L-Citrulline

L‑Citrulline is a non‑essential, conditionally essential α‑amino acid that is not incorporated into proteins but serves as a metabolic precursor to L‑arginine and nitric‑oxide (NO) production. It is most commonly taken as a dietary supplement to support vascular function, exercise performance, and overall metabolic health.

• L‑Citrulline (L‑2‑amino‑5‑ureidopentanoic acid) is a non‑proteinogenic α‑amino acid with the molecular formula C₆H₁₃N₃O₃ and a molecular weight of 175.19 g/mol.
• Its IUPAC name is (2S)-2‑amino‑5‑ureido‑pentanoic acid.
• The molecule contains a standard α‑amino acid backbone, a carboxyl group, and a side‑chain ureido (‑NH‑C(=O)‑NH₂) moiety, which confers high polarity and water solubility (≈10 g/100 mL at 25 °C).
• The L‑enantiomer is the biologically active form; the D‑form is not naturally occurring and lacks metabolic conversion to L‑arginine.
• Its pKa values are ~2.0 (carboxyl), ~9.1 (α‑amino), and ~12.5 (ureido), influencing its absorption and distribution.

• Commercial L‑citrulline is primarily produced by microbial fermentation of glucose using engineered Corynebacterium or E. coli strains, yielding a high‑purity (>99 %) product.
• Natural dietary sources contain low amounts of citrulline: watermelon (Citrullus lanatus) provides ~150 mg per 100 g fresh fruit; other sources include cucumbers, gourds, and some melons.
• For supplements, the amino acid is isolated as a crystalline powder or combined with malic acid (citrulline‑malate) to improve solubility and taste.
• Quality considerations include verification of ≥99 % purity, absence of heavy metals or residual solvents, and compliance with GMP or USP standards.
• Third‑party testing (e.g., NSF, Informed‑Choice) is recommended to ensure product integrity and accurate labeling.