The use of MM/QM calculations of 13C chemical shifts in the conformational analysis of some monosaccharides and sucrose

Abstract

The ¹³C NMR chemical shifts of some common pyranose monosaccharides in D₂O solution were predicted using a combined molecular mechanics (Pcmod 9.1/MMFF94) and ab initio (GIAO (B3LYP/DFT, 6-31G(d))) model. This method has been successfully used for a range of organic molecules in CDCl₃ solution but has not previously been used for aqueous solutions. The method gave such good agreement with experiment that the populations of the conformers at C₆ of the monosaccharides could be obtained. This has been attempted previously by various methods with diverse results. In all the compounds studied the α and β anomers have similar populations. In glucose the populations of the GG and GT conformers are ca equal with TG a minor component, in galactose GT has the largest population and GG the smallest, in mannose GG is preferred with TG a minor component and in talose GT and TG are favoured with GG a minor constituent. The results are in general agreement with previous work. To our knowledge this is the first time that ¹³C shifts have been used to determine sugar conformations in solution. The conformation of sucrose in D₂O solution was examined by both ¹H and ¹³C NMR. The complete analysis of the ¹H NMR spectrum has been achieved and provides information on the preferred conformations of the CH₂OH fragments at G6 (GG) and F6 (GG, GT). Calculations of the ¹³C shifts confirm these results and show also that the conformation at F1 is mainly GG plus TG. The sucrose conformational profile in solution includes the crystal structure together with the above conformations at F1 and F6.