Improved labeling strategy for C-13 relaxation measurements of methyl groups in proteins

Selective incorporation of C-13 into the methyl groups of protein side chains is described as a means for simplifying the measurement and interpretation of C-13 relaxation parameters. High incorporation (>90%) is accomplished by using pyruvate (3-C-13, 99%) as the sole carbon source in the growth media for sr protein overexpression in E. coli. This improved labeling scheme increases the sensitivity of the relaxation experiments by approximately fivefold when compared to randomly fractionally C-13-labeled protein, allowing high-quality measurements on relatively dilute (<1 mM) protein samples at a relatively low cost.