Radiation-induced racemization and amplification of chirality: implications for comets and meteorites 1
The action of high-energy radiation on prebiotic chiral molecules plays against the preservation of chirality. Chiral molecules incorporated in comets and meteorites are bombarded for billions of years by cosmic rays and by the high-energy radiation due to the decay of naturally occurring radionuclides. The action of cosmic rays on the surface of comets and meteorites causes the complete radiation processing of the surface of these bodies, but at depths of 20 m or so the cosmic rays are completely shielded and the radiation should derive only from the decay of radionuclides. In 4.6×109 yr the radiation dose supplied by the radionuclide decay to the organic molecules present inside the cometary or meteoritic body is equivalent to 14000 kGy. Our studies on the radiolysis of a series of naturally occurring chiral molecules, the terpenes, have shown that although all undergo the radioracemization reaction, the extent of radioracemization is such that a significant fraction of chiral excess and chiral molecules can survive a radiation dose equivalent to 14000 kGy. A unique exception is represented by the terpene β-(−)-pinene which, instead of the expected radioracemization reaction, undergoes a radiation-induced polymerization. The resulting poly-β-pinene, having an highly ordered supramolecular structure, displays an optical activity which is 1.7 times higher than the starting monomer. Thus, in this specific case, the optical activity is not reduced but enhanced by the action of radiation and remains locked into a polymer which displays a considerable radiation resistance and may act as a chiral template and as a chiral surface for prebiotic chemistry.(Received September 18 2006)
(Revised December 29 2006)
(Accepted December 29 2006)
(Published Online February 28 2007)
Key Words: chirality; radiation; radiolysis; racemisation; chiral amplification; prebiotic chemistry.
1 Part of this work has been presented as invited lecture at the Workshop ‘The Molecular Universe. Physics and Chemistry of Complex Molecules: The Laboratory Approach’, 11 May 2006, Aarhus, Fuglsocentret, Denmark. It has also been presented at the VII Convegno Nazionale di Scienze Planetarie, 5–9 September 2006, San Felice Circeo, Latina, Italy.