Improved 3D triple resonance experiments, HNN and HN(C)N, for H-N and N-15 sequential correlations in (C-13, N-15) labeled proteins: Application to unfolded proteins

Two triple resonance experiments, HNN and HN(C)N, are presented which correlate H-N and N-15 resonances sequentially along the polypeptide chain of a doubly (C-13, N-15) labeled protein. These incorporate several improvements over the previously published sequences for a similar purpose and have several novel features. The spectral characteristics enable direct identification of certain triplets of residues, which provide many starting points for the sequential assignment procedure. The experiments are sensitive and their utility has been demonstrated with a 22 kDa protein under unfolding conditions where most of the standard triple resonance experiments such as HNCA, CBCANH etc. have limited success because of poor amide, C-alpha and C-beta chemical shift dispersions.