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Title: Kinetic and Equilibrium Constants of Phytic Acid, Ferric and Ferrous Phytate Derived From Nuclear Magnetic Resonance Spectroscopy

Author
item HEIGHTON, LYNNE - UNIV OF MD, COLLEGE PARK
item Schmidt, Walter
item SIEFERT, RONALD - US NAVAL ACADEMY

Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2008
Publication Date: 6/9/2008
Citation: Heighton, L., Schmidt, W.F., Siefert, R.L. 2008. Kinetic and Equilibrium Constants of Phytic Acid, Ferric and Ferrous Phytate Derived From Nuclear Magnetic Resonance Spectroscopy. Journal of Agriculture and Food Chemistry. 56:9543-9547.

Interpretive Summary: Strategies to mitigate the impact of industrialized agriculture on the environment rely on incomplete knowledge of the mechanisms describing the biogeochemical phosphorus cycle. The primary transport processes of the phosphorus cycle involve non-point source movement from soil systems to surface waters. Non-point source transport mechanisms are believed to be primarily overland flow of sediment attached phosphates. Although overland flow processes are the primary mode of phosphorus transport, subsurface leaching may also play an important role in many ecosystems. Factors controlling phosphorus movement into the dissolved phase include the oxidation reduction potential, the amount of iron oxides associated with the soil and the fraction of phosphorus present as an organic phosphorus species. The formation constants of ferric and ferrous complexes of phytic acid were measured by proton nuclear magnetic resonance. The experimental kinetic constants derived from H-NMR although of the same magnitude indicate that ferrous phytate forms faster and dissociates at a slower rate then ferric phytate. The H-NMR derived findings are supported by the overall enzymatic dephosphoralation recoveries of phosphate over a two day period. In anoxic ground water, ferrous phytate has the potential to resist microbial decomposition from the enzyme phytase. This ability to persist may enable ferrous phytate to be transported by subsurface leaching.

Technical Abstract: Inositol phosphates are metabolically derived organic phosphates that increasingly appear to be an important sink and source of phosphate in the environment. Inositol hexakis dihydrogen phosphate or phytic acid is the most common inositol phosphate in the environment. Iron is abundant in many terrestrial systems. Mobility of phytic acid iron complexes are potentially pH and redox responsive. Ferric and ferrous complexes of phytic acid were investigated by proton nuclear magnetic resonance spectroscopy and enzymatic dephosphoralation. Ferrous phytate was found to form quickly and persist for a longer period then ferric phytate. Dissociation constants were 1.113 and 1.186 and formation constants were 0.899 and 0.843 for ferric and ferrous phytate respectively. Enzymatic dephosphoralation recoveries supported the magnitude of the kinetic and equilibrium rate constants.