Question

Consider the following statements about amino acids: (1) nitrous acid liberates nitrous oxide from amino acids (2) an important sensitive test for the detection of amino acids is the ninhydrin colour test (3) glycine is the only amino acid which does not possess a chiral centre. (4) all the natural amino acids that occur as constituents of proteins belong to the L-series Which of these statements are correct? (a) 1,2 and 3 (b) 1,2 and 4 (c) 2,3 and 4 (d) 2 and 3 only

Short answer

(c) 2, 3, and 4

Step-by-step solution

Analyze Statement 1

The statement claims that nitrous acid liberates nitrous oxide from amino acids. However, the reaction of nitrous acid with amino acids typically results in deamination, releasing nitrogen gas rather than nitrous oxide. Hence, statement 1 is incorrect.

Analyze Statement 2

Statement 2 states that ninhydrin is an important test for detecting amino acids. Ninhydrin reacts with amino acids to produce a purple-blue color, known as Ruhemann's purple. This is indeed a sensitive test for amino acids, making statement 2 correct.

Analyze Statement 3

Statement 3 asserts that glycine is the only amino acid without a chiral center. Glycine has two hydrogen atoms attached to its central carbon, making it achiral. Other amino acids have four different groups around the central carbon, giving them chirality. Therefore, statement 3 is correct.

Analyze Statement 4

Statement 4 claims that all natural amino acids in proteins belong to the L-series. This is true for proteinogenic amino acids as they are all L-enantiomers, making statement 4 correct.

Determine Correct Options

Since statements 2, 3, and 4 are correct, the correct option is (c) 2, 3, and 4.

Key concepts

Chirality

Amino acids are known for their role in forming proteins, and a fascinating aspect of their structure is chirality. Chirality means that a molecule exists in two forms that are mirror images of each other, just like your left and right hands. This occurs when a carbon atom in the molecule is bound to four different atoms or groups. Due to this unique arrangement, two forms or enantiomers can exist. One is called the D-form and the other the L-form. Although they have the same chemical composition, these enantiomers can have vastly different properties in biological systems.

• Most amino acids used by living organisms are found in the L-form, meaning their mirror image (D-form) is rarely found in proteins.
• Chirality is crucial because it affects how amino acids, and consequently proteins, interact with other biological molecules.
• In proteins, the specific 3D structure is essential for their function, and even a small change in chirality can disrupt this and lead to malfunction.

This molecular handedness is a key concept in biochemistry and essential for the proper functioning of many biological processes.

Ninhydrin Test

The ninhydrin test is an important chemical test commonly used to detect amino acids and proteins. When amino acids react with ninhydrin, they form a colored complex known as Ruhemann's purple. This reaction is highly sensitive and can detect even minute quantities of amino acids.

• The process involves ninhydrin reacting with the *alpha*-amino group of free amino acids.
• As a result, a deep blue or purple-colored compound forms, which is visible and can be quantified.
• Sometimes, different amino acids might produce slightly varied coloration, making the test useful in both detection and differentiation.

This test is widely used in laboratories, including those involved in criminology for fingerprint detection and in food industry quality control, ensuring the presence and concentration of proteins are within required limits.

L-series

In the realm of biochemistry, the L-series refers to the configuration of amino acids that are naturally incorporated into proteins. This is part of a naming convention known as the Fischer projection, which categorizes molecules based on the orientation of atoms around a central carbon atom.

• All naturally occurring amino acids in proteins, termed "proteinogenic amino acids," belong to the L-series.
• The notation does not stand for Left or Right but signifies the relative spatial positioning derived from glyceraldehyde.
• This consistent use of L-amino acids in nature helps ensure that proteins can fold into their correct functional forms, allowing cells to perform necessary life processes efficiently.

Nature's preference for L-amino acids is a fascinating evolutionary trait, ensuring uniformity and compatibility across all living organisms. Understanding this concept is essential for grasping how proteins are assembled and function at a molecular level.