Three-bond C-O-C-C spin-coupling constants in carbohydrates: Development of a Karplus relationship

A range of C-13-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 (3)J(COCC), 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 (3)J(COCC) values were in close agreement, thus allowing use of the latter to better define the effect of carbohydrate structure on (3)J(COCC) magnitude. In addition to dihedral angle, the disposition of terminal electronegative substituents along the C-O-C-C coupling pathway significantly affects (3)J(COCC) values, and structural motifs have been identified where these effects may be encountered in oligosaccharides. A simple Karplus equation for trans-O-glycoside (3)J(COCC) values is proposed and has been applied in the reanalysis of trans-O-glycosidic couplings in C-13-labeled methyl beta-lactoside and sucrose. The behavior of trans-O-glycosidic (2)J(COC) and (3)J(COCH) values, which provide structural information complementary to that derived from (3)J(COCC) values, is also discussed.