TITLE: " Why we are made of only left-handed amino acids: the origin of chirality on Earth and in the solar system in beyond"

SPEAKER: Professor Alexandra MacDermott,

TIME: 4 PM, Thursday, April 15, 1999

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

Most biomolecules are "chiral" or handed, that is to say they exist in two left and right-handed mirror image forms. But biology only uses one hand, i.e. it is "homochiral". One of the greatest puzzles in biophysics is the question of why life on Earth is based on left-handed (L) amino acids and right-handed (D) sugars - why not "mirror life" based on right-handed (D) amino acids and left-handed (L) sugars? The answer may lie in fundamental physics: the parity-violating weak neutral current produces a very slight energy difference between left and right handed molecules, which may become amplified over an evolutionary timescale, and our calculations of this energy difference show that the natural L-amino acids are indeed more stable than their "unnatural" D mirror images. This parity-violating energy difference or "PVED" between mirror image molecules is important not only in biology but also as a "molecular footprint" of fundamental physics: future measurements of the PVED could in effect give us "table-top particle physics", yielding values of the Weinberg angle and other important parameters of fundamental physics much more cheaply with a new generation of spectrometers rather than a new generation of particle accelerators. Homochirality is such a characteristic signature of life that finding molecules all of one hand on other planets could be a signature of life or prebiotic chemistry, and we are building a polarimeter to Search for Extra-Terrestrial Homochirality (SETH) on missions to Mars and other solar system bodies. We are also looking to Search for EXtra-SOlar Homochirality (SEXSOH) by looking for circular polarization in light reflected from planets round other suns in the next century.