On the utility of 13C-n.m.r. spectroscopy in the identification of the primary structures of manno-oligosaccharides and glycopeptides

NMR chemical shift prediction of glycans: application of the computer program CASPER in structural analysis

Carbohydrate molecules have highly complex structures and the constituent monosaccharides and substituents are linked to each other in a large number of ways. NMR spectroscopy can be used to unravel these structures, but the process may be tedious and time-consuming. The computerized approach based on the CASPER program can facilitate rapid structural determination of glycans with little user intervention, which results in the most probable primary structure of the investigated carbohydrate material. Additionally, (1)H and (13)C NMR chemical shifts of a user-defined structure can be predicted, and this tool may thus be employed in many aspects where NMR spectroscopy plays an important part of a study.

Computer-assisted structural analysis of regular glycopolymers on the basis of 13C NMR data

A computer-assisted approach to the prediction of the primary structures of regular glycopolymers is described. The analysis is based on comparing the calculated 13C NMR spectra of all the possible structures of the repeating unit (for the given monomeric composition) to an experimental 13C NMR spectrum. The spectra generation is based on the spectral database containing information on the 13C chemical shifts of monomers, di- and trimeric fragments. If the required data are missing from this database, the special database for average glycosylation effects is used. The analysis reveals those structures with the calculated 13C NMR spectrum most close to observed. The structures of repeating units of any topology containing up to six residues linked by glycosidic, amidic or phospho-diester bridges can be predicted. Unambiguous selection of the proper structure from the output list of possible structures may require additional experimental data. Testing the created program and databases on bacterial polysaccharides and their derivatives containing up to three non-sugar residues (alditols, amino acids, phosphate groups etc.) per repeating unit revealed the good convergence of prediction with independently obtained structural data.

Computer-assisted structural analysis of polysaccharides with an extended version of CASPER using 1H- and 13C-n.m.r. data

The computer program CASPER, used in the structural analysis of polysaccharides composed of repeating units, has been extended. The extended version uses either unassigned 1H- or 13C-n.m.r. chemical shifts or the complete unassigned C,H-correlation spectrum, and can predict the structure of linear and branched oligo- and poly-saccharides. The number of possible structures, consistent with sugar and methylation analysis, can be decreased by the use of 1JC,H and 3JH,H values. The database, which contains 1H- or 13C-n.m.r. chemical shift data for monosaccharides and 1H- or 13C-glycosylation shifts for all types of glycosidic linkages obtained by combination of the monosaccharides, has been increased and now also contains correction values for sugar residues present in branch-point regions. The program has been tested on four polysaccharides of known structure but with different degrees of complexity. For three polysaccharides, the correct structure was suggested; for the fourth, two structures were consistent with the n.m.r. data, one of them being correct.