Phosphorus-31 Nuclear Magnetic Resonance Spectral Assignments of Phosphorus Compounds in Soil NaOH–EDTA Extracts

Abstract

Soil P composition can be conveniently determined in alkaline extracts using solution 31P nuclear magnetic resonance (NMR) spectroscopy, but spectral assignments are based on fragmentary literature reports of model compounds in various extraction matrices. We report solution 31P NMR chemical shifts of model P compounds, including inorganic phosphates, orthophosphate monoesters and diesters, phosphonates, and organic polyphosphates, determined in a standardized soil P extractant (0.25 M NaOH and 0.05 M EDTA). Signals from nucleic acids (DNA −0.37 ppm, RNA 0.54 ppm) and phospholipids (phosphatidyl choline 0.78 ppm, phosphatidyl serine 1.57 ppm, phosphatidyl ethanolamine 1.75 ppm) could be differentiated in the orthophosphate diester region, and were identified in a sample of cultured soil bacteria. Inorganic and organic polyphosphates could be differentiated by the presence of a signal at −9 ppm from the α phosphate of organic polyphosphates. Some orthophosphate diesters, notably RNA and phosphatidyl choline, degraded rapidly to orthophosphate monoesters in NaOH–EDTA although DNA, other phospholipids, and orthophosphate monoesters were more stable. Changes in probe temperature had a marked influence on signal intensities and the relative magnitude of signals from orthophosphate monoesters and inorganic orthophosphate, and we suggest that solution 31P NMR spectroscopy of soil extracts be performed at 20°C.