The Karplus relations for vicinal J couplings, which relate the peptide backbone angles phi and psi to (3)J(H(a)lpha-N) and (3)J(H-N-C’), have been reparametrized on the basis of measurements made on uniformly C-13/N-15-enriched human ubiquitin and backbone angles derived from the 1.8 Angstrom X-ray structure of this protein (Vijay-Kumar, S.; Bugg, C. E.; Cook, W. J. J. Mol. Biol. 1987, 194, 531-544). For (3)J(H(a)lpha-N), values measured using an E.COSY-type HCACO experiment fall in the -1.9 to 0.1 Hz range and are described by the following : (3)J(H(a)lpha-N) = -0.88 cos(2)(psi + 60 degrees) - 0.61 cos(psi + 60 degrees) - 0.27. The root-mean-square difference (rmsd) between measured values and the Karplus relation is 0.16 Hz. Values measured for (3)J(H-N-C’) using an E.COSY-type HNCA experiment fall in the -0.4 to 3.6 Hz range, are described by 4.0 cos(2)(phi) + 1.1 cos(phi) + 0.1, and fit the observed J couplings with an rmsd of 0.45 Hz.