Submitted to: Journal of Food Agriculture and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2007
Publication Date: 1/12/2008
Citation: Heighton, L., Schmidt, W.F., Rice, C., Siefert, R.L. 2008. Electrospray Ionization Mass Spectroscopy Shows the Speciation of Phytate is pH Dependent. Journal of Food Agriculture and Environment. 6:402-407. Interpretive Summary: Phytic acid is a rich source of organic phosphate ubiquitous in agricultural plant production and consequently in livestock manure. It is the prevalent form of organic phosphate in many soil systems but its fate and transport is not completely understood. The charge based speciation of phytic acid as a function of pH was calculated using acid dissociation constants over the aqueous pH scale. A model of the fractional contribution of each charged species at environmentally relevant pH may contribute to the understanding of the enzymatic, redox and chemical reaction potential of the molecule and aid in predicting the environmental fate and transport of phosphate derived from the organic phosphate phytic acid. Electrospray ionization mass spectroscopy (ESI-MS) was used to positively compare the theoretical charge based distribution of peaks with spectral peaks generated from solutions of phytic acid and cupric and ferric chlorides at pH 2.8, 6 and 13.
Technical Abstract: Phosphorus (P) fate and transport is an emergent problem impacting environmental resources. Myo-inositol hexkis phosphate or more commonly, phytic acid, is an organic phosphate molecule with twelve acidic protons. The acid dissociation constants (pKa) are 1.9(3), 2.4(2), 3.2(1), 5.2(1), 6.3(1), 8.0(1), 9.2(1), and 9.5(2). The charged species fractions were calculated as a function of pH using the acid dissociation constants in order to model the potential enzymatic, redox and chemical reactions of phytic acid over the aqueous pH scale. As many as three different charged species of phytic acid can be present at environmentally relevant pH. The theoretical distribution of charged species was positively correlated to ionization-mass spectroscopy of solutions of a dodeca-sodium salt of phytic acid in the presence of iron and copper chlorides at pH 2.8, 6 and 13.