Table 1.

Isotopically labeled methyl residue precursors for NOESY experiments.

Site of 13CH3	Precursor Name	Precursor Structure	1L single density growth	Additional Supplements	Notes/Potential issues	Suppliers	Ref.
I-δ1	α-Keto-butyrate		60 mg/L	–	Incompatible with A labeling.	CIL, Isotec, NMR-Bio	[1]
	2-(S)-Hydroxy-2-ethyl-3-keto-butyrate		60 mg/L	–	Compatible with A labeling	NMR-Bio	[45]
M-ε	l-Methionine		250 mg/L	–	–	CIL, Isotec, NMR-Bio	[72]
L-δ/V-γ racemic pro-R pro-S	α-Keto-isovalerate		120 mg/L	–	–	CIL, Isotec, NMR-Bio	[4]
L-δ1, δ2/V-γ1, γ2	α-Keto-isovalerate		120 mg/L	–	This labeling scheme works best to establish intra-residue resonance pairings for L and V, less so for long range NOEs	Isotec	[2]
V-γ	α-Keto-isovalerate	Either of the above two precursors	120 mg/L	10% deuterated Bioexpress or Isogro	Addition of Bioexpress/Isogro has been observed to decrease I-δ1 and T-γ2 incorporation. Use to distinguish L/V resonances If also labeling I-δ1, add α-acetolactate 1 h before induction and α-ketobutyrate 20 min before induction	CIL, Isotec, or NMR-BioCIL, Isotec for supplement NMR-Bio	[52]
L-δ/V-γ pro-S	2-(S)-Aceto-lactate		300 mg/L	–			[50]
A-β	l-Alanine		600 mg/L	200 mg/L deuterated α-ketoisovalerate, 60 mg/L deuterated isoleucine (unless labeling L/V or I)	Scrambling to I-γ2 makes this label incompatible with α-ketobutyrate I labeling for NOE studies. 2-(S)-hydroxy-2-ethyl-3-ketobutyrate must be used instead for this scenario	CIL, Isotec, NMR-Bio	[45,49]
T-γ2	l-Threonine		50 mg/L	60 mg/L deuterated α-ketobutyrate (unless also labeling I-δ1) and 100mg/L deuterated Gly	–	CIL, Isotec, NMR-Bio	[46]