Mystery of natural optical selection stirred

When shaking hands with someone, you have to reach across the body to take their right hand with your own. In other words, the left hand of a person facing you is on the same side as your right. This is the same relationship that exists between you and your image in a mirror.

Many chemical compounds, including naturally occurring ones such as sugars and amino acids, exist in two configurations that are mirror images of each other. Often the only way to tell them apart is the way they influence light. Waves of ordinary light vibrate in all possible planes or directions from their point of origin but, when light is passed through a sheet of Polaroid film or certain types of crystals, all but one plane is blocked out and the light vibrates in only that one plane. It is now called plane polarized light and is the operating principle behind a research instrument called a polarimeter.

In 1848, Louis Pasteur noticed there was a subtle difference in crystal structure between two forms of a compound from wine called tartaric acid. He found that, when a solution of these separate forms were placed in a polarimeter, one would twist or rotate polarized light clockwise to the right (D-form) while the other twisted it to the left in a counterclockwise manner (L-form). If equal amounts of the two forms are combined, the resulting mixture does not affect the light beam. Hundreds of compounds are found in nature whose structures are identical except for the way they affect polarized light. This unique property might seem to be of interest only to chemists, however it has profound implications in medicine and lies at the heart of a major biochemical mystery.

Albert Chan, writing in the March 1993 issue of Chemtech, tells how one optical form of a drug can have a completely different effect in the body than does its mirror image. Starting in the 1950s, thalidomide was used as an aid to combat morning sickness. Within a few years, it was found also to be the cause of horrific birth defects. Chan says that one optical form of thalidomide acts as a sedative and mild hypnotic while the other is a powerful teratogen responsible for birth defects. Thalidomide was sold as a mixture of both optical forms with one causing birth defects while the other alleviated symptoms of morning sickness.

Another example is the artificial sweetener Aspartame. One form is 200 times sweeter than sugar while the other is bitter to the taste. Aspartame has to be sold in the form that is sweet, but most optically active compounds are sold as mixtures even though only one form has the desired activity. Ibuprofen, the widely used analgesic and anti-inflammatory drug, is an example. In fact, Daniel Deutsch, in the March 1991 issue of Chemtech, says that 17 of the 20 top-selling drugs in the United States are sold as mixtures. The issue is cost of separation and pharmaceutical companies are investing heavily to find ways of manufacturing only the desired optical form.

In nature, as in the laboratory, any compound that is capable of having two optical forms should occur in equal amounts of each. In actuality, almost all of the sugars that make up the backbone of DNA are right-handed while the amino acids that constitute protein all rotate light to the left. Is this mysterious predilection of nature for one optical form over the other a precondition for the appearance of life, or did it come about as a consequence of life’s biochemical processes?

Jon Cohen, writing in the March 3, 1995, issue of Science on this question, says there are passionate advocates for each point of view but, as yet, no firm answers. The resolution of the question can only come about with the discovery of some physical process involved with chemical synthesis that selects one optical form of a compound over the other.

The most popular candidate is circularly polarized light, i.e. light that spirals either clockwise or counterclockwise and is thought to come from supernova. Astrophysicist Jeremy Bailey et al, in the July 31, 1998, issue of Science, says that the infrared light coming from the Orion Nebula contains sufficient circularly polarized light to favor one optical form over the other in chemical synthesis. Another candidate, writes Laurence Barron in the June 22, 2000, issue of Nature, is magnetic fields. Chemical reactions, initiated by light and carried out in a static magnetic field, have been shown to favor one optical form over the other. Robert Service reports that a third possibility is the weak nuclear force that holds subatomic particles together in atomic nuclei. Service says, in the Nov. 12, 1999, issue of Science, that electrons emitted when protons convert to neutrons always have a left-hand spin and may influence compound formation – a long shot at best.

But it may be that researchers are overlooking an extremely simple reason for the preferential selection of optical forms. Spanish chemist Josep Ribo is lead author of a paper that has just appeared in the June 15 issue of Science. His team managed to get one form of a naturally occurring molecule to form over the other by the simple expedient of briskly stirring the solution! The direction of the vortex, clockwise or counterclockwise, appears to play a major role in the optical form that is synthesized. Could the mystery of natural optical selection be this simple?

Clair Wood taught physics and chemistry for more than a decade at Eastern Maine Technical College in Bangor.