CONSIDERATIONS AND SPECULATION
Mirror Images of an Amino Acid
An illustration of chirality (handedness)
Chirality is a term familiar to the scientific community, but not as familiar to the lay-public. The word, chiral, is from the ancient Greek language for hand; and it bears significance because of life’s unique fingerprint. Our proteins are (for the most part) made of left-handed amino acids. What does this mean? In the hunt for extra-terrestrial microbes, finding microbes beyond Earth that do not possess any resemblance to our own would be revolutionary. It would be one small answer to an age-old question: is human life the only chemistry of life–or if we are not alone why have we not seen anyone else? Perhaps our bio-chemistry is so unique, we did not know where to look?
In Figure 1, we can discern a certain asymmetry between the molecules; they are mirror images of one another. Pasteur made early contributions to the science when recognizing that a molecule may possess a mirror image of itself. Furthermore, he recognized that–on occasion– mirror images may possess different properties. In his experimentation, he separated mirror images of tartaric acid crystals. By today’s standards, his methodology may be considered crude, but it was a complete stroke of genius for his time. Painstakingly, Pasteur separated the right-handed crystals from the left–with a magnifying glass and tweezers.
Tartaric Acid Crystals
It is often said that individual mirror image molecules differ in bio-physical properties, the difference serves as the basis for the debate on why our type of life favors left-handed amino acids. Biologists have argued that life would not have evolved without the current preferences seen in bio-molecular systems. Others have argued for a physical basis for a left-handed preference among bio-molecular systems. In any light, the scientific basis for chirality falls into two hotly contested areas: (1) the physical universe confers a preference for left-handed molecules, (2) left-handed molecules were favored on the Earth through an unknown process.
Experiments have been performed to simulate conditions that molecules experience in the Solar System; under those conditions, 50 percent mixtures of amino acids became an excess of left-handed amino acids. In the year 2014, the European Space Agency (ESA) will land its probe (from the Rosetta mission) on comet 67P/Churyumov-Gerasimenko; it will sample the molecular constituency of the comet. Hopefully this will give a better answer to the question of handedness in the solar system. In fact the ESA probe will have visited a few solar system objects by the 2014 rendezvous date (see Figure 3 for probe trajectory).
Trajectory of ESA probe: Rosetta
For information regarding Rosetta: