At pH=4, what is the net charge of the peptide PLAID?
The Correct Answer and Explanation is :
To determine the net charge of the peptide PLAID at pH 4, we first need to understand the structure of the peptide and the ionization states of its constituent amino acids at this pH.
Peptide Structure
The peptide PLAID consists of the following amino acids:
- P: Proline (Pro)
- L: Leucine (Leu)
- A: Alanine (Ala)
- I: Isoleucine (Ile)
- D: Aspartic acid (Asp)
Ionization States at pH 4
- Amino Group (NH3⁺): The amino group at the N-terminus of the peptide (attached to Pro) will be protonated at pH 4, giving it a charge of +1.
- Carboxylic Acid Groups:
- The carboxylic acid group from the C-terminus (attached to Asp) will be deprotonated at pH 4, giving it a charge of -1.
- The carboxylic acid side chain of aspartic acid will also be deprotonated at this pH, contributing another -1 charge.
- Side Chains of Other Amino Acids:
- Proline, leucine, alanine, and isoleucine are neutral at pH 4, meaning they do not contribute to the net charge.
Calculation of Net Charge
Now, we can summarize the contributions to the net charge:
- Amino group (N-terminus): +1
- C-terminal carboxyl group: -1
- Aspartic acid side chain: -1
- Other amino acids (P, L, A, I): 0
Thus, the net charge can be calculated as follows:
[
\text{Net Charge} = (+1) + (-1) + (-1) + 0 + 0 + 0 = -1
]
Conclusion
At pH 4, the peptide PLAID has a net charge of -1. This negative charge is mainly due to the deprotonated carboxyl groups from the C-terminus and the side chain of aspartic acid, which outbalance the positive charge from the amino group at the N-terminus. Understanding the ionization states of the amino acids in the peptide is crucial for predicting its behavior in biochemical environments.