Origin of Chirality in the Universe

Abstract

O rigin of C hirality in the U niverse Shivaani Gandhi B S J Louis Pasteur was a French scientist of the 19th century with a passion to make a lasting contribution to science. In 1848, he postulated that a compound called racemic acid formed two distinct types of salt crystals due to some structural difference between the molecules the acid was composed of. He made solutions of the two different salt crystals and shone polarized light--light whose waves only move in one directional plane--through both, and found that each solution caused the light’s direction to rotate an equal amount, but in opposite directions. Figure 1. : Chiral compound rotating plane-polarized light. His original postulation was later confirmed with the discovery of enantiomers, or molecules composed of the same atoms but with structures that are not the same, yet are mirror images of each other. (Newton, 2012) Such molecules are said to exhibit chirality. Sometimes, each enantiomer of a chiral pair is called “left-handed” or “right-handed,” since human hands are also non-identical mirror images of each other. Chirality and optical activity--the ability of chiral molecules to rotate plane-polarized light--may seem like minor details that only chemists and maybe physicists would be concerned with. However, one simple difference in molecular geometry, even just switching the position of two atoms relative to the other ones in a molecule, can drastically change its properties. Some can be harmless, like in carvone: the right- handed enantiomer smells like caraway seeds, but the left- handed one smells like peppermint. Other times, the switch can have less benign effects: for the molecule thalidomide, the left-handed enantiomer relieves morning sickness, but the right-handed one induces birth defects (Schirber, 2009). Taking these differences as examples, it may not seem surprising that chiral molecules in organisms exist almost exclusively as single enantiomers. In a lab, most syntheses will yield the left- and right-handed enantiomers in equal amounts, but in fact, most enantiomers in nature exist in the left-handed form (Blackmond, 2010). Finding the origin of the increased left-handed and decreased right-handed concentrations is vital to understanding life. Finding out which chemical reactions sparked the creation of life on Earth can tell us what we might want to expect from life on other planets. Will they be left- handed molecules, like on Earth, or will their structures — and thus their characteristics — be completely different? Most scientists believe that homochirality is a precondition for life, with the argument that one hundred left- or right-handed gloves arranged in a sequence would have a well-defined structure, whereas a random mixture of both would be a mess if you tried to arrange them in a similar sequence (Schirber, To attempt to account for the homochirality of biological molecules, scientists have created models that amplify an initial imbalance between the amount of two enantiomers in a system, which results in very large amounts of one enantiomer. These models are meant to model an amplification that, over time, may have resulted in the enantiomeric excess that we observe today. Proposed model systems include small initial imbalances in meteorites or other extraterrestrial sources, as well as random fluctuations in the physical and chemical environment that might account for a preference towards left- or right-handed molecules. Regardless of whether the excess was started off by chance or by design, an amplification mechanism is useful in understanding how the excess of left-handed enantiomers in our world got to this point (Blackmond, 2010). This article explores a few notable mechanisms that may explain the asymmetry in chirality in our world. In 1966, David Cline, a professor at UCLA at the time, proposed a model by which organic materials, or carbon-containing compounds, were delivered to Earth from an interstellar source, such as meteorites (Cline, 1996). The homochirality observed in organic molecules may be a result of interactions with radioactive decay or supernova explosions in a dense molecular cloud. Chiral subatomic particles resulting from beta decay or effects of weak currents promotes asymmetric dominance, but this is on a very small scale. The amplification of this small asymmetry happens though a bifurcation process over a long period of time, or when a large number of these interactions happen over a short amount of time. Often, both may occur. The latter arises when radioactive decay or supernova explosions occur 1 • B erkeley S cientific J ournal • S ymmetry • F all 2015 • V olume 20 • I ssue 1