Final answer:
The isoelectric point (pI) of amino acids varies due to the distinct chemical properties of their side chains, which affect how they accept or donate protons at different pH levels. Basic amino acids have higher pI, while acidic amino acids have lower pI.
Step-by-step explanation:
The isoelectric point (pI) of an amino acid is the particular pH at which the amino acid exists in solution as a zwitterion, which means the molecule has no net electric charge. This occurs because the positive and negative charges on the amino acid balance each other out. Different amino acids have different isoelectric points because they have side chains (R groups) with diverse chemical properties that can accept or donate protons to varying degrees. For example, amino acids with side chains that are neutral generally have isoelectric points between 5.0 to 6.5. In contrast, basic amino acids have side chains that can gain a positive charge at physiological pH and thus have higher isoelectric points. Conversely, acidic amino acids have side chains that can acquire a negative charge at physiological pH, resulting in lower isoelectric points. The gain or loss of protons by these side chains is subject to equilibrium processes that depend on the pH of the solution.