Nitrogen-detected CAN and CON experiments as alternative experiments for main chain NMR resonance assignments.

Heteronuclear direct-detection experiments, which utilize the slower relaxation properties of low gamma nuclei, such as (13)C have recently been proposed for sequence-specific assignment and structural analyses of large, unstructured, and/or paramagnetic proteins. Here we present two novel (15)N direct-detection experiments. The CAN experiment sequentially connects amide (15)N resonances using (13)C(alpha) chemical shift matching, and the CON experiment connects the preceding (13)C' nuclei. When starting from the same carbon polarization, the intensities of nitrogen signals detected in the CAN or CON experiments would be expected four times lower than those of carbon resonances observed in the corresponding (13)C-detecting experiment, NCA-DIPAP or NCO-IPAP (Bermel et al. 2006b; Takeuchi et al. 2008). However, the disadvantage due to the lower gamma is counteracted by the slower (15)N transverse relaxation during detection, the possibility for more efficient decoupling in both dimensions, and relaxation optimized properties of the pulse sequences. As a result, the median S/N in the (15)N observe CAN experiment is 16% higher than in the (13)C observe NCA-DIPAP experiment. In addition, significantly higher sensitivity was observed for those residues that are hard to detect in the NCA-DIPAP experiment, such as Gly, Ser and residues with high-field C(alpha) resonances. Both CAN and CON experiments are able to detect Pro resonances that would not be observed in conventional proton-detected experiments. In addition, those experiments are free from problems of incomplete deuterium-to-proton back exchange in amide positions of perdeuterated proteins expressed in D(2)O. Thus, these features and the superior resolution of (15)N-detected experiments provide an attractive alternative for main chain assignments. The experiments are demonstrated with the small model protein GB1 at conditions simulating a 150 kDa protein, and the 52 kDa glutathione S-transferase dimer, GST.