L-Methionine

L‑Methionine is an essential, sulfur‑containing α‑amino acid that must be obtained from the diet because humans cannot synthesize it de novo. It serves as a building block for protein synthesis and as a key donor of methyl groups in numerous metabolic reactions, including the synthesis of cysteine, taurine, and the antioxidant glutathione.

• L‑Methionine (L‑2‑amino‑4‑(methylthio)butanoic acid) is a non‑polar, sulfur‑containing amino acid.
• Molecular formula: C₅H₁₁NO₂S.
• Molecular weight: 149.21 g mol⁻¹.
• IUPAC name: (2S)-2‑amino‑4‑(methyl‑sulfanyl)‑butanoic acid.
• It possesses an α‑amino group, a carboxylate group, and a thioether side chain (‑CH₂‑CH₂‑SCH₃) that confers lipophilicity and serves as the sulfur donor in downstream metabolism.
• The L‑configuration corresponds to the natural stereoisomer found in proteins.
• Soluble in water (≈2 g/100 mL at 20 °C) and stable at pH 5–8, it exhibits a pKa₁ ≈2.2 (carboxyl) and pKa₂ ≈9.2 (amino).
• In the solid state, it crystallizes as a zwitterionic powder with a melting point of ~280 °C (decomposes).

• Commercial L‑Methionine is produced via two main routes:
  • (1) Fermentation of Corynebacterium glutamicum or E. coli engineered to overproduce the amino acid, yielding a high‑purity, food‑grade product.
  • (2) Chemical synthesis from ethylene, carbon disulfide, and ammonia through a multi‑step process (e.g., the Strecker synthesis), often used for pharmaceutical‑grade material.
• Natural dietary sources include animal proteins (fish, meat, dairy) and, to a lesser extent, nuts, seeds, and legumes.
• For supplements, GMP‑certified fermentation is preferred due to lower heavy‑metal risk and consistent stereochemistry (L‑form).
• Quality assessment includes verification of ≥ 99 % L‑isomer purity, absence of residual solvents, and compliance with USP/NF specifications for microbial limits.