Solid-state 17O NMR study of α-d-glucose: exploring new frontiers in isotopic labeling, sensitivity enhancement, and NMR crystallography.

Jiahui Shen,Andreas Brinkmann,Victor Terskikh,Jochem Struppe,Zhehong Gan,Alia Hassan,Ivan Hung,Martine Monette,Gang Wu

doi:10.1039/d1sc06060k

Abstract

We report synthesis and solid-state 17O NMR characterization of α-d-glucose for which all six oxygen atoms are site-specifically 17O-labeled. Solid-state 17O NMR spectra were recorded for α-d-glucose/NaCl/H2O (2/1/1) cocrystals under static and magic-angle-spinning (MAS) conditions at five moderate, high, and ultrahigh magnetic fields: 14.1, 16.4, 18.8, 21.1, and 35.2 T. Complete 17O chemical shift (CS) and quadrupolar coupling (QC) tensors were determined for each of the six oxygen-containing functional groups in α-d-glucose. Paramagnetic Cu(ii) doping was found to significantly shorten the spin–lattice relaxation times for both 1H and 17O nuclei in these compounds. A combination of the paramagnetic Cu(ii) doping, new CPMAS CryoProbe technology, and apodization weighted sampling led to a sensitivity boost for solid-state 17O NMR by a factor of 6–8, which made it possible to acquire high-quality 2D 17O multiple-quantum (MQ) MAS spectra for carbohydrate compounds. The unprecedented spectral resolution offered by 2D 17O MQMAS spectra permitted detection of a key structural difference for a single hydrogen bond between two types of crystallographically distinct α-d-glucose molecules. This work represents the first case where all oxygen-containing functional groups in a carbohydrate molecule are site-specifically 17O-labeled and fully characterized by solid-state 17O NMR. Gauge Including Projector Augmented Waves (GIPAW) DFT calculations were performed to aid 17O and 13C NMR signal assignments for a complex crystal structure where there are six crystallographically distinct α-d-glucose molecules in the asymmetric unit.

Highlights

The element of oxygen is a key constituent of organic and biological molecules.Oxygen-containing functional groups are often directly involved in chemical reactions including biological transformation such as enzyme catalysis
The 17O chemical shift (CS) and quadrupolar coupling (QC) tensors were determined for each of the six oxygen sites in α-D-glucose from an analysis of solid-state 17O NMR spectra obtained at multiple magnetic fields
By combining the paramagnetic doping effect with the new CPMAS CryoProbe technology and apodization weighted sampling at high magnetic fields, we have achieved a significant sensitivity boost that allowed us to obtain the first set of 17O 3QMAS spectra ever reported for carbohydrate compounds

Summary

Introduction

The element of oxygen is a key constituent of organic and biological molecules. Oxygen-containing functional groups are often directly involved in chemical reactions including biological transformation such as enzyme catalysis. We report synthesis of a total of six site 17O-labeled D-glucose compounds and their full solid-state 17O NMR characterization. For the latter part, because crystallization of D-glucose into a pure anomeric form (α or β) often encounters low yields, we decided to prepare all solid samples of D-glucose in the form of a D-glucose/NaCl/H2O (2/1/1) cocrystal. (Bottom) different D-glucose tautomers present in aqueous solution Another objective of the present work is to demonstrate utilization of the current state-of-the-art solid-state 17O NMR technologies achieving unprecedented sensitivity and spectral resolution for studying organic and biological molecules. Because these four methods produced essentially the same results, we will focus on the results obtained with the PBE method and report the complete results from all four methods in the ESI

Results and discussion

Conclusions

16.4 T exp exp sim sim