Comprehensive determination of (3)J(HNHα) for unfolded proteins using (13)C′-resolved spin-echo difference spectroscopy

An experiment is presented to determine (3)J(HNHα) coupling constants, with significant advantages for applications to unfolded proteins. The determination of coupling constants for the peptide chain using 1D (1)H, or 2D and 3D (1)H-(15)N correlation spectroscopy is often hampered by extensive resonance overlap when dealing with flexible, disordered proteins. In the experiment detailed here, the overlap problem is largely circumvented by recording (1)H-(13)C′ correlation spectra, which demonstrate superior resolution for unfolded proteins. J-coupling constants are extracted from the peak intensities in a pair of 2D spin-echo difference experiments, affording rapid acquisition of the coupling data. In an application to the cytoplasmic domain of human neuroligin-3 (hNlg3cyt) data were obtained for 78 residues, compared to 54 coupling constants obtained from a 3D HNHA experiment. The coupling constants suggest that hNlg3cyt is intrinsically disordered, with little propensity for structure.