Question

Why is glycine not optically active?

Short answer

Glycine is not optically active because its α-carbon has only three different groups attached to it, as the R group is a hydrogen atom identical to the hydrogen already attached. Thus, glycine lacks a chiral center as it cannot form non-superimposable mirror image enantiomers that interact differently with polarized light.

Step-by-step solution

Understand chirality and optical activity

Chirality is a property of molecules where a molecule and its mirror image (enantiomers) are non-superimposable. Chiral molecules form pairs of enantiomers that have different shapes and interact differently with polarized light. If a molecule has at least one chiral center or stereocenter (usually a carbon atom with four different substituents), it can be optically active. Optically active substances rotate the plane of polarized light either to the left (levorotatory) or to the right (dextrorotatory).

Examine the structure of glycine

Glycine is an amino acid with the chemical formula NH2-CH2-COOH. The central carbon atom (called the α-carbon) is bonded to four different groups: an amino group (NH2), an alpha-carboxyl group (COOH), a hydrogen atom (H), and a lone hydrogen as the R group. Here, the R group is the side chain unique to each amino acid.

Determine if glycine has a chiral center

As previously mentioned, a molecule is chiral if the central carbon atom is connected to four different groups. For glycine, its α-carbon has the following attached groups: 1. Amino group (NH2) 2. Carboxyl group (COOH) 3. Hydrogen atom (H) 4. R group (also a hydrogen atom, H) Since both group 3 and group 4 are identical hydrogen atoms, the α-carbon of glycine is not connected to four different groups.

Conclude if glycine is optically active

Given that glycine's α-carbon is not connected to four different groups and does not have a chiral center, it does not exhibit chirality. Consequently, glycine is not optically active because it cannot form enantiomers that interact differently with polarized light.

Key concepts

Chirality

Chirality plays a crucial role in chemistry and biochemistry, affecting how molecules behave and interact with each other. A molecule is considered to be chiral if it is not superimposable on its mirror image—just like how your left hand is not superimposable on your right hand. This property arises when a molecule has an asymmetrical arrangement of atoms. To visualize chirality, imagine a molecule and its mirror image; if you cannot align all the parts of both molecules, despite rotating them in space, they are chiral. This is important, as chiral molecules can have different biological effects and chemical properties.

Chirality in Nature

Many biological molecules are chiral, including amino acids, sugars, and DNA. In nature, chirality can determine the smell of molecules, the way drugs interact with our body, and even the taste of food. For instance, one enantiomer of limonene smells like oranges, while the other smells like lemons.

Chirality and Optical Activity

A direct consequence of chirality is optical activity, where a chiral molecule can rotate the plane of polarized light. Optical activity is measured using a polarimeter, and the direction and degree of rotation can provide valuable information about the structure of the molecule.

Enantiomers

Enantiomers are a set of molecules that are mirror images of each other but cannot be superimposed, much like left and right hands. These are a type of stereoisomers—molecules with the same molecular formula and sequence of bonded atoms (constitution), but with a different three-dimensional orientation of their atoms in space. Enantiomers arise due to the presence of stereocenters in molecules, typically carbon atoms with four different substituents.

Distinguishing Enantiomers

Enantiomers, while similar, often have drastically different biological activities. A famous example is the drug thalidomide, where one enantiomer was a sedative and the other caused birth defects. This difference in biological activity is due to the specific three-dimensional shape of the enantiomer and how it interacts with biological targets like enzymes.

Identifying Chiral Centers

To identify enantiomers, chemists look for chiral centers within molecules. If a molecule has one or more chiral centers, it is likely to have enantiomers. The presence of enantiomers also imparts optical activity to compounds, which can be studied using techniques like polarimetry.

Glycine

Glycine is one of the twenty standard amino acids used to build proteins in organisms. It is unique because its side chain consists of only a hydrogen atom, which is why it is the simplest amino acid. This simplicity results in glycine lacking a chiral center, setting it apart from other amino acids that typically have four distinct groups attached to their α-carbon atoms.

Glycine's Structure and Implications

Due to its side chain being a hydrogen, which is the same as its α-hydrogen, glycine's α-carbon is not a stereocenter. This means that glycine cannot have enantiomers and is not optically active. The inability of glycine to rotate polarized light makes it achiral, which has implications in its role in proteins. Being symmetrical, glycine can fit into tight places within protein structures where other, bulkier, amino acids might not, thus influencing the overall shape and function of proteins.

Stereocenter

A stereocenter, also known as a chiral center, is an atom that bears groups such that an interchange of any two groups will produce a stereoisomer. This makes the stereocenter a focal point for understanding the three-dimensional arrangement of atoms within a molecule. While stereocenters are often tetrahedral carbon atoms with four different substituents, they can also be other types of atoms like nitrogen, sulfur, or phosphorus.

Significance of Stereocenters in Molecules

For a molecule to exhibit chirality and consequently, optical activity, it must have at least one stereocenter. However, the presence of a stereocenter doesn't always mean that a molecule will be chiral; molecules with more than one stereocenter can be achiral if they have planes of symmetry.

Role of Stereocenters in Drug Design

The presence or absence of stereocenters greatly affects the way a drug interacts with biological targets. As a result, understanding and manipulating stereocenters is an essential aspect of pharmaceutical chemistry, given that the right enantiomer can mean the difference between a safe, effective drug and one that is harmful.