Question

Use your knowledge of amino acid characteristics to order the peptides below according to the net charge contributed by their side chains at physiological pH (~pH 7). Each peptide contains eight amino acids. Use the single-letter amino acid designations to generate your list, placing the most negatively charged peptide on the left and the most positively charged peptide on the right. In addition, for each peptide, list the total number of positive and negative charges. Remember that, at neutral pH, the amino terminus carries a positive charge and the carboxyl terminus carries a negative charge.

A. YGAKKRA
B. ARRKSTRK
C. DERKQNST
D. DDAEIYSA E. NQSTYEEG

Answer 1

The peptides are ordered from most negatively charged to most positively charged based on their side chain charges at physiological pH as: C, D, E, A, B, with Peptide C having 2 negative and 1 positive charge, and Peptide B having 4 positive charges.

Step-by-step explanation:

To determine the net charge of peptides at physiological pH (~pH 7), we must consider the ionizable amino acids and their pKa values in relation to the pH. Basic amino acids such as Arginine (R) and Lysine (K) have positively charged side chains, while acidic amino acids like Aspartic acid (D) and Glutamic acid (E) carry negatively charged side chains. Other amino acids have neutral charges on their side chains. Considering these properties, the peptides can be ordered based on their net side chain charges.

Here's the correct order when considering the net charge of side chains from the most negatively charged to the most positively charged:

1. Peptide C (DERKQNST): 2 negative charges, 1 positive charge.
2. Peptide D (DDAEIYSA): 2 negative charges, no positive charges.
3. Peptide E (NQSTYEEG): 2 negative charges, no positive charges.
4. Peptide A (YGAKKRA): 2 positive charges, 1 negative charge.
5. Peptide B (ARRKSTRK): 4 positive charges, no negative charges.

Note, for each peptide, the amino terminus is considered to be positively charged, and the carboxyl terminus is negatively charged at physiological pH, which also contributes to the net charge calculation.