Methyl and Amino Acid-Type Labeling
Selective methyl probes that push NMR structural studies into high-molecular-weight territory
Protonated carbon-13 methyl groups in otherwise deuterated proteins are excellent probes for structure and dynamics, and are crucial when studying high-molecular-weight proteins and protein complexes. Methyl labeling introduces either 13CH3 or 13CHD2 groups into an otherwise uniformly deuterated protein.
Methyl labeling can be achieved in both prokaryotic and eukaryotic expression systems by adding the relevant amino acid(s) in sufficient amounts directly to the cell culture medium prior to protein induction.
The ILV Labeling Scheme
For E. coli cultures, the most widespread strategy — known as ILV labeling — achieves selective protonation on the methyl groups of isoleucine, leucine, and valine by adding α-keto acid precursors to D2O-based cell culture medium one hour prior to induction:
- α-Ketoisobutyrate is converted into isoleucine.
- α-Ketoisovaleric acid is converted into leucine and valine.
Combined with 13C6,D7-glucose and 15NH4Cl as carbon and nitrogen sources, this scheme allows the production of uniformly deuterated, 13C- and 15N-labeled proteins with protonated ILV methyl probes — at a fraction of the cost of full amino acid labeling.
Why Methyl Probes Matter
Isoleucine, leucine, valine, alanine, threonine, and methionine are the six natural amino acids bearing methyl groups — together accounting for roughly 20% of residues in a typical protein. Combined with methyl-TROSY (transverse relaxation-optimized spectroscopy) experiments, methyl labeling has dramatically increased the useful molecular weight range accessible to NMR:
Beyond ILV: Expanding Labeling Options
Stereospecific Leu/Val Labeling
Pro-S and Pro-R precursors resolve the diastereotopic methyl groups of leucine and valine, more than doubling signal-to-noise compared to standard α-ketoisovaleric acid labeling.
Alanine Methyl Labeling
Extends methyl-TROSY probes onto a deuterated background, providing additional structural reporters for high-molecular-weight proteins.
ILVA & ILVMA Kits
Ready-to-use combinatorial kits for labeling multiple methyl-bearing residue types in a single E. coli expression run.
Extensive deuteration is essential to this approach: it reduces T2 relaxation rates, maximizing the magnetization available at the start of signal collection and enabling high-resolution spectra of very large assemblies.
Related Products
Alpha-Keto Acids
α-Ketoisobutyrate and α-ketoisovaleric acid precursors for ILV methyl labeling in E. coli.
Methyl-Labeling Kits & Amino Acids
13CH3-labeled deuterated amino acids and combinatorial kits (ILVA, ILVMA) for methyl-TROSY experiments.
Minimal Media Reagents
D2O-based minimal media components required to support in vivo methyl labeling strategies.
