Within the general question of the chemical origin of life lies the question of handedness, or chirality (a term derived from the Greek word for hand). Critically important molecules that make up life — including DNA, RNA, polysaccharides and proteins — are chiral. This means there are mirror image forms that are not identical, much as one’s left and right hands are different and are mirror reflections.
“The molecules of life have mirror images, which can be synthesized by organic chemists, but the mirror images don’t exist in life any more than a person’s mirror image does,” explains Mark M. Green, professor of chemistry at Polytechnic Institute of New York University (NYU-Poly).
But why should this be the case? Scientists have tried to answer this for many years.
One theory is that at the very beginning, there existed both mirror forms of the molecules that would lead to life, but with a tiny excess of perhaps one extra left-handed molecule in a hundred or even a thousand or more right- and left-handed molecules. If that small excess gets amplified or concentrated, the mirror form that’s in excess could reasonably become the basis of the chirality in the life that evolved. “Ronald Breslow at Columbia University has recently supported this idea by discovering chemistry mechanisms in which tiny excesses of the chirality seen in meteors raining down on earth, prevailed to control all of the chirality in life we see today,” says Green.
Green recently showed how the properties of polymers support a different theory. In a paper with his student Vipul Jain, featured in an “Editor’s Choice” section of the scholarly journal Science, he proposed that both mirror forms of primordial life could have begun the evolutionary path but that random events led to the survival of only one of these living mirror images. Green calls the theory “Two Equal Runners, One Tripped.”
“It’s possible that life originated with both forms, and one died out and the other continued,” he says.
This theory arises from knowledge of the behavior of polymers, first gained from Green’s participation in the Polymer Research Institute, which was founded at Polytechnic Institute of Brooklyn (now NYU-Poly) in 1946 by Herman F. Mark, who is known as “the father of polymer science.” Moreover, when Green looked back in the records of the Institute, he discovered that Nobel Prize winner George Wald had lectured in the 1950s at Poly and proposed the same idea: that the primordial molecules of life originated in both mirror forms. Wald discussed his idea with Albert Einstein, who suggested that there could have been a struggle between left and right, parallel to a possible “fight” between negative and positive electrons. Wald even suggested that when our species finally gets around to investigating other planets, humans will discover life on the opposite side of the mirror from our own. Many years ago another distinguished scientist, Hans Kuhn, suggested that fossils of early life might yield evidence of mirror image life.
When there are chiral choices to be made, life virtually always chooses one mirror form over the other, never both — although there are rare exceptions, Green says. Making the molecules that life depends on of a single mirror form, or homochirality, allows for predictable and efficient behavior. Moreover, natural polymers are more stable when they are made of chains of molecules that are entirely left- or entirely right-handed. However, Green points out that homochiral life on both sides of the mirror could have co-existed.
After the thalidomide disaster of the late 1950s, the entire pharmaceutical industry was forced to pay more attention to the fact that our bodies react differently to mirror-image molecules (enantiomers). Both mirror images of thalidomide were marketed as a mixture of the two; some worked as intended and relieved morning sickness during pregnancy, while their opposites caused severe birth defects in thousands of babies around the world.
These days, pharmaceutical companies always consider whether mirror forms differ in their effects. Some medications are able to utilize both mirror image forms. In the case of ibuprofen, for example, the body is able to convert the mirror image that is not a pain killer to its mirror image, the pain killer. The pharmaceutical industry can therefore market a mixture of enantiomers of ibuprofen.
Chirality is important in all aspects of the physical sciences. It is one of the foundation principles. Quite a bit depends on this property, which we see every time we glance at our hands. Certainly it is a central principle of life.
Vipul Jain undertook his doctoral research at NYU-Poly in Green’s group. He now works in the advanced polymer research laboratories of Rohm and Haas.
Green’s research in polymers has been funded over the years by the National Science Foundation, The Petroleum Research Fund of the American Chemical Society and by the Office of Naval Research.
About Polytechnic Institute of New York University
Polytechnic Institute of New York University (formerly The Polytechnic Institute of Brooklyn), an affiliate of New York University, is one of New York City’s most comprehensive schools of engineering, applied sciences, technology, and research, and is rooted in a 156-year tradition of invention, innovation, and entrepreneurship: i2e.
The institution, founded in 1854, is one of the nation’s oldest private engineering schools. In addition to its main campus at MetroTech Center in downtown Brooklyn, it offers programs at sites throughout the region and around the globe. NYU-Poly has centers in Long Island, Manhattan and Westchester County; globally, it has programs in Israel, China and will be an integral part of NYU's campus in Abu Dhabi opening in autumn 2010.