A Karplus equation for 3JHCCN in amino sugar derivatives

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 3JHCCN 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: 3JHCCN = 3.1 cos2 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 cos2 φ term found to be numerically smallest for 3JHCCN.

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