## Step 1: Drawing Three Different Tetrapeptides ### Explanation: To create three different tetrapeptides, each **must contain serine, lysine, proline, and methionine**. The amino acids can be in any order, but all four must be present At their **isoelectric point (pI)**, the overall charge of the peptide is zero, so you must show the amino and carboxyl ends as they would appear at pI (N-terminus as NH₃⁺ or NH₂, C-terminus as COO⁻ or COOH, depending on the pI). #### **Peptide 1: Ser-Lys-Pro-Met** - N-terminus: Typically NH₃⁺ at pI. - C-terminus: Typically COO⁻ at pI. - Internal residues: All neutral except for side chains (Lys has a charged side chain at neutral pH, but at its pI, the net charge is zero). #### **Peptide 2: Met-Ser-Pro-Lys** - Change the order to ensure uniqueness. #### **Peptide 3: Pro-Lys-Met-Ser** - Another unique sequence. ### **At the isoelectric point:** - The N-terminus is typically **NH₃⁺**. - The C-terminus is typically **COO⁻**. - Side chains are shown in their predominant forms at pI. --- ## Step 2: Drawing the Tetrapeptides ### Explanation: Draw the peptide bonds, showing the N- and C- termini in their isoelectric forms. For each, write the sequence and show the main chain with the correct terminal charges. #### **Example for Peptide 1:** ``` NH₃⁺-Ser-Lys-Pro-Met-COO⁻ ``` - Serine (Ser): side chain -CH₂OH - Lysine (Lys): side chain -(CH₂)₄NH₃⁺ (at pI, may be neutral depending on context) - Proline (Pro): side chain forms a ring with the backbone N - Methionine (Met): side chain -CH₂CH₂SCH₃ #### **Example for Peptide 2:** ``` NH₃⁺-Met-Ser-Pro-Lys-COO⁻ ``` - Same approach, just change the order. #### **Example for Peptide 3:** ``` NH₃⁺-Pro-Lys-Met-Ser-COO⁻ ``` - Again, just change the order. --- ## **Final Answer** 1. **Peptide 1:** **NH₃⁺-Ser-Lys-Pro-Met-COO⁻** 2. **Peptide 2:** **NH₃⁺-Met-Ser-Pro-Lys-COO⁻** 3. **Peptide 3:** **NH₃⁺-Pro-Lys-Met-Ser-COO⁻** **Each tetrapeptide contains serine, lysine, proline, and methionine in a unique sequence, with the termini drawn as they would appear at the isoelectric point.**