The total number of negative charges in the tetrapeptide, Gly-Glu-Asp-Tyr, at pH 12.5 will be ______ — Biomolecules Chemistry Question

💡 Solution & Explanation

**Step 1: Identify ionizable groups in the tetrapeptide Gly-Glu-Asp-Tyr** - N-terminus (pKa ≈ 9.5) - C-terminus (pKa ≈ 3.5) - Glu side chain (pKa ≈ 4.2) - Asp side chain (pKa ≈ 3.9) - Tyr side chain (pKa ≈ 10.1) **Step 2: Apply the Henderson-Hasselbalch principle at pH 12.5** When pH >> pKa, the group is deprotonated and carries its dissociated charge. When pH << pKa, the group is protonated and neutral. **Step 3: Determine ionization state of each group at pH 12.5** - C-terminus (pKa 3.5): pH >> pKa → **deprotonated, -1 charge** - Glu side chain (pKa 4.2): pH >> pKa → **deprotonated, -1 charge** - Asp side chain (pKa 3.9): pH >> pKa → **deprotonated, -1 charge** - N-terminus (pKa 9.5): pH >> pKa → **deprotonated, 0 charge** (becomes NH₂) - Tyr side chain (pKa 10.1): pH > pKa → **deprotonated, -1 charge** **Step 4: Sum total negative charges** Total negative charges = 1 + 1 + 1 + 1 = **4** Therefore, the answer is 4.00.