Acid-Base Chemistry of Amino Acids
High-Yield Summary
- Amino acids are amphoteric — every one has at least 2 ionizable groups (α-carboxyl, pKa₁ ≈ 2; α-amino, pKa₂ ≈ 9-10). Ionizable side chains add a third pKa (pKa(R)).
- Below a group's pKa → mostly protonated. Above it → mostly deprotonated.
- At physiological pH (~7.4), most amino acids exist as a zwitterion: –COO⁻ (deprotonated) and –NH₃⁺ (protonated) — net charge zero, not "uncharged."
- Isoelectric point (pI) = the pH of zero net charge = average of the two pKa values that flank the zwitterion on the titration curve.
- No side-chain pKa (e.g., glycine): pI = (pKa₁ + pKa₂) / 2.
- With a side-chain pKa: pI = average of whichever two pKa values immediately surround the zwitterionic form — NOT simply the two lowest or two highest.
Key Terms
- Amphoteric
- Can act as either an acid (donate H⁺) or a base (accept H⁺), depending on pH.
- Zwitterion
- A molecule with both a positive and negative charge on different atoms but zero net charge — the dominant form of most amino acids near pH 7.4.
- pKa
- The pH at which exactly half of a given ionizable group's molecules are deprotonated.
- Isoelectric point (pI)
- The specific pH at which an amino acid carries no net charge (exists entirely as the zwitterion).
Glycine's Titration Curve (Worked Example)
- 1Fully protonated start (low pH): –COOH and –NH₃⁺, net positive charge.
- 2First buffering region: carboxyl group (lower pKa, more acidic) loses its proton first; curve flattens around pKa₁ = 2.34.
- 3Isoelectric point at pH 5.97: fully neutral zwitterion, no buffering, curve rises steeply.
- 4Second buffering region around pKa₂ = 9.60: amino group loses its proton (–NH₃⁺ → –NH₂).
Calculating pI
pI = (pKa₁ + pKa₂) / 2
- pKa₁ = lower pKa flanking the zwitterion (e.g., α-carboxyl, or a side-chain carboxyl)
- pKa₂ = higher pKa flanking the zwitterion (e.g., α-amino, or a side-chain amino group)
- Glycine (no side-chain pKa): pI = (2.34 + 9.60)/2 = 5.97.
- Glutamic acid (2 carboxyls + 1 amino): zwitterion sits between the two carboxyl pKa's, so pI = (2.19 + 4.25)/2 = 3.22 — the α-amino pKa (9.67) is NOT used.
- Lysine (2 aminos + 1 carboxyl): zwitterion sits between the two amino pKa's, so pI ≈ (8.95 + 10.53)/2 = 9.74 — the α-carboxyl pKa is NOT used.
Common MCAT Trap
- pI is never just "average the two lowest" or "average the two highest" pKa's when there are three — it's whichever two flank the neutral zwitterion, which depends on the identity of the side chain (acidic side chain → average the two acidic pKa's; basic side chain → average the two basic pKa's).
- Zwitterion = net charge zero, but it is NOT uncharged at the atomic level — it carries both a positive and negative charge simultaneously.
Quick Recall
What form does an amino acid take at its isoelectric point?
How do you calculate pI for glutamic acid (pKa₁ = 2.19, pKa(R) = 4.25, pKa₂ = 9.67)?
Below a group's pKa, is it protonated or deprotonated?