Mugineic acid(1), isolated from the roots of water-cultured barley (Hordeum vulgare L. var. Minorimugi), is the first compound to be shown to play a role in the uptake and transport of iron in higher plants. It is capable of effectively solubilizing Fe(OH)_3 within the pH range 4-9. The potentiometric titration, spectroscopic, electrochemical, and X-ray diffraction studies on the mugineic acid-Fe(III) and -Co(III) complexes clarified that in both complexes the coordination involves the azetidine nitrogen, secondary amine nitrogen, both terminal carboxylate oxygen, hydroxyl oxygen, and intermediate carboxylate oxygen in a nearly octahedral configuration (Fig. 1). The structures of Cu(II) and Zn(II) complexes were also elucidated by X-ray crystallographic and PMR studies. The coordinated copper and zinc atoms adopt the teragonally distorted octahedral geometry (Fig. 1). The Mossbauer and ESR parameters for the mugineic acid-Fe(III) complex are typical of high-spin ferric type. The salient feature of the mugineic acid-Fe(III) complex is high reduction potential(E_<1/2>=-102mV vs. NHE), that is, this Fe(III)-complex is thermodinamically readily reducible by physiologically available reductants such as NAD(P)H. Thus, the present results indicate that the mechanism of iron-uptake and -transport in Graminaceous plant involves the excretion of mugineic acid(1) from the roots, which aids Fe(III)-solubilization and reduction of stable Fe(III)-complex (log K=18.1) to the weakly bound Fe(II)-complex (log K=8.1).
