Diffusion-Ordered Nuclear Magnetic Resonance Spectroscopy (DOSY-NMR): A Novel Tool for Identification of Phosphorus Compounds in Soil Extracts

Abstract

Liquid-state, one-dimension P nuclear magnetic resonance spectroscopy (NMR) has greatly advanced our understanding of the composition of organic phosphorus in the environment. However, the correct assignment of signals is complicated by overlapping and shifting signals in different types of soils. We applied therefore for the first time diffusion-ordered spectroscopy (DOSY) to soil extracts, allowing us to separate phosphorus components in the second domain based on their translational diffusion coefficients. After successful application to a mixture of 14 model compounds, diffusion rates correlated closely with the molecular weight of the individual compound in aqueous solution (R = 0.97). The method was then applied to NaOH/EDTA extracts of a grassland soil, of which paramagnetic contaminations were removed with sodium sulfide following high-velocity centrifugation (21 500g, 45 min) at 4 °C. Diffusion rates in soil extracts were again closely related to molecular weight (R = 0.98), varying from 163.9 to 923.8 Da. However, our DOSY application failed for a forest soil with low organic phosphorus content. Overall, DOSY did help to clearly identify specific NMR signals like myo- and scyllo-inositol hexakisphosphate. It thus provides a more confident signal assignment than 1D P NMR, although currently the ubiquitous use of this novel methodology is still limited to soils with high organic phosphorus content.

The structural composition of soil phosphomonoesters as determined by solution 31P NMR spectroscopy and transverse relaxation (T2) experiments

“…The longest T2 time of all organic P species was usually that of scyllo-IHP. Results for myo-IHP and scyllo-IHP are consistent with visual observations (Figure 1) and expected differences in T2 values according to molecular motion/weight (McLaren et al, 2015a;Wang et al, 2017).…”

“…DNA) rapidly decayed and had short T2 times (Table 3). This is consistent with the NMR signal of DNA being broad and contained in large molecular weight fractions (> 10 kDa) (McLaren et al, 2015a;Wang et al, 2017). The T2 time of 2aminoethylphosphonate, and an unknown phosphonate, was long in the VES soil, which is expected given its small molecular weight (< 1 kDa).…”

Determination of individual chain length and chain-length distribution of polyphosphates in microalgae by 31P-DOSY-NMR

“…DOSY is gaining popularity for P studies, for instance in soil samples [25]. In contrast with soils typically containing 0.1 -3 mg .…”

The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

“…Multidimensional NMR approaches, such as 31 P-1 H correlation spectroscopy, can offer a route to firm identification of species. Diffusion NMR has recently been shown to be an effective additional spectroscopic tool for such analysis [223], using a mixture of 14 different model phosphorus compounds, including phosphonic acids, phosphate and pyrophosphate salts. Three "unknown" compounds were also included in the sample: lipoteichoic acid (LTP) from Staphylococcus aureus, RNA, from torula yeast Type VI, and DNA, from salmon testes.…”