Problem 91 - Chemistry: The Central Science in SI Units, Global Edition

Question

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.

Step-by-Step Solution

Answer

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

Step 1: 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

Step 2: 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

Step 3: 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.

Key Concepts

Amino Acid Charge

Amino acids are the building blocks of peptides and proteins. Each amino acid consists of an amino group, a carboxyl group, and a side chain known as the R group. The charge of an amino acid is influenced by its side chain, which can be acidic, basic, or neutral.

In general, amino acids can be classified into three categories based on the nature of their side chains:

• Acidic amino acids (e.g., Glutamic acid, Aspartic acid) have side chains that can donate a proton, resulting in a negative charge.
• Basic amino acids (e.g., Lysine, Arginine, Histidine) have side chains that can accept a proton, leading to a positive charge.
• Neutral amino acids (e.g., Alanine, Phenylalanine, Threonine) generally do not have a charge under physiological pH conditions.

For example, in the peptide Asn-His-Arg, Histidine (His) and Arginine (Arg) have basic side chains, making them positively charged. Understanding these properties is crucial to predicting the overall charge of peptides.

pH Level Effects

The pH level of a solution significantly impacts the overall charge of amino acids and peptides. Each amino acid has a unique pKa value, which determines the pH at which the molecule exists in equilibrium between its protonated and deprotonated forms.

Here is how pH affects amino acids:

• At low pH (acidic conditions), more protonation occurs, making amino acids more positively charged.
• At high pH (basic conditions), deprotonation occurs, resulting in amino acids becoming more negatively charged or neutral.
• At neutral pH (around 7), acidic side chains might lose protons (negative charge), while basic side chains might gain protons (positive charge).

For instance, at pH 7, Histidine can be positively charged because its pKa value is close to the pH, allowing it to remain protonated. In contrast, Glutamic acid will likely be deprotonated, reflecting a negative charge. Recognizing these changes is key to assessing how a peptide's charge shifts with varying pH levels.

Peptide Charge Calculation

To determine the net charge of a peptide at a given pH, it is essential to calculate the sum of individual amino acid charges. This process involves identifying if each amino acid in a peptide is neutral, negatively charged, or positively charged at that specific pH.

Here's a step-by-step guide:

• List the sequence of amino acids in the peptide.
• Identify the charge each amino acid contributes at the given pH, based on their side chain properties.
• Add the individual charges to find the peptide's total net charge.

Let's explore this with an example: the peptide Asn-His-Arg at pH 7.

• Asparagine (Asn) is neutral, contributing 0 to the charge.
• Histidine (His), a basic amino acid, contributes a +1 charge.
• Arginine (Arg), another basic amino acid, also contributes a +1 charge.

Thus, the net charge at pH 7 is 0 (Asn) + 1 (His) + 1 (Arg) = +2. This calculation confirms that the peptide Asn-His-Arg has a net positive charge. Understanding how to calculate peptide charges helps in predicting the behavior of peptides in different environments, which is crucial for biological and biochemical applications.