A Karplus equation for 3JHCCN in amino sugar derivatives

Abstract

The 1H– 15N coupling constants of a suite of organic-soluble amino sugar derivatives have been measured by one-dimensional and two-dimensional 1H/ 15N heteronuclear single quantum, multiple bond correlation (HSQMBC), and the values so obtained are compared with those measured by analysis of 1H spectra of 15N-labeled amino sugar derivatives. A number of bicyclic amino sugar models have been studied, including methyl 2- (and 3-)amino-4,6- O-benzylidene-2- (and 3-)deoxy-α- d-hexopyranosides in chair or skew conformations, and methyl 2,6-anhydro-3-deoxy-3-phthalimido-α- d-mannopyranoside in a locked, almost classical boat conformation. The magnitudes of the vicinal 1H– 15N coupling constants 3 J HCCN have been correlated with 1H/ 15N dihedral angles φ computed for the favored conformations by molecular dynamics with molecular mechanics energy minimization. Non-linear regression of the coupling constants on the dihedral angles has yielded a Karplus equation: 3 J HCCN = 3.1 cos 2 φ − 0.6 cos φ + 0.4. The coefficients of the terms in this equation have been compared with those reported for 15 other pairs of nuclei, and the coefficient of the important cos 2 φ term found to be numerically smallest for 3 J HCCN.