Molecular Parity Violation via Comets? UWE MEIERHENRICH, 1 WOLFRAM H.-P. THIEMANN, 1 * AND HELMUT ROSENBAUER 2 1 Universita ¨t Bremen, Institut fu ¨r Physikalische Chemie 2 Max-Planck-Institut fu ¨r Aeronomie, Katlenburg-Lindau ABSTRACT Recent theoretical and experimental investigations referring to the ori- gin of homochirality are reviewed and integrated into the hitherto known state of the art. Attention is directed to an extraterrestrial scenario, which describes the interaction of circularly polarized synchrotron radiation with interstellar organic matter. Following this Bonner-Rubenstein hypothesis, optically active molecules could be transferred to Earth via comets. We plan to identify any enantiomeric enhancement in organic molecules of the cometary matter in situ. The present preliminary experimental study intends to optimize gas-chromatographic conditions for the separation of racemates into their en- antiomer constituents on the surface of the comet 46P/Wirtanen. Underivatized racemic pairs of alcohols, diols, and phenyl-substituted amines have been separated with the help of a stationary trifluoroacetyl-cyclodextrin phase. We are still developing a technique that will enable us to detect any enantiomeric enhancement of specific simple organic mol- ecules both in cometary or Martian matter in situ and in meteorites found on Earth. Chirality 11:575–582, 1999. © 1999 Wiley-Liss, Inc. KEY WORDS: asymmetric synthesis; chirality; cyclodextrin; enantiomer separation; gas chromatography; origin of optical activity; 46P/Wirtanen INTRODUCTION Homochirality is one characteristic sign of the organic molecules which living organisms—plants as well as ani- mals—are made of. In sharp contrast to this, abiotically produced enantiomers are normally racemic. A key prob- lem to understand the first steps of biology or the origin of life on Earth is consequently the origin of the mentioned homochirality. Biotic as well as abiotic mechanisms 1 were proposed in the literature. So-called biotic or selection theories were preferred in the 1950s. Very early in biochemical or biological evolution, two kinds of organisms could co-existed. One type con- sisted of proteins (and nucleic acids) composed of L-amino acids (and/or D-sugars) and the other one of D-amino acid (and/or L-sugar) enantiomers. Both optical antipodes co- existed for a certain time, and the more favorable enantio- mer, having a certain evolutionary advantage, could have been selected by interacting with other nonracemic sub- stances. 2 This theory suggests that life arose in a racemic environment and that homochirality as such developed later. 3 More evidence has been compiled about the fact that in contrast to “normal” proteins, D-amino acids were discovered in a number of specific biological samples such as bacterial cells, 4 vertebrate brains, 5 and others, which emphasizes the biological selection process of specific an- tipodes during evolution. In contrast to this, experiments and a qualitative model 6 proposed that pure homochiral protein macromolecules and pure polynucleotide chains are not self-replicable in a racemic medium. In short, life requires homochirality. Nowadays various theories of an abiogenic origin of the biomolecular homochirality dating back to the prebiologi- cal chemical stage of evolution due to physico-chemical processes are more widely accepted: Random mechanisms interpret the appearance of optical activity by a mere chance process—like the parity violating precipitation from supersaturated racemic solutions, 7 en- antioselective adsorption on quartz, 8 or many other similar scenarios—and its amplification by suitable mechanisms. Stereospecific autocatalysis is proposed as process of am- plifying capacity: The presence of one enantiomer encour- ages the generation of the same one but inhibits automati- cally the synthesis of its corresponding mirror image, lead- ing ultimately to a state of homochirality. 9 Promising mathematical formalisms 10 and experimental models 11 were proposed and tested, but all of them lacked experi- mental verification. Determinate mechanisms describe the interaction of chi- ral physical driving forces with racemates that are causing prevalence of one enantiomer. Cogencies like circularly polarized light 12 or a magnetic field, 13 for example on the surface of a liquid crystal during a photoreaction, 14 or on the surface of dealuminated zeolites 15 were examined. However, a paper with spectacular results had to be with- drawn due to “manipulation” of the data. 16 Apart from this, “parity-violating energy differences” 17,18 Contract grant sponsors: Max-Planck-Society: European Space Agency *Correspondence to: W.H.-P. Thiemann, Universita¨t Bremen, NW Il, Leo- benerstrasse, D-28359 Bremen, Germany. E-mail: thiemann@uni- bremen.de. Received for publication 28 October 1998; Accepted 2 March 1999 CHIRALITY 11:575–582 (1999) © 1999 Wiley-Liss, Inc.