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 (Lys) and the C-terminal amino acid is Alanine (Ala).

Step-by-step solution

Identify the individual amino acids and their structures

First, identify the individual amino acids and their structures. Lysine (Lys), phenylalanine (Phe), and alanine (Ala) are the three amino acids in the given tripeptide. Their general structure is: Lysine: NH3+ - CH2-CH2-CH2-CH2-NH2 - COO- Phenylalanine: NH3+ - CH2 - C6H5 - COO- Alanine: NH3+ - CH3 - COO-

Peptide bond formation in the tripeptide

Now, we form peptide bonds between the amino acids to create the tripeptide. A peptide bond is formed between the carboxyl group of one amino acid (C-terminus) and the amino group of the next amino acid (N-terminus). The order of the amino acids in the tripeptide is Lys-Phe-Ala. Form peptide bonds between Lys-Phe and Phe-Ala to get the tripeptide structure: NH3+ - CH2-CH2-CH2-CH2-NH - CO - CH2 - C6H5 - NH - CO - CH3 - COO-

Identifying N- and C-terminal amino acids

The N-terminal amino acid is the one with the free amino group (NH3+), which is at the beginning of the tripeptide. In this case, the N-terminal amino acid is Lysine (Lys). The C-terminal amino acid is the one with the free carboxyl group (COO-), which is at the end of the tripeptide. In this case, the C-terminal amino acid is Alanine (Ala).

Calculating the net charge at pH 6.0

We calculate the net charge of the tripeptide at pH 6.0 by analyzing each ionizable group of amino acids and determining if they are protonated or deprotonated at the given pH. The ionizable groups and their pKa values are as follows: Lysine: - α-amino group (NH3+): pKa ~ 9.0 - α-carboxyl group (COO-): pKa ~ 2.2 - ε-amino group (NH2): pKa ~ 10.5 Phenylalanine: - α-amino group (NH3+): pKa ~ 9.0 - α-carboxyl group (COO-): pKa ~ 2.5 Alanine: - α-amino group (NH3+): pKa ~ 9.0 - α-carboxyl group (COO-): pKa ~ 2.3 At pH 6.0: Lysine: - α-amino group: Protonated (NH3+) - α-carboxyl group: Deprotonated (COO-) - ε-amino group: Protonated (NH3+) Phenylalanine: - α-amino group: Protonated (NH3+) - α-carboxyl group: Deprotonated (COO-) Alanine: - α-amino group: Protonated (NH3+) - α-carboxyl group: Deprotonated (COO-) Since in the tripeptide, one N-terminal amino group (Lys) and one C-terminal carboxyl group (Ala) are free, the other ionizable groups involved in peptide bond formation are not considered. The net charge of the tripeptide at pH 6.0 is: Net charge = (+1 Lys-NH3+) + (+1 Lys-ε-NH3+) + (-1 Ala-COO-) = +1 So, the net charge of the Lys-Phe-Ala tripeptide at pH 6.0 is +1.