Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

All so far known crystalline forms of all positional isomers of methyl β-D-xylopyranosyl-β-D-xylopyranoside were prepared and their high resolution ¹³C NMR spectra measured by the magic angle rotation (MAR) method using cross-polarization (CP). According to these spectra only two modifications of methyl 4-O-(β-D-xylopyranosyl)-β-D-xylopyranoside (modifications IVβ and IVγ) contain more than one molecule in the crystal elementary cell; in all other studied disaccharide samples all molecules are equivalent. The majority of carbon-13 chemical shifts could be assigned, at least partially, by a comparison with solution spectra. Some of the glycosidic carbons in the units with methyl aglycone (ring A) exhibit surprisingly large paramagnetic shifts with respect to the solution chemical shifts. Apparently, molecular structures at these sites in solids differ most from the average structures the molecules assume in the liquid phase. Assuming that solid state chemical shifts are affected also by similar factors as the shifts in solution, the methyl aglycone carbon chemical shifts indicate that the frozen conformation of the ring A in the solid is closer either to ¹C₄ conformation (modifications IIIα and II) or to ⁴C₁ conformation (modification IVα) than are the respective equilibrium conformations in solution. Thermal analysis has proven, however, that the xylobioside forms with two nonequivalent molecules in the cell similarly as some other forms are indeed hydrates. Thus is shown that all the methyl xylobiosides are monomorphous, all the polymorphs are not true polymorphs but forms hydrated to a different extent.