Answer

Isoelectric Point (pI) of Aspartic Acid

What is the Isoelectric Point?

The isoelectric point (pI) is the pH at which an amino acid carries no net electric charge. At this pH, the amino acid exists primarily in its zwitterionic form.

Ionizable Groups in Aspartic Acid

Aspartic acid contains three ionizable groups:

• α-carboxyl group (COOH) → pKa₁ = 1.88
• Side-chain carboxyl group (R–COOH) → pKa_R = 3.65
• α-amino group (NH₃⁺) → pKa₂ = 9.60

pH-Dependent Charge States of Aspartic Acid

pH < 1.88

• α-COOH: neutral
• R–COOH: neutral
• NH₃⁺: positive
• Net charge: +1

pH between 1.88 and 3.65

• α-COO⁻: negative
• R–COOH: neutral
• NH₃⁺: positive
• Net charge: 0

pH between 3.65 and 9.60

• α-COO⁻: negative
• R–COO⁻: negative
• NH₃⁺: positive
• Net charge: -1

pH > 9.60

• α-COO⁻: negative
• R–COO⁻: negative
• NH₂: neutral
• Net charge: -2

Determining the Isoelectric Point (pI)

The zwitterionic (neutral) form occurs between pKa₁ and pKa_R:

pI = (pKa₁ + pKaR) / 2 = (1.88 + 3.65) / 2 = 2.77