Question

Which one of the following statements is correct? \([2012]\) (a) All amino acids except glycine are optically active (b) All amino acids are optically active (c) All amino acids except lysine are optically active (d) All amino acids except glutamic acid are optically acitve

Short answer

Option (a) is correct; all amino acids except glycine are optically active.

Step-by-step solution

Understanding Optically Active Amino Acids

Amino acids are considered optically active if they have chiral centers, which means they have four different groups attached to a central carbon atom. Glycine, however, has two hydrogen atoms attached to its central carbon, making it non-chiral and therefore not optically active.

Analyzing Each Option

- Option (a) states that all amino acids except glycine are optically active. This is true because glycine is the only amino acid without a chiral center. - Option (b) suggests all amino acids are optically active, which is incorrect as glycine is not optically active. - Option (c) and (d) suggest specific amino acids (lysine and glutamic acid, respectively) are not optically active. Both these amino acids have chiral centers, making them optically active, so options (c) and (d) are incorrect.

Key concepts

Chirality in Chemistry

In chemistry, chirality refers to the geometric property where a molecule and its mirror image are non-superimposable. Think of it like your left and right hands, which are mirror images but cannot be perfectly aligned when placed one on top of the other. This characteristic is crucial in understanding the behavior of molecules in biological systems. Chiral molecules have a central atom bonded to four different groups, making them asymmetric.

This lack of symmetry is what makes the molecule chiral, and it becomes optically active, meaning it can rotate plane-polarized light. Chirality is significant in chemistry because it can influence how a substance interacts with biological molecules, which are often chiral themselves. In drug development, for instance, the chirality of the compound can greatly affect how it works in the body.

Amino Acids

Amino acids are the building blocks of proteins and play a vital role in almost all biological processes. They not only construct proteins but also have roles in metabolism and neurotransmission. There are 20 standard amino acids, each with a unique side chain that defines its properties and function.

Most amino acids are chiral, meaning they have a central carbon (the alpha carbon) bonded to four different groups: an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group). This chiral nature leads them to exhibit optical activity. However, glycine is the exception. It has a hydrogen atom as its side chain, resulting in two hydrogen atoms attached to the alpha carbon. This makes glycine achiral, and subsequently, not optically active.

Optically Active Compounds

Optically active compounds have the ability to rotate plane-polarized light. This property occurs due to the presence of chiral centers, which are carbon atoms attached to four different groups, forming an asymmetric structure. When light passes through a solution containing these molecules, the light is rotated, which can be measured with a polarimeter.

Understanding optical activity is important in various fields, such as pharmacology, where the optical isomers, or enantiomers, of a compound can have different therapeutic effects. In the context of amino acids, if an amino acid has a chiral center, it will generally be optically active. Glycine, though, is not optically active due to its lack of chirality, making it the sole outlier among standard amino acids.