At pH 9.0, what is the major form of each ionizable group?
N-terminus: ammonium; lysine R group: ammonium; C-terminus: carboxylate; aspartic acid R group: carboxylate
N-terminus: amine; lysine R group: ammonium; C-terminus: carboxylic acid; aspartic acid R group: carboxylic acid
N-terminus: amine; lysine R group: ammonium: C-terminus: carboxylic acid; aspartic acid R group: carboxylate
N-terminus: amine; lysine R group: ammonium: C-terminus: carboxylate; aspartic acid R group: carboxylate
The correct answer and explanation is :
The correct answer is:
N-terminus: amine; lysine R group: ammonium; C-terminus: carboxylate; aspartic acid R group: carboxylate.
Explanation:
At pH 9.0, the protonation state of ionizable groups depends on their respective pKa values:
- N-terminal amine (pKa ≈ 9.0-10.0)
- The pKa of the N-terminal amine is around 9.0-10.0.
- At pH 9.0, this value is close to its pKa, meaning it will be in equilibrium between protonated (( \text{-NH}_3^+ )) and deprotonated (( \text{-NH}_2 )) forms.
- Since deprotonation starts occurring at this pH, the dominant form is neutral amine (-NH₂).
- Lysine R-group (pKa ≈ 10.5)
- The lysine side chain has a pKa of around 10.5.
- Since pH 9.0 is below its pKa, the group remains in its protonated (+1 charge) ammonium (-NH₃⁺) form.
- C-terminal carboxyl group (pKa ≈ 2-3)
- The carboxyl group has a much lower pKa (around 2-3).
- Since pH 9.0 is much higher than its pKa, the carboxyl group will be deprotonated (-COO⁻).
- Aspartic acid R-group (pKa ≈ 3.9-4.1)
- The aspartic acid side chain has a pKa of around 3.9-4.1.
- Since pH 9.0 is much higher than its pKa, the group is deprotonated (-COO⁻).
Summary of Major Forms at pH 9.0:
- N-terminus: Neutral amine (-NH₂)
- Lysine R-group: Protonated ammonium (-NH₃⁺)
- C-terminus: Deprotonated carboxylate (-COO⁻)
- Aspartic acid R-group: Deprotonated carboxylate (-COO⁻)
Thus, the correct answer is:
N-terminus: amine; lysine R group: ammonium; C-terminus: carboxylate; aspartic acid R group: carboxylate.