Chiral enantiomers—molecules that, like our left and right hands, are mirror images of each other but aren’t superimposable—are troublesome to differentiate. Identical in chemical formula, they behave interchangeably for the most part but distinguish themselves in the presence of other chiral molecules or fields.
Chiral enantiomers differ, for example, in how they interact with circularly polarized light (CPL). Like a screw, CPL’s polarization vector describes either a clockwise (right-handed) or counterclockwise (left-handed) helix in space; opposite-handed beams tend to veer in opposite directions as they refract through a chiral material. (See PHYSICS TODAY, December 2006, page 22.)
Chiral molecules also absorb opposite-handed CPL at different rates. That difference is the basis for what’s known as circular dichroism (CD) spectroscopy. By measuring the dissymmetry factor, the difference between absorption rates of left- and right-handed CPL, normalized by the mean, scientists can discern the enantiomeric composition of, say, a biomolecule...
