Density functional theory (DFT) has been used to investigate the structural dependencies of NMR spin-coupling constants (J-couplings) involving the exchangeable hydroxyl protons of saccharides. 3JHCOH, 3JCCOH, and 2JCOH values were calculated at different positions in model aldopyranosyl rings as a function of one or more torsion angles, and results support the use of a generalized Karplus equation to treat 3JHCOH involving the non-anomeric OH groups. The presence of O5 appended to the H1-C1-O1-H coupling pathway introduces asymmetry in 3JH1,O1H Karplus curves due to internal electronegative substituent effects on the gauche couplings, thus requiring separate equations to treat this coupling. 3JCCOH values depend not only on the C-C-O-H torsion angle but also on the orientation of terminal substituents on the coupled carbon, similar to 3JCOCC studied previously (Bose et al., J. Am. Chem. Soc. 1998, 120, 11158-11173). "In-plane" oxygen increased 3JCCOH by approximately 3-4 Hz, whereas "in-plane" carbon gave more modest enhancements ( approximately 1 Hz). Three Karplus equations were derived for non-anomeric 3JCCOH based on the nature and orientation of substituents on the coupled carbon. Like 3JH1,O1H, 3JC2,O1H is also subject to internal electronegative substituent effects on the gauche couplings, thus necessitating separate equations to treat this coupling. 2JCOH values were found not to be useful probes of C-O torsions as a result of their nonsystematic dependence on these torsions. Experimental measurements of 3JHCOH and 3JCCOH in doubly 13C-labeled methyl beta-lactoside 20 and its constituent 13C-labeled methyl aldopyranosides in H2O/acetone-d6 at -20 degrees C showed that some C-O torsion angles are influenced by molecular context and do not experience complete rotational averaging in solution. A strong bias in the H3-C3-O3-H torsion angle in the Glc residue of 20 favoring a gauche conformation suggests the presence of inter-residue H-bonding between O3HGlc and O5Gal. Quantitative analysis of 3JHCOH and 3JCCOH values in 20 indicates that approximately 85% of the forms in solution have geometries consistent with H-bonding. These results suggest that H-bonding between adjacent and/or remote residues may play a role in dictating preferred glycosidic bond conformation in simple and complex oligosaccharides in aqueous solution.

Density functional theory (DFT) is a powerful computational tool to enable structural interpretations of NMR spin-spin coupling constants ( J-couplings) in saccharides, including the abundant (1)H-(1)H ( JHH), (13)C-(1)H ( JCH), and (13)C-(13)C ( JCC) values that exist for coupling pathways comprised of 1-4 bonds. The multiple hydroxyl groups in saccharides, with their attendant lone-pair orbitals, exert significant effects on J-couplings that can be difficult to decipher and quantify without input from theory. Oxygen substituent effects are configurational and conformational in origin (e.g., axial/equatorial orientation of an OH group in an aldopyranosyl ring; C-O bond conformation involving an exocyclic OH group). DFT studies shed light on these effects, and if conducted properly, yield quantitative relationships between a specific J-coupling and one or more conformational elements in the target molecule. These relationships assist studies of saccharide structure and conformation in solution, which are often challenged by the presence of conformational averaging. Redundant J-couplings, defined as an ensemble of J-couplings sensitive to the same conformational element, are particularly helpful when the element is flexible in solution (i.e., samples multiple conformational states on the NMR time scale), provided that algorithms are available to convert redundant J-values into meaningful conformational models. If the latter conversion is achievable, the data can serve as a means of testing, validating, and refining theoretical methods like molecular dynamics (MD) simulations, which are currently relied upon heavily to assign conformational models of saccharides in solution despite a paucity of experimental data needed to independently validate the method.

Density functional theory (DFT) calculations show that internal electronegative substituents perturb 3JCOCC Karplus curves by phase shifting them with respect to analogous pathways devoid of this substitution. Thus, the J-coupling maximum, which normally is observed near 180 degrees , is shifted by approximately 15 degrees . These findings suggest that the two types of inter-residue C-O-C-C coupling pathways observed in many oligosaccharides cannot be treated using a generalized 3JCOCC Karplus equation. Quantitative interpretations of trans-glycoside J-couplings to evaluate linkage conformations will need to take this effect into account.

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