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A Systematic NMR Determination of α-D-Glucooligosaccharides, Effect of Linkage Type, Anomeric Configuration and Combination of Different Linkages Type on13C Chemical Shifts for the Determination of Unknown Isomaltooligosaccharides

“…13 C complete assignments referenced to acetone (acetone δC 31.45 ppm and δH 2.225 ppm) for the eight standard glucooligosaccharides are shown in Table 1. 1 H assignments are in accordance with previous studies [7][8][9][10][11][12][13][14][15]18,[23][24][25][26][27][28][29][30] including the most recent one. 14 Two approaches are available for determination of anomeric configuration.…”

“…In our study, as a preliminary work, their 1 H assignments are confirmed at a slightly different temperature and by a wider range of techniques making them unambiguous. These assignments can then be used in 2D heteronuclear spectra to confirmed 13 C assignments but moreover as a support for 13 C chemical shift deviation interpretations as proton configuration and interactions highly influences these deviations. 13 C chemical shifts can then be used as IMO structural determination tools, as they are more spread out, therefore less overlapped, and chemical shifts changes due to anomeric configuration and glycosylation are more marked than in the case of 1 H. These last considerations are of a great importance taking into account the wide variety of linkage type combinations found in IMO preparations.…”

“…IMOs are enzymatically produced glucooligosaccharides linked by at least one α-(1→6) linkage, or in a lower proportion α-(1→3) (nigerooligosaccharides) or α-(1→2) (kojioligosaccharides) glucosidic linkages depending on the substrate and the origin of the transglucosidase which specifies its tendency to form specific linkages. [7][8][9][10][11][12][13] The transglucosidase removes a Glcp residue linked through a α- (1)(2)(3)(4) linkage from the non-reducing end of a glucooligosaccharide (preferably maltose) and transfers it to the non-reducing end of any glucooligosaccharide present in the medium This flexibility of the transglucosylation mechanism gives rise to a large variety of structurally close molecules in IMO preparations characterized at the same time by their DP value (from 2 to ~10), linkage types (α-1→2, 3, 4 or 6) and the proportion and position of each type of linkage.…”

“…In the case of homooligosaccharides, such as IMOs and MOs, with different types of linkages, the NMR structural analysis is more complicated than for heterooligosaccharides with a repeating unit which generally have well-separated anomeric 1 H and 13 C signals for each residue of the repeating unit. 14 The full 1 H and 13 C assignments must thus be obtained by employing a variety of 1D and 2D NMR techniques Moreover, taking into account that oligosaccharide chemical shifts are more influenced by slight structural changes than polysaccharides, the structural origin of chemical shifts differences and the study of substitution patterns, effect of anomeric configuration and diastereotopic effect can give us potential tools for unknown IMO structural determination. The anomeric effect is the tendency of an electronegative substituent at the anomeric carbon to assume the axial rather than the equatorial conformation, 16,17 thus, in our case, α-form rather than β-form for reducing end residue.…”

“…Inversion of an equatorial O to axial O is associated with increased shielding of the 13 C nucleus to which it is bonded, of adjacent 13 C nuclei, and of those in the next position. The anomeric effect does not arise through instability of the equatorial anomeric C-O bond since the destabilizing interaction appears to cause polarization of most C-H bonds in the molecules, 13 C becoming more shielded, and 1 H less shielded, suggesting a concerted means for delocalizing the instability. Moreover, free-energy difference between the α and β anomers is related to the degree of axial asymmetry of the chemical shift tensor in the plane containing the O-5-C-1-O-1 bonds, showing that the anomeric effect has an electronic origin.…”