Question

Which of the following is correct about $\alpha$-amino acids?

• A. All $\alpha$-amino acids contain an asymmetric carbon atom
• B. In nature, they are always present in optically active form
• C. They are always exist in L forms
• D. In nature, they are present in both L and D forms

Hint:

In nature, amino acids primarily exist in the L-form, and only the L-forms are incorporated into proteins. However, D-forms of some amino acids are found in certain biological systems, like bacterial cell walls.

Answer 1

The Correct Option is D

$\alpha$-amino acids are the building blocks of proteins and contain a central carbon atom bonded to an amino group (-NH$_2$), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group).
1. Option (1) is incorrect. Not all $\alpha$-amino acids contain an asymmetric carbon atom. For example, glycine, the simplest $\alpha$-amino acid, does not have an asymmetric carbon.
2. Option (2) is also incorrect. While $\alpha$-amino acids are optically active when they contain an asymmetric carbon, not all of them exist in optically active forms in nature.
3. Option (3) is incorrect because $\alpha$-amino acids exist in both L and D forms in nature. However, L-form amino acids are predominantly used in the synthesis of proteins.
4. Option (4) is correct. $\alpha$-amino acids exist in both L- and D-forms. The L-form is primarily found in nature, especially in proteins, while the D-form can be found in some bacterial cell walls.
Thus, the correct answer is option (4): In nature, they are present in both L and D forms.