Optimal methyl labeling for studies of supra-molecular systems.

Selective methyl labeling combined with HMQC spectroscopy that exploits a TROSY effect in (13)CH(3) spin systems has significantly extended the utility of solution NMR spectroscopy in studies of high molecular weight particles. Herein we compare the utility of (13)CH(3)- versus (13)CHD(2)-labeling of Ile, Leu, Val probes in supra-molecular systems through quantification of relative signal-to-noise ratios in optimized spectra of highly deuterated, (13)CH(3)- and (13)CHD(2)-labeled samples of the half proteasome (alpha(7)alpha(7), 360 kDa). It is shown that the sensitivity of spectra recorded on Ile, Leu, Val (13)CH(3)-labeled samples is between 1.5 and 2 fold higher than the corresponding data sets obtained on alpha(7)alpha(7) with (13)CHD(2) probes. Thus, labeling of supra-molecules with (13)CH(3) isotopomers remains the method of choice, but in applications where (13)CHD(2) moieties are required, sensitivity will in general not be limiting.