A Systematic NMR Determination of α-D-Glucooligosaccharides, Effect of Linkage Type, Anomeric Configuration and Combination of Different Linkages Type on 13C Chemical Shifts for the Determination of Unknown Isomaltooligosaccharides

Prebiotic isomaltooligosaccharide preparations contain α-D-glucooligosaccharides comprising isomaltooligosaccharides (IMOs) and non-prebiotic maltooligosaccharides (MOs). They are both glucose oligosaccharides characterized by their degree of polymerization (DP) value (from 2 to ~10), linkage types and positions (IMOs: α-(12, 3, 6 and in a lower proportion internal 14) linkages, Mos: -(14) linkages). Their structure is the key factor for their prebiotic potential. In order to determine and elucidate the exact structure of unknown IMOs and Mos, unambiguous assignments of 13C and 1H chemical shifts of commercial standards, representative of IMOs and Mos diversity, have been determined using optimized standard one and two-dimensional experiments such as 1H NMR, 13C NMR, APT and 1H-1H COSY, TOCSY, NOESY and 1H-13C heteronuclear HSQC, HSQC-TOCSY, and HMBC.
Here we point out the differential effect of substitution by a glucose residue at different positions on chemical shifts of anomeric as well as ring carbons together with the effect of the reducing end configuration for low DP oligosaccharides and diasteroisotopic effect for H-6 protons. From this study, structural 13C specific spectral features can be identified as tools for structural analysis of isomaltooligosaccharides.