Assignment strategies in homonuclear three-dimensional 1H NMR spectra of proteins

The increase in dimensionality of three-dimensional (3D) NMR greatly enhances the spectral resolution in comparison to 2D NMR. It alleviates the problem of resonance overlap and may extend the range of molecules amenable to structure determination by high-resolution NRM spectroscopy. Here, we present strategies for the assignment of protein resonances from homonuclear nonselective 3D NOE-HOHAHA spectra. A notation for connectivities between protons, corresponding to cross peaks in 3D spectra, is introduced. We show how spin systems can be identified by tracing cross-peak patterns in cross sections perpendicular to the three frequency axes. The observable 3D sequential connectivities in proteins are tabulated, and estimates for the relative intensities of the corresponding cross peaks are given for alpha-helical and beta-sheet conformations. Intensities of the cross peaks in the 3D spectrum of pike III parvalbumin follow the predictions. The sequential-assignment procedure is illustrated for loop regions, extended and alpha-helical conformations for the residues Ala 54-Leu 63 of parvalbumin. NOEs that were not previously identified in 2D spectra of parvalbumin due to overlap are found.