21 first understand polarized light. In unpolarized light – such as sunlight – the electric field vibrates in many random directions perpendicular to the light’s direction. However, in plane-polarized light, the electric field vibrates in only one fixed direction (or on a single plane) as the light travels forward. In early 1800s, scientists have already discovered that when plane-polarized light is passed through the solution of natural tartaric acid or its salts, the plane of polarization rotates clockwise [5]. One day, an industrial chemist, Phillippe Kestner, discovered a mysterious acid from the winemaking process [5]. The mysterious acid, later named paratartaric acid, appeared to share the same chemical composition with natural tartaric acid (at that time they didn’t know the chemical structures) but showed no rotation [6]. This was perplexing because both acids should have behaved the same way. A French chemist (later known as “the father of microbiology”), Louis Pasteur, approached this mystery from a novel angle. He examined the crystals formed by paratartaric acid under a magnifying glass, and observed that crystals occurred in two shapes that were mirror images of each other (Figure 2) [5, 7]. They were almost identical – but like left and right hands that could not be perfectly placed on top of the other. Pasteur separated these crystals with a tweezer and dissolved them to make two solutions. He found that one solution rotated polarized light to the left while the other rotated it to the right. When mixed in equal amounts, their rotations cancelled each other out. This discovery introduced the concept of chirality to chemistry. Scientists further deduced that the chirality of tartaric acid crystals might stem from the dissymmetric structure of the molecules [5]. The hypothesis was not proved until 1940s; the structure of tartaric acid was revealed with the availability of rigorous X-ray diffraction analysis [7]. Why Chirality Matters Intriguingly, chirality matters in nature. A classic example comes from amino acids, the building blocks of proteins. Most amino acids (except glycine) consist of four different atoms or groups of atoms bonded to the central carbon, so there are left-handed amino acids and right-handed amino acids (Figure 3), which rotate polarized light to left and right respectively. Interestingly, almost all proteins in mammals are constructed exclusively by left-handed amino acids [8]. This remarkable preference of handedness also applies to other molecules, like sugars. Enzymes and cell receptors bind strongly to their substrates only when their molecular handedness matches. This explains Figure 2 The two types of chiral crystals observed by Louis Pasteur [5].
RkJQdWJsaXNoZXIy NDk5Njg=