Searching for Life in the Solar System ... And Beyond
A Research Discussion Meeting
London, UK - 31 October 1996

Chirality as a signature of bio-organic synthesis

A.J. MacDermott
University Chemical Laboratory, Lensfield Road, Cambridge

Most biomolecules are chiral or handed, existing in two mirror image forms called enantiomers. Non-living systems normally contain equal numbers of the left (L) and right (D) handed forms. But a characteristic hallmark of life is its homochirality: biochemistry uses only one hand and not the other - on Earth L-amino acids and D-sugars. So a search for extra-terrestrial biology can be approached as a Search for Extra-Terrestrial Homochirality - SETH. The signature of homochirality - or at least an enantiomeric excess - is optical rotation, which could be detected on other planets using a novel miniaturised space polarimeter called the SETH Cigar. We hope to find an enantiomeric excess in the Martian subsurface - and perhaps also in the Mars meteorite ALH84001 - as a relic of extinct life. As excess of L amino acids has recently been found in the Murchison meteorite, suggesting advanced pre-biotic chemistry on the parent asteroid or pre-solar nebula. But why L and not D? Life's handedness may be a feature of fundamental physics, a result of selection by the weak force, which makes L-amino acids very slightly more stable. Finding the same hand on different planets would tend to support the role of the weak force, but even finding the "wrong" hand would be of enormous significance because it could still be the homochiral signature of biology.