Estimating Side-Chain Order in [U-2H;13CH3]-Labeled High Molecular Weight Proteins from Analysis of HMQC/HSQC Spectra

A simple approach for quantification of methyl-containing side-chain mobility in high molecular weight methyl-protonated, uniformly deuterated proteins is described, based on the measurement of peak intensities in methyl H-1-C-13 HMQC and HSQC correlation maps and relaxation rates of slowly decaying components of methyl H-1-C-13 multiple-quantum coherences. A strength of the method is that [U-H-2;(CH3)-C-13]-labeled protein samples are required that are typically available at an early stage of any analysis. The utility of the methodology is demonstrated with applications to three protein systems ranging in molecular weight from 82 to 670 kDa. Although the approach is only semiquantitative, a high correlation between order parameters extracted via this scheme and other more established methods is nevertheless demonstrated.