How many ionizable groups are present in the peptide Leucine-Valine-Aspartate-Tryptophan-Lysine?

• A. 6
• B. 5
• C. 4
• D. 3

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the concept of ionizable groups. Ionizable groups in peptides are functional groups that can gain or lose protons depending on the pH. Common ionizable groups include the amino group (N-terminus), carboxyl group (C-terminus), and side chains of certain amino acids such as Aspartate, Glutamate, Lysine, Arginine, Histidine, Tyrosine, and Cysteine.
2. Step 2: Identify the ionizable groups in the peptide sequence. The peptide Leucine-Valine-Aspartate-Tryptophan-Lysine has the following amino acids: Leucine, Valine, Aspartate, Tryptophan, and Lysine. Check the side chains of each amino acid to determine if they are ionizable.
3. Step 3: Analyze the N-terminus and C-terminus. The N-terminus (amino group) of the peptide is ionizable, and the C-terminus (carboxyl group) is also ionizable. These are always present in peptides.
4. Step 4: Examine the side chains of the amino acids. Aspartate has a carboxyl group in its side chain that is ionizable. Lysine has an amino group in its side chain that is ionizable. Tryptophan, Leucine, and Valine do not have ionizable side chains.
5. Step 5: Count the total number of ionizable groups. Add the ionizable groups from the N-terminus, C-terminus, and the side chains of Aspartate and Lysine. This will give the total number of ionizable groups in the peptide.