Chapter 3, Problem 1RE

RECALL How do D-amino acids differ from L-amino acids? What biological roles are played by peptides that contain D-amino acids?

Interpretation Introduction

Interpretation:

The difference between d-amino acid and l-amino acid, and the biological role played by peptides that contain d-amino acids is to be explained.

Concept information:

Amino acids are the building blocks of proteins.

These are more than 900 amino acids, but among them only 20 amino acids are found in proteins.

The structure of amino acids contains a carboxyl group and an amino group, both bonded to the $\alpha -$ carbon. The α-carbon is also bonded to a hydrogen atom and a R (side chain) group. The general structure of amino acids is as follows:

Answer to Problem 1RE

The stereochemistry of d-amino acids and l-amino acids are different around the α-carbon of the amino acid structure, and the peptide that contains d-amino acids protects us from bacterial infection.

Explanation of Solution

All amino acids except glycine have two forms. These two forms of the same amino acid are referred to as stereoisomers. Amino acids having an active chiral carbon (carbon bonded with four different groups) show stereoisomerism. These stereoisomers are referred to as d-amino and l-amino acids.

d-amino acids have the amino acid group on the right side of the α-carbon, whereas l-amino acids have the amino acid group on the left side of the $\alpha -$ carbon.

The l-form and d-form of amino acid are as follows:

l-Amino acid is the most common form of amino acid which occur in proteins. d-Amino acids are not common in proteins, but are found in bacterial cell wall, and also used as antibiotics which indicates that it plays a role in the protection from bacterial infections.

Conclusion

The difference between l- and d- amino acids is due to the position of the amino acid group around the $\alpha -$ carbon of the amino acid. Moreover, d-amino acids are used against bacterial infections by incorporating them in antibiotics.