Amino acids — common mistakes

Examiners frequently ask you to draw the structure of an amino acid at different pH values. Always start by drawing the zwitterion, then decide whether to add H⁺ (in acid) or remove H⁺ (in alkali). Remember that the –COO⁻ group is protonated first in acid, and the –NH₃⁺ group is deprotonated first in alkali.

When asked to form a dipeptide, pay close attention to which amino acid's carboxyl group reacts with which amino acid's amine group. If not specified, remember there are two possible products if the amino acids are different. For a tripeptide from three different amino acids (A, B, C), there are 3! = 6 possible arrangements (e.g., ABC, ACB, BAC, etc.).

Because the amine group (–NH₂) is attached to the alpha-carbon, which is the first carbon atom immediately adjacent to the carboxyl (–COOH) functional group.

A peptide is a short chain of amino acids. A dipeptide has two, a tripeptide has three. A polypeptide is a long, continuous, unbranched chain of amino acids. A protein consists of one or more polypeptides that have been folded and coiled into a specific three-dimensional structure.

A mixture of amino acids is placed on a support medium (like paper or gel) buffered to a specific pH, and an electric field is applied. At a pH below its pI, an amino acid is positively charged and moves towards the negative electrode (cathode). At a pH above its pI, it is negatively charged and moves towards the positive electrode (anode). An amino acid at its pI is neutral (a zwitterion) and does not move. This allows for separation based on charge.

No. While both enantiomers are chemically possible, virtually all proteins in living organisms are made exclusively from L-amino acids. This is an example of stereospecificity in biological systems.