Eschweiler-Clarke N-methylation (formaldehyde + formic acid)
Formaldehyde + formic acid + amine → N-methylamine + CO₂ + H₂O (the Eschweiler-Clark reaction). Both forms are common DMF degradation products, so this reaction can run in trace amounts during long syntheses. +14 Da, N-methylated amine.
- +28.0314 — +28 Da, N,N-dimethylation
Why it happens (mechanism)
HCHO + R-NH₂ → R-N=CH₂ (imine). Formic acid protonates and reduces by hydride transfer: R-NH-CH₃ + CO₂. Repeat once: R-N(CH₃)-H → R-N(CH₃)₂ + CO₂ + H₂O. So +14 (mono-Me) and +28 (di-Me) both possible, with same residue.
When it strikes (triggers)
Old DMF (releases HCHO + formic acid). Long synthesis cycles where cumulative trace exposure adds up. Pd-catalyzed hydrogenation in alcohol solvent that's photo-oxidized to traces of HCHO. Free Lys ε-amine is the most reactive site.
How to spot it (MS signature)
+14.02 Da (one methyl) or +28.03 Da (two methyls). Distinguish from Trt-residue (+242) or Pbf (+252) by location and the rest of the peak ladder. NMR of the Lys side chain confirms (-CH₂-N(CH₃)₂ singlet).
How to prevent it
- Use peptide-grade fresh DMF.
- Protect Lys as Lys(Boc) until needed; the Boc shields it from Eschweiler-Clarke.
- Avoid prolonged Pd/H₂ in methanol when peptides have free amines.
If it already happened (salvage)
- N-methylation is permanent. Re-synthesize.
Source
Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 7, §7.4.3.1; 14.3.2.