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Chapter 24: Problem 91

Which of the following peptides have a net positive charge at $\mathrm{pH} 7 ?$ (a) Asn-His-Arg, (b) Glu-Lys-Phe, (c) Thr-Leu-Ala.

Short Answer

Expert verified
Only peptide (a) Asn-His-Arg has a net positive charge at pH 7, with a net charge of +2.

Step by step solution

01

Identify the charges of amino acids at pH 7

The three peptides given in the prompt contain the following amino acids: (a) Asn: Neutral, His: Basic, Arg: Basic (b) Glu: Acidic, Lys: Basic, Phe: Neutral (c) Thr: Neutral, Leu: Neutral, Ala: Neutral
02

Calculate the net charge of each peptide.

Now, we will calculate the net charge of each peptide by summing up the charges on their constituent amino acids at pH 7: (a) Asn: 0, His: +1, Arg: +1; Net charge: 0 + 1 + 1 = +2 (b) Glu: -1, Lys: +1, Phe: 0; Net charge: -1 + 1 + 0 = 0 (c) Thr: 0, Leu: 0, Ala: 0; Net charge: 0 + 0 + 0 = 0
03

Determine which peptides have a net positive charge at pH 7.

Comparing the net charges of the peptides, we can see the following: (a) Asn-His-Arg has a net charge of +2, which is positive. (b) Glu-Lys-Phe has a net charge of 0, which is not positive. (c) Thr-Leu-Ala has a net charge of 0, which is not positive. Based on this analysis, only peptide (a) Asn-His-Arg has a net positive charge at pH 7.

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