Problem 91 - Chemistry: The Central Science

Question

Which of the following peptides have a net positive charge at pH 7 (a) Gly- Ser-Lys, (b) Pro-Leu-lle, (c) Phe-Tyr-Asp.

Step-by-Step Solution

Answer

Only peptide (a) Gly-Ser-Lys has a net positive charge at pH 7.

Step 1: Peptide (a) - Gly-Ser-Lys

1. Glycine (Gly) has a neutral side group, so it does not contribute any charge at pH 7. 2. Serine (Ser) has a neutral side group, so it also does not contribute any charge at pH 7. 3. Lysine (Lys) has a basic side group, which is positively charged at pH 7. The net charge for Gly-Ser-Lys at pH 7 is +1.

Step 2: Peptide (b) - Pro-Leu-lle

1. Proline (Pro) has a neutral side group and does not contribute any charge at pH 7. 2. Leucine (Leu) has a neutral side group and does not contribute any charge at pH 7. 3. Isoleucine (Ile) has a neutral side group and also does not contribute any charge at pH 7. The net charge for Pro-Leu-Ile at pH 7 is 0.

Step 3: Peptide (c) - Phe-Tyr-Asp

1. Phenylalanine (Phe) has a neutral side group and does not contribute any charge at pH 7. 2. Tyrosine (Tyr) has a neutral side group and also does not contribute any charge at pH 7. 3. Aspartic acid (Asp) has an acidic side group, which is negatively charged at pH 7. The net charge for Phe-Tyr-Asp at pH 7 is -1. Out of the given peptides (a) Gly-Ser-Lys, (b) Pro-Leu-Ile, and (c) Phe-Tyr-Asp, only peptide (a) Gly-Ser-Lys has a net positive charge at pH 7.

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins and peptides. Each amino acid has a core structure consisting of an amino group (-NH\(_2\)), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain or R group that determines its properties.
Amino acids can be classified based on the nature of their side chains into various categories, such as:

• Polar and non-polar
• Acidic, basic, or neutral
• Aromatic (containing a ring structure)

Understanding the R group's nature is crucial because it affects how the amino acid behaves at different pH levels. For example, lysine has a basic R group that carries a positive charge when it is at neutral pH, while aspartic acid has an acidic side group that carries a negative charge under the same conditions.
The combination of different amino acids in a peptide chain determines the overall properties and functionality of the peptide.

Acid-Base Properties

The acid-base properties of amino acids are related to their ability to accept or donate protons (H\(^+\)). At any given pH, some amino acids can act as acids or bases, affecting the overall charge of the peptide they are a part of.
Each amino acid has specific pKa values for its functional groups. The pKa is the pH at which half of the molecules of that group are in their charged state. For instance:

• The amino group of an amino acid has a pKa around 9-10, meaning it is usually protonated and carries a positive charge at physiological pH (around 7).
• The carboxyl group has a pKa around 2-3 and is usually deprotonated, thus carrying a negative charge at physiological pH.

The side chains of some amino acids, such as lysine and aspartic acid, have their own pKa. These side chains can gain or lose protons, contributing to the overall charge of a peptide.
By analyzing these acid-base properties in context with the environment's pH, we can predict how the amino acids and the peptides they form will behave in terms of their charges.

Peptide Net Charge

To determine the net charge of a peptide at a specific pH, we need to consider the charges of all ionizable groups within the peptide chain. At pH 7, the charges of the amino acid side chains and the peptide ends will determine the overall net charge.
Here's how we can determine the net charge of a peptide:

• Identify the charged components: terminal ends and any charged side chains.
• Consider that the N-terminus (amino end) is usually positively charged, and the C-terminus (carboxyl end) is often negatively charged at pH 7.
• Evaluate the side chains: basic side chains like lysine (K) and arginine (R) will be positively charged, while acidic side chains like aspartic acid (D) and glutamic acid (E) will be negatively charged.

For example, the peptide Gly-Ser-Lys at pH 7 has lysine's positively charged side chain, resulting in a net positive charge. This contrast with Phe-Tyr-Asp, where aspartic acid’s side chain is negatively charged, leading to a net negative charge for the peptide.
Understanding the net charge helps in predicting peptide solubility, interaction with other molecules, and behavior in different environments.