Amino acids

An α-amino acid has both an –NH₂ group and a –COOH group on the same (α) carbon: H₂N–CHR–COOH. Because it has both an acidic group and a basic group, it is amphoteric — it reacts with both acids and bases: $\text{H}_2\text{NCH}_2\text{COOH} + \text{HCl} \rightarrow \text{ClH}_3\text{NCH}_2\text{COOH}$ $\text{H}_2\text{NCH}_2\text{COOH} + \text{NaOH} \rightarrow \text{H}_2\text{NCH}_2\text{COONa} + \text{H}_2\text{O}$

Within the molecule, the –COOH transfers its H⁺ to the –NH₂, giving an internal salt called a zwitterion: ⁺H₃N–CHR–COO⁻. It has a positive and a negative charge but no overall charge.

The charge of an amino acid depends on the pH:

• In acid (low pH): the –COO⁻ is protonated → ⁺H₃N–CHR–COOH, a cation (net +).
• In alkali (high pH): the –NH₃⁺ loses H⁺ → H₂N–CHR–COO⁻, an anion (net −).
• At the isoelectric point: the pH at which the amino acid has no net charge, existing as the zwitterion.

Electrophoresis uses these charges to separate amino acids. At a chosen pH, an amino acid below its isoelectric point is positive and migrates to the negative electrode (cathode); one above its isoelectric point is negative and migrates to the positive electrode (anode); one at its isoelectric point does not move. Because different amino acids have different isoelectric points, they separate.

Two amino acids join when the –COOH of one reacts with the –NH₂ of the other, losing a molecule of water (a condensation reaction). The link formed is an amide bond, which in this context is called a peptide bond (–CONH–), giving a dipeptide.

Repeating this builds polypeptides and proteins — they are condensation polymers of amino acids. Most amino acids also have a chiral centre (the α-carbon bears four different groups), so they are optically active; the same chirality of amino acids makes proteins, and the enzymes built from them, stereospecific.