**Question:** Amino acid with double chiral carbon-

A. Lysine
B. Tyrosine
C. Valine
D. Arginine

**Correct Answer:** A. Lysine

**Core Concept:** A chiral center is an atom in a molecule that has four different substituents (groups attached to it). In organic chemistry, chiral centers can cause a molecule to rotate plane-polarized light in a specific direction, which is referred to as optical activity. Chiral centers can lead to the existence of two enantiomers, non-superimposable mirror images of each other, that cannot interconvert by any chemical means.

**Why the Correct Answer is Right:** Lysine is an amino acid with a double chiral carbon (also known as an allenic bond) at its carbon-7 position. This double bond results in two enantiomers: (R)-lysine and (S)-lysine. Due to this chiral center, both enantiomers rotate plane-polarized light in the same direction, which is why lysine does not exhibit optical activity.

**Why Each Wrong Option is Incorrect:**

B. Tyrosine - Tyrosine is an amino acid, but it does not have a double chiral carbon. Therefore, it does not have enantiomers and does not exhibit optical activity.

C. Valine - Valine is an amino acid with a single chiral center, not a double chiral carbon. This results in two enantiomers that rotate plane-polarized light in opposite directions, allowing valine to exhibit optical activity.

D. Arginine - Arginine is an amino acid with a single chiral center. This results in two enantiomers that rotate plane-polarized light in opposite directions, allowing arginine to exhibit optical activity.

**Clinical Pearl:** Understanding optical activity and enantiomers is crucial in drug discovery and development. Many drugs exist as a mixture of enantiomers, and enantiomers can have different pharmacological activities or side effects. Enantiomer separation techniques, such as chiral chromatography or crystallization, are often employed to obtain a single enantiomer for clinical trials and medicine production.