We present a highly sensitive pulse sequence, carbonyl carbon label selective 1H-15N HSQC (CCLS-HSQC) for the detection of signals from 1H-15N units involved in 13C′-15N linkages. The CCLS-HSQC pulse sequence utilizes a modified 15N CT evolution period equal to 1/(21JNC′) (∼33 ms) to select for 13C′-15N pairs. By collecting CCLS-HSQC and HNCO data for two proteins (8 kDa ubiquitin and 20 kDa HscB) at various temperatures (5-40°C) in order to vary correlation times, we demonstrate the superiority of the CCLS-HSQC pulse sequence for proteins with long correlation times (i.e. higher molecular weight). We then show that the CCLS-HSQC experiment yields assignments in the case of a 41 kDa protein incorporating pairs of 15N- and 13C′- labeled amino acids, where a TROSY 2D-HN(CO) had failed. Although the approach requires that the 1H-15N HSQC cross peaks be observable, it does not require deuteration of the protein. The method is suitable for larger proteins and is less affected by conformational exchange than HNCO experiments, which require a longer period of transverse 15N magnetization. The method also is tolerant to the partial loss of signal from isotopic dilution (scrambling). This approach will be applicable to families of proteins that have been resistant to NMR structural and dynamic analysis, such as large enzymes, and partially folded or unfolded proteins.
- Backbone resonance assignment
- Protein kinase A
- Selective labeling