Arginine ] is an a-amino acid. The L-form is one of the 20 most common natural amino acids. Its codons are CGU, CGC, CGA, CGG, AGA, and AGG. Arginine consists of a 4-carbon aliphatic straight chain, the distal end of which is capped by a complex guanidinium group. With a pKa of 12.48, the guanidinium group is positively charged in neutral, acidic and even most basic environments, and thus imparts basic chemical properties to arginine.