Angewandte Chemie International Edition

Cover image for Vol. 56 Issue 47

Editor: Neville Compton; Editor Emeritus: Peter Gölitz

Online ISSN: 1521-3773

Associated Title(s): Angewandte Chemie, Chemistry - A European Journal, Chemistry – An Asian Journal, ChemistryOpen, ChemPhotoChem, ChemPlusChem, Zeitschrift für Chemie

Press Release

Angew. Chem. Int. Ed. ,
doi: 10.1002/ange.200501311

Nr. 29/2005

Choosing Between Image and Mirror Image

Is “asymmetric” space radiation the reason for life based on L-amino acids?

A recent advertisement (for “right-handed” yogurt) has introduced many of us to the concept that many biomolecules exist in “right-” and “left-handed” versions. Molecules or parts of molecules can be structured as image or mirror image. Image and mirror image can have completely different properties: a different taste or scent, or even different physiological effects. The basic building blocks of life are mostly found in exclusively one form or the other, designated as L or D (from the Latin laevus for left and dexter for right). Natural amino acids, the components of proteins, are found in the L form. The reason for this preference could stem from the irradiation of amino acids in space. A European research team has now found further indications to support this hypothesis.

The origins of our existence seem to lie in the depths of space. Amino acids, the building blocks that presumably played a key role in the emergence of life on Earth, have been found on meteorites. “The foundation for the asymmetry of biomolecules seems to have been laid there,” explains Uwe J. Meierhenrich. Frozen within tiny particles of ice, it is believed these primordial amino acids were exposed to highly energetic, “asymmetric” UV radiation (vacuum UV). The electromagnetic radiation produced by certain white dwarfs can consist of up to 50% such asymmetric light, called circularly polarized light. Dust particles with magnetic fields in reflection nebulas and star-forming regions can also induce circular polarization of light. The photons then have angular momentum (“helicity”), spinning either to the left or the right. (Alternatively, light can be described as an electromagnetic wave that oscillates in all planes. Circular polarized light only oscillates in one plane, which rotates around the direction of propagation of the light.)

Circularly polarized vacuum UV can be produced artificially in a synchroton. A team of researchers from universities in Nice, Paris, and Orléans (France), Bremen (Germany), and Aarhus (Denmark) irradiated samples of the amino acid leucine in the solid phase with right and left circularly polarized light. This revealed that L- and D-leucine have opposite preferences in the absorption of “right-handed” or “left-handed” photons. The absorption of vacuum UV light initiates decomposition of the leucine molecules. In a sample with equal amounts of D- and L-leucine, right-polarized light destroys somewhat more of the L-leucine, while left-polarized light destroys somewhat more of the D form.

“In space, this type of irradiation clearly led to small but significant local enrichment of L-amino acids,” says Meierhenrich. “This excess could have been enough to lead to autocatalytic processes for the formation of life based on L-amino acids.”

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