In studies pertaining to the OH groups of carbohydrates, an NMR spectroscopist faces the problem of rapid chemical exchange of these protons with solvent water, reflected in the broadening of OH signals in 1H-NMR spectra to such an extent that they cannot be observed at ambient temperatures. Yet these OH resonances are potentially the most valuable probes for characterization of intramolecular hydrogen bonding. Researchers have applied mixed solvents (water/acetone or water/methanol) to cool the oligosaccharide sample below 00C, which slows the exchange down to a few times per second, so that OH signals become observable. The presence of organic solvents, however, may affect the formation of hydrogen bonds and, indeed, the oligosaccharide's conformation. Alternatively, the carbohydrate can be dissolved in ~100 ml of pure water (H2O, i.e., not in mixed solvents) in a capillary tube (1.5 mm I.D.); under those conditions the sample can be supercooled to -15 to -200C in the NMR probe while it remains in the liquid state. The OH signals of the oligosaccharide become rather sharp (their scalar couplings become larger than the line widths), facilitating their assignment by COSY and TOCSY (rather than NOESY) techniques. The chemical shifts, line widths, scalar couplings, etc., of OH signals involved in intramolecular hydrogen bonds are significantly different from those simply """"""""dangling"""""""" around the HC-OH bond. This technique is being applied to the study of sucralose, a sucrose analog important to the sweetener industry.
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