Amino Acid Charges at pH 7.0
Question:
What amino acid is most likely to have a net negative charge at a pH of 7.0?
- Glutamine
- Aspartic acid
- Lysine
- Histidine
Answer:
To determine the net charge of each amino acid at pH 7.0, we consider the ionization states of the:
- α-carboxyl group (usually deprotonated at pH 7.0 → -1)
- α-amino group (usually protonated at pH 7.0 → +1)
- Side chain (varies depending on the amino acid)
Glutamine
- α-carboxyl: −1
- α-amino: +1
- Side chain: amide (neutral)
Net charge: 0
Aspartic Acid
- α-carboxyl: −1
- α-amino: +1
- Side chain: carboxyl group, pKa ≈ 3.9 → deprotonated at pH 7.0 (−1)
Net charge: −1 ✅ Most likely to carry a net negative charge at pH 7.0
Lysine
- α-carboxyl: −1
- α-amino: +1
- Side chain: amino group, pKa ≈ 10.5 → protonated at pH 7.0 (+1)
Net charge: +1
Histidine
- α-carboxyl: −1
- α-amino: +1
- Side chain: imidazole, pKa ≈ 6.0 → ~10% protonated at pH 7.0 (~+0.1)
Net charge: ≈ +0.1
🧪 Final Answer:
Aspartic acid is the amino acid most likely to have a net negative charge at pH 7.0, due to its extra deprotonated carboxyl side chain.