What is the net charge of the peptide Arg-Ala-Phe-Leu at ...

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Asked by • 04/14/2023

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To determine the net charge of the peptide Arg-Ala-Phe-Leu at pH 8, we need to analyze the individual amino acids and their side chains' pKa values at the given pH. The options for the net charge are A. -1, B. 0, C. +1, and D. +2. Let's break it down step by step:

1. Arg (Arginine) has a side chain pKa of ~12.5. Since the pH 8 is less than the pKa, the side chain will be protonated and positively charged (+1).
2. Ala (Alanine) has a neutral side chain, so it does not contribute any charge.
3. Phe (Phenylalanine) also has a neutral side chain, so it does not contribute any charge.
4. Leu (Leucine) has a neutral side chain, so it does not contribute any charge.

Now, we need to consider the N-terminus and C-terminus charges. At pH 8:
5. The N-terminus (NH3+) has a pKa of ~9. Since the pH is less than the pKa, it will be protonated and positively charged (+1).
6. The C-terminus (COO-) has a pKa of ~2. Since the pH is greater than the pKa, it will be deprotonated and negatively charged (-1).

Now, let's calculate the net charge by adding all charges together: +1 (Arg) + 0 (Ala) + 0 (Phe) + 0 (Leu) +1 (N-terminus) -1 (C-terminus) = +1.

So, the net charge of the peptide Arg-Ala-Phe-Leu at pH 8 is +1, making the correct answer C. +1.

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The net charge of the peptide Arg-Ala-Phe-Leu at pH 8 is +1. This is determined by analyzing the charges of the individual amino acids and the terminal groups of the peptide. Therefore, the correct answer is C. +1.

Explanation

To determine the net charge of the peptide Arg-Ala-Phe-Leu at pH 8, we need to analyze the individual amino acids and their side chains' pKa values at this pH. The amino acids involved are:

1. Arginine (Arg): This amino acid has a side chain with a pKa of approximately 12.5. Since the pH (8) is lower than the pKa, the side chain will be protonated and thus positively charged (+1).

2. Alanine (Ala): Alanine has a neutral side chain, contributing a charge of 0.

3. Phenylalanine (Phe): Similar to alanine, phenylalanine has a neutral side chain, contributing a charge of 0.

4. Leucine (Leu): This amino acid also has a neutral side chain, contributing a charge of 0.

Next, we consider the terminal charges on the peptide:

1. N-terminus: At pH 8, the amino group of the N-terminus (NH3+) has a pKa of around 9. Since the pH is less than the pKa, it remains positively charged (+1).

2. C-terminus: The carboxylic acid group of the C-terminus (COO-) has a pKa of around 2. At pH 8, which is above this pKa, the group will be deprotonated and negatively charged (-1).

Now we can calculate the total net charge by summing all the charges:

• Arg: +1
• Ala: 0
• Phe: 0
• Leu: 0
• N-terminus: +1
• C-terminus: -1

Calculating the total:

Thus, the net charge of the peptide Arg-Ala-Phe-Leu at pH 8 is +1, making the correct answer C. +1.

Examples & Evidence

For example, if we were to analyze a peptide like Gly-Ser-Lys, we would determine the net charge by looking at the side chains and terminal groups similarly. Lysine would contribute a positive charge, while Gly and Ser would not contribute any charge. Understanding these principles helps clarify how peptide charges vary with pH.

The pKa values for the amino acids and the behavior of carboxylic and amino groups at different pH levels are well established in biochemistry, which supports the accuracy of the charge calculations.

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