Which structure determines the optical activity of an amino acid?

The structure that determines the optical activity of an amino acid is the presence of chiral centers. In chemistry, optical activity refers to the ability of a compound to rotate the plane of polarized light. A chiral center, typically a carbon atom bonded to four different substituents, creates two non-superimposable mirror images known as enantiomers. These enantiomers have distinct optical activities; one may rotate polarized light to the right (dextrorotatory), while the other might rotate it to the left (levorotatory).

In the context of amino acids, most of them have at least one chiral center, which is the alpha carbon atom bonded to an amino group, carboxyl group, hydrogen atom, and a variable side chain (R group). The presence of this chiral center is what enhances the amino acid's capability to be optically active. Thus, understanding chiral centers is crucial for determining the optical properties of amino acids.

Other factors such as the amount of proteins, the configuration of peptide bonds, or the type of side chains do not directly influence the optical activity of an individual amino acid, as optical activity fundamentally arises from chirality rather than these characteristics.