Accurate measurement of (3)J(HNHα) couplings in small or disordered proteins from WATERGATE-optimized TROSY spectra.

Provided that care is taken in adjusting the WATERGATE element of a (1)H-(15)N TROSY-HSQC experiment, such that neither the water magnetization nor the (1)H(α) protons are inverted by its final 180° pulse, (3)JHNHα couplings can be measured directly from splittings in the (1)H dimension of the spectrum. With band-selective (1)H decoupling, very high (15)N resolution can be achieved. A complete set of (3)JHNHα values, ranging from 3.4 to 10.1 Hz was measured for the 56-residue third domain of IgG-binding protein G (GB3). Using the H-N-C(α)-H(α) dihedral angles extracted from a RDC-refined structure of GB3, (3)JHNHα values predicted by a previously parameterized Karplus equation agree to within a root-mean-square deviation (rmsd) of 0.37 Hz with the experimental data. Values measured for the Alzheimer's implicated Aβ(1-40) peptide fit to within an rmsd of 0.45 Hz to random coil (3)JHNHα values.