Heterometallic manganese/zinc-phytate complex as a model compound for metal storage in wheat grains

Abstract

Myo-inositol-1,2,3,4,5,6-hexakisphosphate, also known as phytate, is a natural metal chelate present in cereals, an important feedstock worldwide. This article reports the characterization of three metal storage model complexes: the homometallic Mn(II) myo-inositol-1,2,3,4,5,6-hexakisphosphate (IP6), the heterometallic Zn(II), Mn(II) analogue Na 4MnZn 4(C 6H 6O 24P 6) · (NO 3) 2 · 8H 2O (MnZn 4IP6) and the homometallic Zn(II) metal complex Na 3Zn 5(C 6H 6O 24P 6)OH · 9H 2O (Zn 5IP6). The techniques of high-resolution 23Na, 13C and 31P NMR, electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) were applied in this study. The complexation of Zn(II) and Mn(II) by phosphate groups of IP6 is demonstrated by NMR and XPS results. 13C NMR results show a conformation for IP6 consisting of five equatorial phosphate groups to one axial group showing only one chemical environment for Zn and two for Mn, when characterized by XPS and EPR, in both Mn complexes. These results support, for the first time, a probable supramacromolecular structure for phytate complexes of transition metals. Based on the similarity between the EPR spectra of wheat seeds and that of the MnZn 4IP6 compound, the manganese storage centers in wheat grains can be assigned to similar heterometallic phytate complexes.