Problem 18 - Basic Principles of Organic Chemistry

Question

The tripeptide, eisenine, has only one free carboxyl group, does not react with 2,4 -dinitrofluorobenzene, and on complete hydrolysis yields 2 moles of \(L\) -glutamic acid, 1 mole of \(L\) -alanine, and 1 mole of ammonia. Alanine is indicated to be the C-terminal amino acid. Write a structure for eisenine that is in accord with the above facts.

Step-by-Step Solution

Answer

Eisenine is likely a pyroglutamic acid derivative with glutamic acid - alanine sequence.

Step 1: Analyze the Components

Eisenine is a tripeptide composed of amino acids. We start by identifying these amino acids: 2 moles of L-glutamic acid, 1 mole of L-alanine, and 1 mole of ammonia. Alanine is noted as the C-terminal.

Step 2: Identify the C-terminal

The information specifies that alanine is the C-terminal amino acid, which means that alanine is the last amino acid in the sequence and has the free carboxyl (-COOH) group.

Step 3: Understand the Implications of Ammonia

Eisenine's release of 1 mole of ammonia suggests the presence of a blocked N-terminal amino acid. A blocked N-terminus could mean the amino group is modified or part of a heterocycle, preventing the typical reaction with 2,4-dinitrofluorobenzene.

Step 4: Assemble the Peptide Structure

To form a tripeptide chain given the components, we place an amide bond between the N-terminal-modified amino group of L-glutamic acid, another glutamic acid, and alanine. Since the chain does not react with 2,4-dinitrofluorobenzene, it is likely that the N-terminal of glutamic acid is part of a cyclic structure that includes ammonia.

Step 5: Construct a Hypothetical Structure

Given the data, a plausible structure for eisenine is cyclic between the alpha-amino group of the L-glutamic acid and its side-chain carboxyl group, forming a pyroglutamic acid. This leaves the free carboxyl group of the C-terminal alanine exposed.

Key Concepts

Tripeptide Structure

In organic chemistry, a tripeptide is a peptide consisting of three amino acids linked together by peptide bonds. Each amino acid consists of an amino group (\(-NH_2\)), a carboxyl group (\(-COOH\)), and a unique side chain. The sequence in which these amino acids are linked is crucial, as it defines the peptide's properties and function.

In the case of the tripeptide eisenine, it is composed of two molecules of \(L\)-glutamic acid and one molecule of \(L\)-alanine. The unique arrangement involves linking these amino acids through amide bonds. An amide bond, typically known as a peptide bond, forms when the carboxyl group of one amino acid reacts with the amino group of another. This results in the release of a molecule of water (a condensation reaction).

In eisenine, the conventional linear linkage is complicated by modifications at the N-terminal. Instead of a simple linear chain, eisenine forms a cyclic structure with an amide bond which involves one of its L-glutamic acid molecules. Such structural nuances can dramatically alter the physical and chemical properties of the peptide.

Amino Acid Sequence

Amino acid sequence refers to the particular order in which amino acids are joined to form a peptide or protein. This sequence is crucial as it determines the specific structure and functionality of the molecule. Different sequences can result in peptides with vastly different properties.

For the tripeptide eisenine, the specified sequence terminates with \(L\)-alanine as the C-terminal residue. The task is to determine the order of the two \(L\)-glutamic acid residues and the implications of ammonia release. The sequence in eisenine is affected by the presence of a blocked N-terminal, restricting the modification or degradation path typical to certain reagents.

The release of ammonia suggests an alteration in the amino group of one amino acid, probably due to the incorporation into a cyclic structure. Thus, implying that one \(L\)-glutamic acid residue is modified to form a pyroglutamic structure, which affects the regular sequence recognition. This specific arrangement is essential when creating synthetic peptides for research and pharmaceutical developments.

C-terminal and N-terminal

Peptides are described by their C-terminal and N-terminal ends. The N-terminal refers to the end of the peptide chain where the amino group is free, while the C-terminal refers to the end with a free carboxyl group. These terminals are critical since they define how peptides interact chemically and biologically.

In the case of eisenine, the C-terminal is \(L\)-alanine, meaning alanine resides at the snap where the sequence ends, and its carboxyl group remains free. This confers the usual reactive characteristics of the C-terminal residues, such as participation in further peptide elongation or interactions.

However, the N-terminal in eisenine is unusual due to the structural modification. The typical N-terminal amino group is not free; instead, it suggests modification, possibly forming part of a heterocycle. This formation can inhibit usual reactions such as those with 2,4-dinitrofluorobenzene, commonly used for identifying free amino groups.

Understanding the characteristics of these terminals is vital in peptide synthesis and sequencing, playing a pivotal role in delineating function and reactivity.