NMR spectroscopic study of organic phosphate esters coprecipitated with calcite

Abstract

Organic phosphorus incorporated in calcite during laboratory precipitation experiments and in natural cave deposits was investigated by solid-state NMR spectroscopy. For calcite precipitated in the presence of organic phosphoesters of varying size and functionality, solid-state 31 P{ 1 H} CP/MAS NMR shows that the phosphoesters were incorporated intact into the solid. Systematic changes in the 31 P NMR chemical shift of the phosphate group were observed between the solid phosphoester and that incorporated in the solid precipitate, yielding 31 P NMR chemical shifts of the coprecipitates in the range of +1.8 to -2.2 ppm.These chemical shifts are distinct from that of similarly prepared calcite coprecipitated with inorganic phosphate, 3.5 ppm. Only minor changes were noted in the phosphoester 31 P chemical shift anisotropy (CSA) which suggests no significant change in the local structure of the phosphate group, which is dominated by C-O-P bonding. Close spatial proximity of the organic phosphate group to calcite structural components was revealed by 31 P/ 13 C rotational echo double resonance (REDOR) experiments for coprecipitates prepared with 13 C-labeled carbonate. All coprecipitates showed significant 31 P dephasing effects upon 13 C-irradiation, signaling atomic-scale proximity to carbonate carbon. The dephasing rate for smaller organophosphate molecules is similar to that observed for inorganic phosphate, whereas much slower dephasing was observed for larger molecules having long and/or bulky side-chains. This result suggests that small organic molecules can be tightly enclosed within the calcite structure, whereas significant structural disruption required to accommodate the larger organic molecules leads to longer phosphate-carbonate distances. Comparison of 31 P NMR spectroscopic data from the synthetic coprecipitates with those from calcite moonmilk speleothems indicates that phosphorus occurs mainly as inorganic orthophosphate in the natural deposits, although small signals occur with characteristics consistent with phosphate monoesters. The results of this study indicate that trace-to minor concentrations of dissolved organic molecules can be effectively taken up during calcite precipitation and incorporated in the structure, leaving a resilient record of materials present during crystallization.

Biomimetic Coating of Hydroxyapatite on Glycerol Phosphate-Conjugated Polyurethane via Mineralization

“…Through the competitive reaction between two hydroxyl groups, PU-gp was successfully fabricated and characterized using 31 P NMR (Figure b). The peak was shifted from the zero point of standard phosphoric acid to 6.68 ppm, indicating the presence of the phosphate monoester (R-O-PO­(OH) 2 ) of glycerol phosphates, whereas no peaks were detected in pure PU, as this compound has no P source or linkage …”

“…The peak was shifted from the zero point of standard phosphoric acid to 6.68 ppm, indicating the presence of the phosphate monoester (R-O-PO(OH) 2 ) of glycerol phosphates, whereas no peaks were detected in pure PU, as this compound has no P source or linkage. 22 …”

“…The peak was shifted from the zero point of standard phosphoric acid to 6.68 ppm, indicating the presence of the phosphate monoester (R-O-PO(OH) 2 ) of glycerol phosphates, whereas no peaks were detected in pure PU, as this compound has no P source or linkage. 22 Second, PU-gp-HA was fabricated through treatment in concentrated simulated body fluid (SBF) solutions for 4 and 12 h. During precipitation, amorphous CaP nanoparticles reversibly formed in the solution. The phosphate groups in PU-gp provided nucleation sites for reversibly existing CaP nanoparticles and calcium and phosphorus ions.…”

Formation of calcite in the presence of dissolved organic matter: Partitioning, fabrics and fluorescence

“…Carboxyl groups (COOH -) are common in natural DOM, and can readily be adsorbed onto calcite, which has a net positive charge (via Ca 2+ ). By contrast, an NMR (Nuclear Magnetic Resonance) spectroscopy study revealed that small organic (P containing) molecules can be incorporated in the calcite structure, thus supporting the notion that speleothems may provide a resilient record of organic matter present during crystallisation (Phillips et al, 2016). These results implied that calcite precipitated under natural conditions can accommodate organic molecules as structural defects.…”

Characterization and Geological Implications of Precambrian Calcite‐Hosted Phosphate

“…The presented 1 H/ 31 P REDOR results are indicative of hydrogen phosphate groups (12–15 ppm Figure 1d) and water molecules (peak at 7 ppm). This broad 31 P peak exhibits a strong 13 C/ 31 P REDOR dephasing effect, indicating atomically close proximity to multiple carbonate groups (Phillips et al., 2016). The NMR spectroscopic results from the synthetic calcite samples indicate that calcite‐hosted phosphate accounts for the majority form of phosphate in these samples, thus, the features observed in the XANES spectroscopy of these synthetic calcite samples can be used to fingerprint calcite‐hosted phosphate in geological samples.…”