Question

A pentapeptide on complete hydrolysis yields 3 moles of glycine, 1 mole of alanine, and 1 mole of phenylalanine. Among the products of partial hydrolysis are found \(\mathrm{H} \cdot \mathrm{Ala} \cdot \mathrm{Gly} \cdot \mathrm{OH}\) and \(\mathrm{H} \cdot \mathrm{Gly} \cdot \mathrm{Ala} \cdot \mathrm{OH}\). What structures are possible for this substance on the basis of its giving no nitrogen in the Van Slyke determination?

Short answer

Considering the given conditions, there are two possible structures for the pentapeptide: 1. Gly-Gly-Ala-Gly-Phe 2. Gly-Gly-Gly-Ala-Phe These structures fit the requirements of no nitrogen detection in the Van Slyke determination and account for all amino acids and partial hydrolysis products given in the exercise.

Step-by-step solution

Understand the given information

We know there are a total of 5 amino acids in the pentapeptide - 3 Gly, 1 Ala, and 1 Phe. Two partial hydrolysis products are H·Ala·Gly·OH and H·Gly·Ala·OH. The substance doesn't produce any nitrogen in the Van Slyke determination, which means that there is no free amino group (-NH_2) at the N-terminal of the peptide.

Determine the N-terminal amino acid of the pentapeptide

Since no nitrogen is detected in the Van Slyke determination, the N-terminal amino acid can't have a free -NH_2 group. This suggests that the N-terminal amino acid must be one of the 3 moles of Glycine as it's the only amino acid that can create a residue without a free -NH_2 group when it forms a peptide bond with another Glycine. In other words, the N-terminal end must be Gly-Gly.

Formulate the possible structures considering the partial hydrolysis products

Since we know two partial hydrolysis products: H·Ala·Gly·OH and H·Gly·Ala·OH, it indicates that Gly and Ala residues can interchange positions in the middle of the peptide chain, while preserving the N-terminal Gly-Gly bond. Therefore, consider the following order of amino acids in the peptide chain: 1. Gly-Gly-Ala-Gly-Phe 2. Gly-Gly-Gly-Ala-Phe Both of these possible structures start with the N-terminal Gly-Gly and incorporate all 3 Gly residues, as well as 1 Ala and 1 Phe residue.

Provide the final possible pentapeptide structures

Considering the conditions given in the exercise, there are two possible structures for the pentapeptide: 1. Gly-Gly-Ala-Gly-Phe 2. Gly-Gly-Gly-Ala-Phe Note that these structures fit the requirements of no nitrogen detection in the Van Slyke determination and account for all amino acids and partial hydrolysis products given in the exercise.

Key concepts

Amino Acid Analysis

Amino acid analysis is a crucial first step in understanding peptide structure. By breaking down a peptide into its individual amino acids, we can identify the building blocks that make up the peptide chain.
This method involves complete hydrolysis, where the peptide bonds are broken, and the amino acids are released. For instance, a pentapeptide, when hydrolyzed, may release distinct moles of different amino acids, such as glycine, alanine, and phenylalanine, as in our example.

Once identified, these amino acids give valuable insights into the original peptide structure. For example, knowing the number of each type of amino acid helps researchers predict potential peptide arrangements. When working with a pentapeptide, understanding these individual components is essential to deduce the possible sequences they form.

Hydrolysis Products

Hydrolysis products tell a lot about the peptide sequence. Partial hydrolysis breaks down the peptide into smaller segments, providing insight into the peptide sequence without fully deconstructing it.
For example, one might encounter sequences like H·Ala·Gly·OH and H·Gly·Ala·OH in partial hydrolysis. These smaller segments help identify neighboring amino acids in the peptide chain.

They help in deducing which amino acids are likely to be next to each other. By examining these segments, one can hypothesize about different potential structures of the full peptide. For example, knowing that both Ala and Gly appear next to each other suggests they might be in close succession within the overall peptide chain.

Van Slyke Determination

Van Slyke determination is a technique used to detect free amino groups in peptides. It helps in determining whether the N-terminal amino acid has a free amino group (-NH_2).
If no nitrogen is released in the Van Slyke test, it indicates the absence of a free terminal amino group. This was the case with the pentapeptide, where no nitrogen detection implied no free amino groups at the N-terminal.

This information narrows down the possibilities of peptide sequences. For instance, this can suggest that a glycine residue may be forming a peptide bond without a free amino group, such as in cases where Gly-Gly ends occur. Understanding this test is essential for determining the probable sequence starting points in peptide analysis.

Peptide Bond Formation

Peptide bond formation is a fundamental concept in building peptide chains. These bonds link amino acids through a reaction between the amino group of one amino acid and the carboxyl group of another, releasing water.
In our pentapeptide analysis, the significance of peptide bond formation is seen in the arrangement of amino acids. Knowing that glycine can appear multiple times, it can form bonds creating sequences like Gly-Gly, affecting the order and structure possibilities.

Understanding peptide bonds help establish how amino acids link and are arranged within peptides. This knowledge is essential to predict primary peptide structures and validate them against known hydrolysis products and analytical results, forming the basis of determining the complete peptide arrangement.