Structural Diversity of Metal Phytate Compounds: a Solid State C-13 NMR and FT-IR Spectroscopic Investigation.

Phytate (myo-inositol-1,2,3,4,5,6-hexakisphosphate) is the major organic phosphorus component in the environment. Phytate contains a 6-carbon ring with 1 hydrogen and 1 phosphate attached to each carbon. Each of the 6 phosphate groups is attached in an ester linkage. Phytate exists in aqueous solution in either of two conformations: the low-pH form possesses one axial and five equatorial phosphates (1a/5e), whereas the high-pH form has an inverted 5a/1e structure.  However, no information is available on the conformational forms of solid metal phytate compounds. Consequently, we investigated the spectral features of nine representative metal phytate compounds by solid state C-13 NMR and FT-IR spectroscopic analysis. Five distinct types of FT-IR spectral features from 900 to 1200 cm^-1 were observed for monovalent (Na), hydrogen monovalent (H and K), light divalent (Ca and Mg), heavy divalent (Cd and Zn), and trivalent (Al) metal phytate compounds. A broad resonance peak appeared in all solid state C-13 NMR spectra of hydrogen monovalent, divalent, and trivalent metal phytate compounds. However, the spectra of hydrogen monovalent and divalent compounds could be deconvoluted to two separate resonance peaks. The relative intensity and chemical shift of the two deconvoluted peaks varied among these compounds. Based on the known structures of myo-inositol and Na phytate, we propose the presence of various deformed 1a/5e and 5e/1a conformations in these solid metal phytate compounds. Structural information derived from this research may improve our understanding of phytate compound stability and solubility in the environment.