See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: The isoelectric point (pI) of an amino acid is defined as the pH at which the amino acid exists predominantly as the zwitterion (dipolar ion) form, carrying no net electrical charge. At the isoelectric point, the concentration of the zwitterionic species is at its maximum because the rates of protonation and deprotonation are balanced such that neither the fully protonated cationic form nor the deprotonated anionic form predominates. Reasoning: For alanine, the isoelectric point is given as pH = 6. At this pH, the amino group (-NH3+) is protonated and the carboxyl group (-COO-) is deprotonated, resulting in the zwitterion form +H3N-CH(CH3)-COO-. The zwitterion concentration is maximized exactly at the isoelectric point because this is the pH at which the molecule has zero net charge and exists most stably in the dipolar form. Why other options fail: - (a) pH > 6: At pH above the isoelectric point, the solution is more basic, causing the -NH3+ group to lose a proton, shifting the equilibrium toward the anionic form (-NH2, -COO-), thus reducing zwitterion concentration. - (b) pH < 6: At pH below the isoelectric point, the solution is more acidic, causing the -COO- group to become protonated, shifting equilibrium toward the cationic form (-NH3+, -COOH), thus reducing zwitterion concentration. - (d) pH = 7: pH = 7 is above the isoelectric point (pI = 6), so the anionic form predominates over the zwitterion at this pH. Therefore, the correct answer is C.