Question

Which of the following amino acids does not have a chiral carbon? (a) glycine; (b) alanine; (c) threonine; (d) lysine; (e) none of these.

Short answer

The amino acid which does not have a chiral carbon is glycine.

Step-by-step solution

Understanding the Structure of Glycine

Glycine is the simplest amino acid. Its R group is a hydrogen atom, which is identical to the other hydrogen atom already attached to the alpha carbon. Therefore, glycine does not have a chiral carbon.

Analyzing Alanine, Threonine, and Lysine

Unlike glycine, the R groups of alanine (methyl group), threonine (hydroxyl group), and lysine (amino group) are not identical to any of the other groups attached to the alpha carbon. Therefore, all three of these amino acids have a chiral carbon.

Conclusion based on Steps 1 and 2

Based on the analysis of each amino acid, it can be concluded that the only amino acid from the provided list which does not have a chiral carbon is glycine.

Key concepts

Glycine Structure

When examining amino acids, their unique structures dictate their properties and functionalities in chemistry and biochemistry. Glycine, the simplest amino acid, stands out because of its side chain, which is a single hydrogen atom. Uniquely, this hydrogen side chain is identical to another hydrogen already attached to the alpha carbon, the central carbon in the amino acid structure.

Since a chiral carbon requires four different groups attached to it, glycine does not qualify as a chiral molecule. Its symmetrical structure means it is achiral; when placed in a symmetrical environment, it does not produce a mirror image that is non-superimposable. This distinctive feature of glycine contributes to its less constrained structural role in proteins, allowing tight packing and flexibility in protein chains.

Amino Acid Stereoisomerism

Stereoisomerism in amino acids plays a pivotal role in their biological functionality. The term 'stereoisomerism' refers to the three-dimensional arrangement of atoms within a molecule which can exist in multiple forms. For amino acids, this is typically due to the existence of a chiral carbon, an asymmetric carbon atom with four distinct groups attached to it.

Except for glycine, all standard amino acids have at least one chiral carbon, leading to two potential configurations: L (levo) and D (dextro). In nature, almost all amino acids incorporated into proteins are in the L-configuration, highlighting the significance of chirality in biological systems. The subtle differences in spatial arrangement between these stereoisomers can lead to dramatic differences in the way they interact with other biological molecules, influencing processes from enzyme function to drug interactions.

Chirality in Chemistry

The concept of chirality is crucial to understanding the diverse behaviors of molecules in chemistry. A chiral molecule is one that has a non-superimposable mirror image, often compared to the relationship between left and right hands. This property arises when a molecule has an asymmetric carbon atom—a carbon atom bound to four distinct groups. Chirality is not limited to amino acids and is a fundamental concept across various fields of chemistry including pharmaceuticals, where the chirality of a drug can affect how it interacts with the body.

Chiral molecules are optically active, meaning they rotate the plane of polarized light, an effect used to distinguish between enantiomers—two isomers that are mirror images of each other. Chirality is integral to the study of enantiomerism and the influence of molecular asymmetry on the physical and chemical properties of organic compounds.