analysis of the -glycosidic linkages of oligosaccharides using nonconventional NMR -couplings: and MD models of .

analysis (Meredith , 2022, , 3135-3141) is a new NMR-based method to treat ensembles of redundant NMR spin-coupling constants (-couplings) to obtain experiment-based probability distributions of molecular torsion angles in solution. Work reported to date on modeling the conformations of -glycosidic linkages of oligosaccharides using three conventional -coupling constraints ( , , ) has shown that the method gives mean torsion angles and circular standard deviations (CSDs) for in very good agreement with those obtained by MD simulation. On the other hand, CSDs for determined by analysis have consistently been much larger than those determined by MD, calling into question either the reliability of analysis or MD to accurately predict this behavior. Prior work has shown that this discrepancy does not stem from the limitations of DFT-based -coupling equation parameterization where secondary conformational dependencies can introduce uncertainties. The present work re-visits this problem by incorporating a new nonconventional -coupling constraint into analyses of , namely, a geminal (two-bond) -value that exhibits a strong primary dependence on . The latter property pertains explicitly to linkages contributed by GlcNAc pyranosyl rings and pyranosyl rings devoid of substituents at C2 (, deoxy residues) where known secondary contributions to magnitude caused by C-O bond rotation involving the coupled carbon are negligible or absent. The results show that when values are added to the analysis, CSDs reduce considerably, bringing them into better alignment with those obtained by MD simulation. The cause of the discrepancy when only three conventional -couplings are used to treat appears to be associated with the two-bond , which has properties that make it less effective than the non-conventional as a discriminator of different conformational models of .