3] 13C NMR Analysis of Complex Carbohydrates

Abstract

This chapter describes the applications of 13C nuclear magnetic resonance (NMR) using higher field NMR spectrometers to the study of oligosaccharides containing a number of different monosaccharide units with special attention given to the examination of solution conformations. Typically, 13C NMR spectra are obtained using broadband decoupling of protons. Spectra are relatively simple, with a single sharp resonance for each carbon in the compound. At natural abundance levels, 13C–13C coupling is not observable. Proton-coupled 13C spectra are much more complex and show the number of hydrogens covalently bonded to each carbon because each 13C resonance is split by covalently bonded hydrogens into n + 1 lines. Additional splittings or line-broadening because of 2- and 3-bond and long-range coupling also add to the complexity of 1H-coupled 13C spectra. Several experimental options are available on most high-field FT spectrometers that facilitate assignment of 13C resonances and of 13C–1H coupling constants. These include: gated 1H decoupling, off-resonance 1H decoupling, selective 13C saturation combined with gated 1H decoupling and FT difference spectroscopy, and two-dimensional J spectroscopy. The latter may be particularly valuable for complex molecules because it permits 13C chemical shifts and 13C–1H coupling of each 13C nucleus to be displayed separately.