What is an ionized amino acid?

An ionized amino acid is characterized by having a charge due to the processes of protonation or deprotonation. In physiological conditions, the amino acid can exist in zwitterionic form, where it has both a positively charged amino group and a negatively charged carboxyl group. This results in a net charge of zero, but when the amino acid gains or loses protons (H+ ions), it can become fully charged.

When deprotonated, the carboxyl group loses a proton and becomes negatively charged, while the amino group may also become positively charged upon protonation, leading to the amino acid exhibiting an overall charge. This ionization state is crucial for the amino acid's behavior in biochemical reactions, protein folding, and its interactions in biological systems.

The other choices do not accurately describe the properties of an ionized amino acid. The absence of charge does not define ionization, oxidation does not inherently create an ionized state, and the presence of an ester bond is unrelated to the ionization characteristics of amino acids. Thus, ionization is fundamentally linked to the protonation and deprotonation of the functional groups in the amino acid structure.