See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: The isoelectric point (pI) of an amino acid is the pH at which the molecule carries no net electrical charge. For an amino acid with multiple ionizable groups, pI is calculated as the average of the two pKa values that flank the zwitterionic (neutral) form. Identifying the groups: - Group Z (alpha-carboxyl, CO2H attached directly to the alpha carbon): pKa = 2.2 - Group X (side-chain carboxyl, HO2C-CH2-): pKa = 4.3 - +NH3 (alpha-amino group, Y): pKa = 9.7 Determining the zwitterion region: At the isoelectric point, glutamic acid exists as a zwitterion with both carboxyl groups deprotonated and the amino group protonated — wait, let us reconsider. The zwitterion of glutamic acid (an acidic amino acid) has the alpha-amino group protonated (+NH3) and both carboxyl groups... actually at pI only one net charge state is neutral. For an acidic amino acid like glutamic acid, the neutral zwitterion has: alpha-NH3+ (protonated), alpha-COO- (deprotonated), and side-chain -COOH (protonated). This species exists between pKa of the side-chain carboxyl (4.3) and pKa of the alpha-carboxyl (2.2)... Re-examining: Order of ionization from low to high pH: first Z loses proton (pKa=2.2), then X loses proton (pKa=4.3), then Y loses proton (pKa=9.7). Species at various pH: - Below 2.2: +H3N-CH(COOH)-CH2-COOH (charge = +1) - Between 2.2 and 4.3: +H3N-CH(COO-)-CH2-COOH (charge = 0) — this is the neutral/zwitterionic form - Between 4.3 and 9.7: +H3N-CH(COO-)-CH2-COO- (charge = -1) - Above 9.7: H2N-CH(COO-)-CH2-COO- (charge = -2) The neutral (zwitterionic) form exists between pKa(Z) = 2.2 and pKa(X) = 4.3. pI = (pKa(Z) + pKa(X)) / 2 = (2.2 + 4.3) / 2 = 6.5 / 2 = 3.25 Why other options fail: - (a) 7.00: This would be the pI for a neutral amino acid (average of alpha-COOH and alpha-NH3+), not applicable here. - (c) 4.95: This would be the average of pKa(X) and pKa(Y) = (4.3+9.7)/2, which corresponds to the wrong pair of flanking species. - (d) 5.95: Incorrect average, does not correspond to any meaningful flanking pair. Therefore, the correct answer is B.