Light is a pretty amazing thing, when you think about it. As chemists, we’re lucky to have it–what else in the universe has internal dimensions that span orders of magnitude and internal energies that can interact with all manner of molecular movements? Perhaps more amazing than light itself are the things people have figured out to do with light, and all the various spectroscopic methods that have evolved as a result.
The most general mathematical representation of a light wave is as a superposition of sine waves with different planes of oscillation, phases, wavelengths, and amplitudes, but with a common direction of propagation. Explore some of the possibilities, and some interesting results arise: for example, a light wave made of two perpendicular sine waves of equal amplitude oscillating 90 degrees out of phase with each other looks like a helix (or a circle when projected onto its normal plane…hence the term “circularly polarized”). Positive and negative phase differences lead to two kinds of light, right and left circularly polarized. Add a right CP light wave to a left CP one of equal amplitude, and the result is a light wave with a single plane of oscillation (plane-polarized light). As it turns out, this is incredibly useful!
Right and left circularly polarized light are about the most convenient chiral objects in the universe, and not surprisingly, chiral molecules interact differently with these two enantiomeric objects. The fascinating thing is that plane-polarized light, though obviously achiral, “encodes” both right and left circularly polarized light! Fire a plane-polarized beam at a chiral compound, and the result is a superposition of two circularly polarized waves of different amplitude, since the chiral compound interacts with the right- and left-hand components unequally! When projected onto the normal plane, this superposition looks like an ellipse whose major axis coincides with the plane of the input plane-polarized light. The difference between the magnitude of the output right-hand light and the output left-hand light is called the “ellipticity” (not to be confused with The Police’s “Synchronicity”). The skinnier the ellipse, the higher this value and the greater the differential absorption of one enantiomer of light versus the other.
Circular dichroism spectroscopy measures ellipticity as a function of input light frequency. Biochemists have used it to explore protein structure, as secondary structure motifs have their own CD “signatures.” In a recent paper, Izumi and co-workers use it in a conformational analysis of paclitaxel. Somehow they calculated the CD spectrum of the dominant conformations of baccatin III (a substructure of paclitaxel) and noted similarities between the measured CD spectra of paclitaxel and baccatin III. Look for a future post on computational methods for calculating CD spectra!