Three-Bond C−O−C−C Spin-Coupling Constants in Carbohydrates: Development of a Karplus Relationship

Abstract

A range of 13C-labeled carbohydrates containing C−O−C−C coupling pathways having different structures and dihedral angles has been prepared and used to identify structural factors affecting 3JCOCC, especially those across the O-glycosidic linkages of oligosaccharides. Model mono- and disaccharides were geometrically optimized using density functional methods, and scalar couplings involving carbon were calculated using a similar approach coupled with finite-field perturbation theory. Experimental and calculated 3JCOCC values were in close agreement, thus allowing use of the latter to better define the effect of carbohydrate structure on 3JCOCC magnitude. In addition to dihedral angle, the disposition of terminal electronegative substituents along the C−O−C−C coupling pathway significantly affects 3JCOCC values, and structural motifs have been identified where these effects may be encountered in oligosaccharides. A simple Karplus equation for trans-O-glycoside 3JCOCC values is proposed and has been applied in the reanalysis of trans-O-glycosidic couplings in 13C-labeled methyl β-lactoside and sucrose. The behavior of trans-O-glycosidic 2JCOC and 3JCOCH values, which provide structural information complementary to that derived from 3JCOCC values, is also discussed.