See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: The isoelectric point (pI) is the pH at which the amino acid carries no net charge (exists as a zwitterion). For an amino acid with multiple ionizable groups, pI is calculated as the average of the two pKa values that flank the neutral (zwitterionic) form. Step 1: Identify the amino acid. The structure shown is glutamic acid (Glu), which has three ionizable groups: - Alpha-carboxyl group: pKa = 2 - Side chain carboxyl group: pKa = 4 - Alpha-amino group (as NH3+): pKa = 9 Step 2: Determine the charge states as pH increases: - At very low pH (below 2): all groups protonated → +1 overall charge (both COOH protonated, NH3+ charged) - Between pKa 2 and pKa 4: alpha-COOH loses proton → net charge = 0... let's track carefully. Actually, let's track systematically. The fully protonated form (cation) has charge: +1 (NH3+ contributes +1, both COOHs contribute 0). As pH rises: - At pKa = 2: alpha-COOH deprotonates → charge goes from +1 to 0 (net neutral zwitterion? No, side chain COOH still protonated) - At pKa = 4: side chain COOH deprotonates → charge goes from 0 to -1 - At pKa = 9: NH3+ deprotonates → charge goes from -1 to -2 Step 3: The neutral (zwitterionic) species exists between pKa = 2 and pKa = 4, i.e., the form with alpha-COO-, side chain COOH, and NH3+ has net charge = -1 + 0 + 1 = 0. Step 4: The isoelectric point is the average of the two pKa values that bracket the neutral form: pI = (pKa1 + pKa2) / 2 = (2 + 4) / 2 = 6/2 = 3 Step 5: Why other options fail: - (a) 5.5: This would be pI for a simple neutral amino acid (e.g., average of pKa 2 and 9), incorrect for an acidic amino acid. - (b) 6.5: Incorrect; not the average of the two relevant pKa values. - (d) 5: Incorrect arithmetic; not derived from the correct pKa pair. Therefore, the correct answer is C.