Alpha-amino acids: H2N-CHR-COOH. At physiological pH (~7.4), both groups are ionized: NH3+-CHR-COO- $\frac{zwitterion}{dipolar ion}$. This explains high melting points (ionic character), water solubility, and amphoteric behavior.

Isoelectric point (pI): The pH at which the amino acid has zero net charge (exists entirely as zwitterion). No migration in electric field at this pH.

For neutral amino acids (no ionizable side chain): pI = $\frac{pKa1 + pKa2}{2}$ where pKa1 = COOH dissociation (~2.0-2.4) and pKa2 = NH3+ dissociation (~9.0-10.0).

For acidic amino acids (Asp, Glu): pI = $\frac{pKa1 + pKaR}{2}$ where pKaR is the side-chain COOH. Low pI (~3).

For basic amino acids (Lys, Arg): pI = $\frac{pKa2 + pKaR}{2}$ where pKaR is the side-chain NH3+. High pI (~10).

Electrophoresis prediction: if buffer pH < pI → amino acid is cation → migrates to cathode. If pH > pI → anion → migrates to anode. If pH = pI → no migration.