Conformational change of metal phytates: Solid state 1D 13C and 2D 1H-13C NMR spectroscopic investigations

Phytate is an important phosphorus and mineral storage compound in plant seeds. Both the benign and adverse effects of phytate in nutrition and environment are mainly due to its unique conformational structures, where a strong chelating ability makes phytate interact with many cations (such as Zn2+, Ca2+, Mg2+, and Al3+). However, information is scant on the conformational forms of different solid metal phytate compounds although phytate in solution exists in two conformations: one axial and five equatorial phosphates (1a/5e) structure and an inverted 5a/1e structure. Consequently, we investigated the spectral features of nine representative metal phytate compounds by solid state 1D C-13 cross polarization magic angle spinning (CPMAS) and 2D C-13-H-1 heteronuclear correlation (HETCOR) NMR. A broad peak appeared in all solid 1D NMR spectra of hydrogen monovalent, divalent, and trivalent metal phytate compounds. The spectra of hydrogen monovalent and divalent compounds could be deconvoluted to two separate resonance peaks. 2D HETCOR clearly showed distinct C-13-H-1 correlations for inositol C-H moieties in hydrogen metal phytates from non-hydrogen metal phytates. Through spectral comparison, this work demonstrated that different valent cations in solid phytate compounds could induce conformational changes of the inositol ring as pH does on the phytate in solution. Therefore, the current knowledge on the effect of pH on phytate conformation can be helpful in understanding the conformational changes of phytate when it interacts with different valent metals to form less soluble even insoluble compounds in the environment.