Organic Chemistry MCAT Practice Exam

When pH is greater than the isoelectric point (pI) of an amino acid, what is the charge of the amino acid?

1. Neutral

2. Net positive

3. Net negative

4. Depends on the specific amino acid

The correct answer is: Net negative

When the pH of a solution is greater than the isoelectric point (pI) of an amino acid, the amino acid will typically carry a net negative charge. The isoelectric point represents the pH at which the amino acid exists in a form that has no overall charge; this occurs when the positive and negative charges from the amino acid's side chains and the amino and carboxyl groups balance each other out. At a pH higher than the pI, the environment is sufficiently basic that the amino group, which tends to be positively charged (protonated) at lower pH values, loses a proton and becomes neutral. In contrast, the carboxyl group, which is negatively charged (deprotonated) at higher pH levels, remains negatively charged. The result is an overall negative charge on the amino acid, due to the dominance of the negatively charged carboxyl group over the neutral amino group. This concept is crucial in understanding the behavior of amino acids in different pH environments, particularly in biochemical contexts where protein folding, enzyme activity, and interactions with other molecules can be significantly influenced by the charge states of the amino acids involved.