Comparison of 13CH3, 13CH2D, and 13CHD2 methyl labeling strategies in proteins.

A comparison of three labeling strategies for studies involving side chain methyl groups in high molecular weight proteins, using 13CH3, 13CH2D, and 13CHD2 methyl isotopomers, is presented. For each labeling scheme, 1H-13C pulse sequences that give optimal resolution and sensitivity are identified. Three highly deuterated samples of a 723 residue enzyme, malate synthase G, with 13CH3, 13CH2D, and 13CHD2 labeling in Ile delta1 positions, are used to test the pulse sequences experimentally, and a rationalization of each sequence's performance based on a product operator formalism that focuses on individual transitions is presented. The HMQC pulse sequence has previously been identified as a transverse relaxation optimized experiment for 13CH3-labeled methyl groups attached to macromolecules, and a zero-quantum correlation pulse scheme (13CH3 HZQC) has been developed to further improve resolution in the indirectly detected dimension. We present a modified version of the 13CH3 HZQC sequence that provides improved sensitivity by using the steady-state magnetization of both 13C and 1H spins. The HSQC and HMQC spectra of 13CH2D-labeled methyl groups in malate synthase G are very poorly resolved, but we present a new pulse sequence, 13CH2D TROSY, that exploits cross-correlation effects to record 1H-13C correlation maps with dramatically reduced linewidths in both dimensions. Well-resolved spectra of 13CHD2-labeled methyl groups can be recorded with HSQC or HMQC; a new 13CHD2 HZQC sequence is described that provides improved resolution with no loss in sensitivity in the applications considered here. When spectra recorded on samples prepared with the three isotopomers are compared, it is clear that the 13CH3 labeling strategy is the most beneficial from the perspective of sensitivity (gains > or =2.4 relative to either 13CH2D or 13CHD2 labeling), although excellent resolution can be obtained with any of the isotopomers using the pulse sequences presented here.