Question

Draw a structural formula for Lys-Phe-Ala. Label the \(N\)-terminal amino acid and the C-terminal amino acid. What is the net charge on this tripeptide at \(\mathrm{pH} 6.0\) ?

Short answer

Answer: The net charge of the Lys-Phe-Ala tripeptide at pH 6.0 is +1. The N-terminal amino acid is Lysine, and the C-terminal amino acid is Alanine.

Step-by-step solution

Identify the amino acids

Lys-Phe-Ala is the short form for the amino acids Lysine, Phenylalanine, and Alanine. We'll need to know the structures and properties of each individual amino acid to construct the structural formula for this tripeptide and calculate the net charge at pH 6.0.

Draw the structural formula for the individual amino acids

Here are the structural formulas for Lysine, Phenylalanine, and Alanine: Lysine (Lys): -NH\(_2\)-CH\(_2\)-CH\(_2\)-CH\(_2\)-CH\(_2\)-NH\(_2\) (pK\(_a1\)=2.18, pK\(_a2\)=8.95, pK\(_R\)=10.53) Phenylalanine (Phe): -CH\(_2\)-C\(_6\)H\(_5\) (No ionizable side chain) Alanine (Ala): -CH\(_3\) (No ionizable side chain)

Connect the amino acids and draw the tripeptide Lys-Phe-Ala

We need to form peptide bonds between the carboxyl group (-COOH) of the first amino acid and the amino group (-NH2) of the second amino acid. The same goes for the second and third amino acids. After forming peptide bonds, the complete structure will look like this: H\(_3\)N\(^{+}\)-Lys-CO-NH-Phe-CO-NH-Ala-COO\(^{-}\) N-terminal is Lysine and the C-terminal is Alanine.

Calculate the net charge of the tripeptide at pH 6.0

At a certain pH, an amino acid protonates or deprotonates depending on its pK values. Now, let's look at the ionizable groups of Lys-Phe-Ala tripeptide and their pK values: N-terminal amine group (Lysine): pK\(_a1\) = 2.18 R-group of Lysine: pK\(_R\) = 10.53 C-terminal carboxyl group (Alanine): pK\(_a2\) = 8.95 Now, we need to compare the pK values with the pH (6.0). If the pH is lower than the pK value, the group will be protonated; if the pH is higher than the pK value, the group will be deprotonated. N-terminal amine group (Lysine): pH < pK\(_a1\) ⇒ Protonated, charge = +1 R-group of Lysine: pH < pK\(_R\) ⇒ Protonated, charge = +1 C-terminal carboxyl group (Alanine): pH > pK\(_a2\) ⇒ Deprotonated, charge = -1 To find the net charge, add all the charges: Net charge = (+1) + (+1) + (-1) = +1 So, the net charge of the Lys-Phe-Ala tripeptide at pH 6.0 is +1.