Solid state NMR is being developed as an additional method for determining structures of membrane protein complexes. The implementation of the solid state NMR experiments places several requirements on the system. The protein must be uniformly 15N/2H/13C labeled in bacterial expression systems. The protein must cooperate to provide a uniaxis of orientation. This leaves the burden of resolution and assignment to the spectroscopy. A systematic method for making sequential resonance assignments of the uniformly labeled proteins must be developed that is robust and independent of secondary structure elements. Structure calculations can follow once the complete set of resonance frequencies can be measured and assigned. Both the strong and weak homonuclear dipolar couplings provide a mechanism for identifying pairs of nuclei in close spatial proximity, which provides a method for making sequential resonance assignments especially in regions of regular secondary structure like the alpha helices in membrane proteins. The resonances are now fully resolved in multidimensional solid-state NMR spectra of uniformly 15N labeled proteins. Abundant spin-exchange among nearby 1H nuclei in model peptides and dilute spin-exchange among 15N sites in both model systems has been achieved. Recently, three-dimensional dilute spin exchange experiments have been used to assign a significant number of resonances in a uniformly 15N labeled M2 channel peptide in oriented bilayers. The preparation of uniformly 13C/15N labeled samples enables us to develop triple-resonance assignment strategies for these proteins, as well as giving full access to all of the backbone and sidechain sites. A triple resonance pulse sequence has been developed that effects the transfer of magnetization along the peptide backbone from 15N to 13C. The experiment correlates 13C chemical shift anisotropy with 15N-1H dipolar coupling and 15N/13C correlation. By controlling the timing of the correlation of 13C and 15N, the sequence can select the transfer of magnetization from amide nitrogens to directly bonded carbon neighbors or to carbons in sequential residues. This pulse sequence has been demonstrated on a double 15N/13C labeled crystal of acetylated glycine. The potential of acquiring the three dimensional correlation on 15N/13C labeled proteins in oriented lipid bilayers holds the promise of full sequential assignment of the sites in the peptide backbone.
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