Solid-state structures of keto-disaccharides as probed by 13C cross-polarization, “magic-angle” spinning n.m.r. spectroscopy

Abstract

The 13C cross-polarization, magic-angle spinning (CP-MAS) spectra of maltulose · H 2O and anhydrous lactulose were examined at different applied magnetic and proton-decoupling fields B 0 and B 1, respectively. Whereas the spectrum of lactulose was insensitive to these changes, that of maltulose showed significant responses. From selective-relaxation experiments, a mixture of three “forms” (based on the ratio of the C-2 resonances) of both lactulose and maltulose were shown to exist in the solid state. The 360-MHz 1H spectra of the unmutarotated disaccharides in dimethyl sulfoxide solution were used to establish the tautomeric composition of these crystalline solids. The tautomeric composition of lactulose, as determined by the 1H spectrum in dimethyl sulfoxide, correlated well with the CP-MAS data, but the spectrum of unmutarotated maltulose showed only a single, β-pyranose tautomer to be present. Based on the ratio of the lactulose tautomers determined from the 1H spectrum in dimethyl sulfoxide (referenced to fructose), the C-2 resonances corresponding to each anomeric form of lactulose were assigned in the CP-MAS spectrum. A “crossover” in chemical-shift positions of the anomeric resonances was observed in going from solution to the solid state. Furthermore, a pronounced increase in the proportion of the furanoid anomers was noted for crystalline lactulose relative to its solution.