Concept used: optical activity of amino acids

An amino acid will be optically active if it has a chiral carbon atom (an asymmetric carbon attached to four different groups).

General structure of an amino acid

The common structure is: $\mathrm{H_2N-CH(R)-COOH}$

The carbon marked as $\mathrm{CH}$ is the $\alpha$-carbon.

For most amino acids, this $\alpha$-carbon is attached to four different groups:

• $\mathrm{-NH_2}$
• $\mathrm{-COOH}$
• $\mathrm{-H}$
• side chain $R$

Hence, most amino acids are chiral and therefore optically active.

Exception: glycine

For glycine, the side chain is: $R = H$

So its structure becomes: $\mathrm{H_2N-CH_2-COOH}$

Here, the $\alpha$-carbon is attached to:

• $\mathrm{-NH_2}$
• $\mathrm{-COOH}$
• $\mathrm{-H}$
• another $\mathrm{-H}$

Since two groups are identical, the carbon is not chiral. Therefore, glycine is optically inactive.

Check the options

A: All amino acids except lysine are optically active
This is false, because lysine is optically active, while glycine is the exception.

B: All amino acids are optically active
False, because glycine is optically inactive.

C: All amino acids except glycine are optically active
True.

D: All amino acids except glutamic acid are optically active
False, because glutamic acid is optically active; glycine is the exception.

Final answer

The correct statement is: $\boxed{\text{C: All amino acids except glycine are optically active}}$

Comparison with stored correct answer

Stored correct answer: C
Derived answer: C
They match.