Optical activity: optical dichroism

Virtually all natural products - proteins, nucleic acids, sugars, hormones, lipids, vitamins, antibiotics, etc. - exhibit optical activity. Given the important role played by the interactions between these different types of molecules in biological processes, knowledge of their optical activity is crucial to understanding the fundamental phenomena of living organisms. Similarly, determining the chirality or optical purity of certain compounds is extremely important in the pharmaceutical and food industries, because of the different effects that optical isomers can have.

The discovery of natural optical activity dates back to the early 19th century, with Biot and Fresnel, and the first explanation of the origin of this phenomenon came from Pasteur, in 1848. These advances enabled optical activity data to be among the first to be used, in particular for characterizing molecules. Molar rotation at the wavelength of sodium's D line has long been one of the parameters provided in the description of a new product. However, it wasn't until the mid-twentieth century that spectral measurements, in particular optical rotational dispersion, became a routine tool in chemical laboratories. Gradually, this spectroscopy gave way to circular dichroism, which is able to provide equivalent information but is easier to measure and interpret than rotational optical dispersion spectra.

First, we describe light (§ 1 .) and its various forms of polarization (§ 2 .), basing ourselves mainly on wave theory.

After describing the main characteristics of optical activity, we present the two essential methods for demonstrating it: optical rotation and, above all, circular dichroism (§ 3