How do amino acids change at different pH?

Amino acids are amphoteric which means they can act as an acid or a base.

An isoelectric point is the pH at which an amino acid exists as its zwitterion. A zwitterion is the dipolar ionic form of an amino acid. This is formed by a hydrogen ion (H⁺) from the carboxyl group being donated to the amino group. There is no overall charge. The carboxyl group becomes COO^- and the amino group becomes NH3⁺ showing an imbalance of charge that makes it a polar molecule.

If the pH is lower (in acidic conditions) than the isoelectric point then the amino acid acts as a base and accepts a proton at the amino group. This gives it a positive change.

If the pH is higher (in alkaline conditions) than the isoelectric point then the amino acid acts as an acid and donates a proton from its carboxyl group. This gives it a negative charge.