Problem 61 - Chemistry: The Central Science in SI Units, Global Edition

Question

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

Step-by-Step Solution

Answer

The two possible heterodimeric dipeptides formed by condensation reactions between glycine (Gly) and alanine (Ala) are: 1. Glycine-Alanine (Gly-Ala): \(H_2N-CH_2-CO-NH-CH(CH_3)-COOH\) 2. Alanine-Glycine (Ala-Gly): \(H_2N-CH(CH_3)-CO-NH-CH_2-COOH\)

Step 1: Draw the structures of glycine and alanine.

First, draw the structures of both glycine and alanine, which are the two amino acids involved in the condensation reactions. Glycine is the simplest amino acid with an H atom as its side chain and alanine has a CH3 group as its side chain. The general structure of an amino acid consists of a central carbon atom (α-carbon), an amino group (NH2), a carboxyl group (COOH), and a side chain (R). The structures of glycine and alanine are as follows: Glycine (Gly): H2N-CH2-COOH, with R = H Alanine (Ala): H2N-CH(CH3)-COOH, with R = CH3

Step 2: Perform condensation reactions between glycine and alanine.

In a condensation reaction between two amino acids, the amino group of one amino acid reacts with the carboxyl group of the other amino acid, resulting in the loss of water (H2O) and the formation of a peptide bond. There are two possible condensation reactions between glycine and alanine: 1. Glycine-Alanine (Gly-Ala): The amino group on glycine reacts with the carboxyl group on alanine. 2. Alanine-Glycine (Ala-Gly): The amino group on alanine reacts with the carboxyl group on glycine.

Step 3: Draw the structures of the resulting heterodimeric dipeptides.

Now, we'll draw the structures of the two heterodimeric dipeptides formed by the condensation reactions between glycine and alanine, as described in Step 2. 1. Glycine-Alanine (Gly-Ala): The dipeptide structure resulting from the condensation reaction between the amino group on glycine and the carboxyl group on alanine is: H2N-CH2-CO-NH-CH(CH3)-COOH 2. Alanine-Glycine (Ala-Gly): The dipeptide structure resulting from the condensation reaction between the amino group on alanine and the carboxyl group on glycine is: H2N-CH(CH3)-CO-NH-CH2-COOH These are the two possible heterodimeric dipeptides formed by condensation reactions between glycine and alanine.

Key Concepts

Condensation Reaction

Condensation reactions are fundamental processes in organic chemistry, particularly in the formation of peptides in biochemical systems. This type of reaction involves the joining of two molecules with the elimination of a small molecule, often water (H₂O). In the context of forming dipeptides, such as those from glycine and alanine, condensation involves connecting two amino acids.

• One amino acid's amino group (\(NH_2\))
• Another amino acid's carboxyl group (\(COOH\))

These groups join together to form a peptide bond. During this process, a molecule of water is released.
The reaction results in the formation of a bond known as an amide bond, which is stable and forms the backbone of protein structures.
For example, when pairing glycine and alanine, the two possible dipeptides are glycine-alanine and alanine-glycine. These depend on which amino group's NH₂ joins with which carboxyl group's COOH.

Glycine

Glycine is the simplest amino acid, which is characterized by having a single hydrogen atom as its side chain. This makes it unique among the 20 standard amino acids. Glycine's simple structure is expressed chemically as \(H_2N-CH_2-COOH\), where:

• The amino group is \(H_2N\)
• The central \(\alpha\)-carbon is attached to a single hydrogen
• The carboxyl group is \(COOH\)

This minimal side chain allows glycine a great deal of flexibility within protein structures. Its small size enables it to fit into tight spaces in protein structures, often playing a critical role in the flexibility and folding of polypeptides.
In peptide formation, glycine's small size and simple structure allow it to act as a common linker when it's involved in condensation reactions with other amino acids like alanine.

Alanine

Alanine is a small, non-polar amino acid that plays a significant role in protein structure due to its methyl side chain (\(CH_3\)). This side chain gives alanine its characteristically hydrophobic properties, affecting how proteins fold and interact in aqueous environments.
Chemically, alanine is expressed as \(H_2N-CH(CH_3)-COOH\), where:

• The amino group is \(H_2N\)
• Attached to the central \(\alpha\)-carbon
• A carboxyl group \(COOH\)
• A methyl group \(CH_3\) constitutes its side chain

The presence of the \(CH_3\) group makes alanine slightly more bulky than glycine, but its small size still allows it to be introduced into peptide structures without significant steric hindrance.
When alanine undergoes condensation reactions to form dipeptides such as alanine-glycine or glycine-alanine, it helps to stabilize protein structures due to its non-reactive, non-polar side chain that can interact with other hydrophobic units.