Carbonyl carbon label selective (CCLS) ¹H-¹⁵N HSQC experiment for improved detection of backbone ¹³ C-¹⁵N cross peaks in larger proteins

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/(2¹J_NC′) (∼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.