34.4 Amino acids

An amino acid contains both a carboxylic acid and an amine. In water it can transfer a proton internally, forming a zwitterion with –NH₃⁺ and –COO⁻ groups.

At low pH the molecule is more positively charged; at high pH it is more negatively charged. The isoelectric point is the pH at which the average net charge is zero.

Glycine is mainly a zwitterion near its isoelectric point, but becomes cationic in acid and anionic in alkaline solution.

A zwitterion is not electrically neutral at every site: it has internal charges whose sum may be zero.

Condensation between the carboxylic acid group of one amino acid and the amino group of another forms an amide (peptide) bond, –CO–NH–, and removes water.

The order of amino acids matters: the free amino end is the N-terminus and the free carboxyl end is the C-terminus. A dipeptide has two residues; a tripeptide has three.

Glycine + alanine can form Gly–Ala or Ala–Gly, which are different sequences even though they contain the same two amino acids.

Peptide formation is not ionic attraction between zwitterions; it is covalent condensation at the functional groups.

In an electric field, an amino acid or peptide moves toward the electrode of opposite charge. Its direction and speed depend on net charge, pH relative to its isoelectric point, and size/shape.

At pH below pI the species is net positive; above pI it is net negative; at pI it has no net migration. Compare every component at the stated pH.

At a pH below glycine’s pI, glycine moves toward the negative electrode; at a pH above its pI it moves toward the positive electrode.

“All amino acids move to the same side” ignores their different pI values and the chosen buffer pH.