Oligosaccharide trans-glycoside 3JCOCC Karplus curves are not equivalent: Effects of internal electronegative substituents

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°, is shifted by ∼15°. 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.