Problem 61 - Chemistry The Central Science In Si Units

Question

Draw the two possible heterodimeric dipeptides formed by condensation reactions between glycine and alanine.

Step-by-Step Solution

Answer

The two dipeptides are Gly-Ala and Ala-Gly.

Step 1: Understand the Components

We have to consider two amino acids, glycine (Gly) and alanine (Ala). Glycine has the structure of R = H while alanine has R = CH₃. Each amino acid has an amino group (-NH₂) and a carboxyl group (-COOH).

Step 2: Concept of Dipeptide Formation

A dipeptide is formed when two amino acids are joined together through a condensation reaction, which involves the elimination of a water molecule (H₂O). The carboxyl group of one amino acid reacts with the amino group of another.

Step 3: Forming the First Dipeptide: Gly-Ala

The first dipeptide, glycine-alanine (Gly-Ala), is formed by joining the carboxyl group of glycine (Gly) to the amino group of alanine (Ala). The resulting structure is: Gly-Ala: NH₂-CH₂-CO-NH-CH(CH₃)-COOH.

Step 4: Forming the Second Dipeptide: Ala-Gly

The second dipeptide, alanine-glycine (Ala-Gly), is formed by joining the carboxyl group of alanine (Ala) to the amino group of glycine (Gly). The resulting structure is: Ala-Gly: NH₂-CH(CH₃)-CO-NH-CH₂-COOH.

Step 5: Confirm the Condensation Reaction

In both cases, a water molecule (H₂O) is removed to form the peptide bond between the two amino acids. The general formula for the water loss is: -COOH + -NH₂ → -CO-NH- + H₂O.

Key Concepts

Condensation Reaction

A condensation reaction is a chemical process where two molecules combine, resulting in the loss of a small molecule like water. In biology, this type of reaction is essential for forming complex biomolecules, such as proteins. When it comes to forming dipeptides from amino acids, consecutive condensation reactions play a significant role. Here, the carboxyl group (-COOH) of one amino acid interacts with the amino group (-NH₂) of another.

This reaction is crucial for linking amino acids together to build proteins. During the process, a water molecule (H₂O) is removed as the two amino acids combine via a peptide bond. This bond formation is central to dipeptide synthesis where either amino acid can act as the starting material.

• The carboxyl group donates an -OH.
• The amino group donates a hydrogen atom.
• This forms the peptide bond (-CO-NH-), commonly found in proteins.

The removal of water and the creation of this strong covalent bond successfully join the amino acids glycine and alanine, leading to the formation of dipeptides.

Glycine and Alanine

Glycine and alanine are the simplest types of amino acids, both sharing common features like the amino group (-NH₂) and carboxyl group (-COOH).

Glycine, or Gly, is special for being the smallest amino acid. Its side chain is just a hydrogen atom, represented as R = H. This minimalistic structure allows it more flexibility within the polypeptide chain, playing crucial roles in stabilizing proteins.

• Structure for Glycine: NH₂-CH₂-COOH.
• Simple and small, enhancing protein flexibility.

Alanine, abbreviated as Ala, has a slightly larger side chain with R = CH₃. This methyl group provides a bit more bulk compared to glycine, affecting the hydrophobic interactions in proteins.

• Structure for Alanine: NH₂-CH(CH₃)-COOH.
• Contributes to the folding and stability of protein structures.

When glycine and alanine react, they can form dipeptides in two different sequences: Gly-Ala or Ala-Gly. These unique formations alter the properties and functions of the resulting dipeptide.

Peptide Bond

The peptide bond is a crucial link in protein chemistry. It is formed during a condensation reaction between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another.

This bond is strong and covalent, contributing significantly to the secondary and tertiary structures of proteins. During dipeptide formation, each peptide bond adds stability to the compound.

• The formation of a peptide bond results in water being expelled.
• The bond itself is characterized by the linkage: -CO-NH-.

In glycine and alanine, this transformation occurs twice, resulting in formations such as the Gly-Ala and Ala-Gly dipeptides. These bonds not only bridge amino acids together but also create the foundation for longer protein chains.

Additionally, the peptide bond is planar and exhibits resonance, lending it partial double-bond character, which prevents rotation around the bond axis. This feature is critical for the fixed structure and function of proteins, becoming an indispensable part of molecular biology.