, both terminal carboxylate oxygens as basal planar donors, and the hydroxyl oxygen and intermediate carboxylate oxygen as axial donors in nearly octahedral configuration. The Mössbauer (ΔEQ = 0.24 and δ Fe = + 0.39 mm/sec) and ESR (g = 9.4, 4.51, 4.44, and 4.31) parameters of the mugineic acid-Fe(III) complex are characteristic of high-spin (S = 5/2) ferric type. Of special interest is the apparent high reduction potential (E1/2 = − 102 mV vs. NHE) of the mugineic acid-Fe(III) complex, as compared to those of the microbial hydroxamates and ferric enterobactin. The mechanism of iron-absorption and -transport in gramineous plants probably includes Fe(III)-solubilization by mugineic acid and reduction from the thermodynamically stable ferric mugineic acid complex (log K ML M = 18.1) to the weakly bound ferrous complex (log K ML M = 8.1).