Stereochemistry and Isomerism (5B) Practice Test

A peptide-mimetic inhibitor, Molecule K, contains two stereocenters and is synthesized as four stereoisomers. Only K(2R,3S) shows potent inhibition of a chiral protease. The lab isolates K(2R,3S) and K(2S,3R) and notes they have identical melting points and equal-magnitude opposite-sign optical rotations, but sharply different inhibition constants are not observed between these two. Which statement best describes the stereochemical relationship between K(2R,3S) and K(2S,3R) consistent with the physical data?

An optical rotation experiment examined two isomers of Molecule C (a chiral alcohol). A 1.0 dm polarimeter tube containing a 0.20 g/mL solution gives an observed rotation of $+12.0^\circ$ at 589 nm. A second sample prepared at the same concentration and path length gives $-12.0^\circ$. No other differences are detected by NMR. Which configuration is consistent with the observed optical activity relationship between the two samples?