What is the isoelectric point of an amino acid?

The isoelectric point (pI) of an amino acid is indeed defined as the pH at which the molecule exists predominantly in its zwitterionic form, meaning it carries no net electric charge. This occurs when the positive and negative charges on the amino acid balance each other out.

The pI is calculated as the average of the pKa values of the functional groups that are pertinent to this charge balance, specifically the carboxyl and amino groups. At the isoelectric point, the concentration of the positively charged form equals that of the negatively charged form, resulting in a net charge of zero.

Understanding this concept is crucial for interpreting behaviors of amino acids in various biological and chemical contexts, such as their solubility in different pH environments and their interactions with other biomolecules. The other options provided do not adequately describe the isoelectric point, focusing instead on aspects that relate to charge properties at particular pH levels or solubility characteristics, which do not capture the balance of charge present at the pI.