3-BOND C-O-C-C SPIN-COUPLING CONSTANTS IN CARBOHYDRATES - DEVELOPMENTOF A KARPLUS RELATIONSHIP

A range of C-13-labeled carbohydrates containing C-O-C-C coupling path ways having different structures and dihedral angles has been prepared and used to identify structural factors affecting (3)J(COCC), especia lly those across the O-glycosidic Linkages of oligosaccharides. Model mono- and disaccharides were geometrically optimized using density fun ctional methods, and scalar couplings involving carbon were calculated using a similar approach coupled with finite-field perturbation theor y. Experimental and calculated (3)J(COCC) values were in close agreeme nt, thus allowing use of the latter to better define the effect of car bohydrate structure on (3)J(COCC) magnitude. In addition to dihedral a ngle, the disposition of terminal electronegative substituents along t he C-O-C-C coupling pathway significantly affects (3)J(COCC) values, a nd structural motifs have been identified where these effects may be e ncountered in oligosaccharides. A simple Karplus equation for trans-O- glycoside (3)J(COCC) values is proposed and has been applied in the re analysis of trans-O-glycosidic couplings in C-13-labeled methyl beta-l actoside 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.