NMR Spin-Couplings in Saccharides: Relationships Between Structure, Conformation and the Magnitudes ofJHH,JCH andJCC Values

Abstract

NMR spin–spin coupling constants (scalar couplings, J-couplings) are highly abundant in saccharides, with multiple values often reporting on the same structural domain (redundancy). While conventional analyses of J-couplings typically involve studies of these parameters in relation to individual structural elements, future treatments are more likely to involve simultaneous analyses of large numbers of redundant (and in some cases correlated) J-couplings sensitive to multiple structural domains, providing a more complete appraisal of local molecular structure in solution. These analyses require quantitative relationships between J-couplings and saccharide structure. This chapter provides a detailed look at structural information encoded in one-bond (1J), two-bond (2J), three-bond (3J), four-bond (4J), and dual pathway (2+3J, 3+3J) scalar couplings involving hydrogen and carbon as coupled nuclei in saccharides. Experimental and computational data are integrated to illustrate correlations between saccharide structure, J-coupling magnitude and J-coupling sign. Topics ranging from selective isotopic labeling to enable J-coupling measurements, experimental methods to determine J-coupling magnitudes and signs, and the effects of specific types of molecular motions on the behaviors of saccharide J-couplings are treated. While work published over the past fifteen years comprise the main discussion, new data are included to augment or refine prior findings, notably on long-range 4JHH values in aldopyranosyl rings and across O-glycosidic linkages, and J-couplings pertinent to conformational analyses of glycosidic linkages.

Conformational Populations of β-(1→4) O-Glycosidic Linkages Using Redundant NMR J-Couplings and Circular Statistics

“…The above findings confirm those of earlier studies 30-32,34,38,39,107 that different orientations ( i.e. , axial vs equatorial) of electronegative (hydroxyl) groups affect the parametrization of J -coupling equations when these groups are attached to internal and terminal carbons in the coupling pathway (Figures 3 and 5).…”

“…26-34 In this study, systematic effects caused by the second torsion angle were observed for some 3 J COCC values. Modeling these dual dependencies requires equations with at least one two-dimensional component to describe the significant features of the data adequately.…”

“…30,34,105 The C1′–O1′–C4–C5 torsion angle is ~180° at ψ = ~300°, and the associated calculated 3 J C1′,C5 values lie between 5.5 and 6.0 Hz (Figure 4A). In the latter geometry, the in-plane O5 is expected enhance 3 J C1′,C5 values relative to the analogous 3 J C1′,C3 values for C1′–O1′–C4–C3 coupling pathways that lack this feature (see below).…”

“…30-34,39,104 These differences affect curve amplitudes and/or the locations of curve maxima and minima.…”

“…J -couplings may also be affected by the conformations of C–O bonds, and possibly of C–C bonds, especially when these bonds involve coupling pathway carbons. 32,34 DFT calculations were conducted on disaccharides 2 and 4–14 in which all hydroxyl group conformations ( i.e. , exocyclic C–O bond torsions) were fixed at discrete values (Scheme S2, Supporting Information).…”