Optical activity occurs when chiral molecules rotate plane-polarized light, a key property in biochemistry for identifying enantiomers. At 100 mM in water, only solutions of chiral, non-racemic compounds show this effect. The correct answer is (B) Solution of D-Glucose and (D) Solution of L-Proline.

What is Optical Activity?

Optical activity measures a substance’s ability to rotate polarized light due to chirality, quantified by specific rotation $[\alpha ]$. Achiral or racemic mixtures lack this property as their rotations cancel out. In dilute aqueous solutions like 100 mM, measurements remain valid without interference from high ionic strength.

Option Analysis

• (A) NaCl: Sodium chloride is achiral with a symmetric ionic lattice, showing no optical activity in water. Salts like NaCl do not rotate light even at 100 mM, as confirmed by polarimetry studies.

• (B) D-Glucose: This D-enantiomer of glucose has multiple chiral centers and exhibits strong optical activity. Pure D-glucose solutions rotate light positively, with [α]D≈+52.7∘ in water, observable at 100 mM.

• (C) Glycine: Glycine, the simplest amino acid, has no chiral carbon (NH₂-CH₂-COOH), making it achiral. Its solutions show zero optical rotation at any concentration, including 100 mM.

• (D) L-Proline: L-Proline’s five-membered ring creates a chiral center, leading to optical activity. It rotates light negatively, with [α]D≈−86∘, detectable at 100 mM in water.

Correct Answer Breakdown

Options (B) and (D) show optical activity because both are enantiomerically pure chiral molecules. NaCl lacks chirality, and glycine is achiral. In MCQs, “show(s)” often implies multiple selections, so select B and D. This tests understanding of molecular asymmetry in biological contexts.