Amino acids are called acids because they contain a carboxyl group (-COOH), which is an acidic group. The carboxyl group can lose a hydrogen ion (H+), which gives the amino acid a net negative charge. This negative charge is what gives amino acids their acidic properties.

Amino acids also contain an amino group (-NH2), which is a basic group. The amino group can accept a hydrogen ion (H+), which gives the amino acid a net positive charge. This positive charge is what gives amino acids their basic properties.

The balance between the acidic and basic groups in amino acids determines their overall charge. At neutral pH, most amino acids have a net negative charge because the carboxyl group is more acidic than the amino group. However, some amino acids, such as lysine and arginine, have more basic groups than acidic groups and therefore have a net positive charge at neutral pH.