The 'natural' asymmetry of biological molecules may have come from space

Jan 11, 2011
The circularly polarized ultraviolet light (UV-CPL) beam on the DESIRS beamline at the SOLEIL synchrotron facility, shown as it passes through a gas filter filled with xenon. Credit: Thomas Lannes

Certain molecules do exist in two forms which are symmetrical mirror images of each other: they are known as chiral molecules. On Earth, the chiral molecules of life, especially amino acids and sugars, exist in only one form, either left-handed or right-handed. Why is it that life has initially chosen one form over the other?

A consortium bringing together several French teams led by Louis d'Hendecourt, CNRS senior researcher at the Institut d'astrophysique spatiale, has for the first time obtained an excess of left-handed molecules (and then an excess of right-handed ones) under conditions that reproduce those found in . This result therefore supports the hypothesis that the asymmetry of on Earth has a cosmic origin. The researchers also suggest that the solar nebula formed in a region of massive stars. This work has just been published on-line on the web site of The . The experiment was carried out at the SOLEIL synchrotron facility.

Chiral molecules are molecules that can exist in two forms (enantiomers) which are symmetrical mirror images of each other, one left-handed and the other right-handed. For instance, our hands are chiral since they come in two forms, the left hand and the right hand, that are symmetrical with their but not super imposable on it. Biological molecules are mostly chiral, with some forms being favored over others. For instance, the that make up proteins only exist in one of their two enantiomeric forms, the left-handed (L) form. On the other hand, the sugars present in the DNA of living organisms are solely right-handed (D). This property that organic molecules have of existing in living organisms in only one of their two structural forms is called homochirality.

What is the origin of such asymmetry in ? There are two competing hypotheses. One postulates that originated from a mixture containing 50% of one enantiomer and 50% of the other (known as a racemic mixture), and that homochirality progressively emerged during the course of evolution. The other hypothesis suggests that asymmetry leading to homochirality preceded the appearance of life and was of cosmic origin. This is supported by the detection of L excesses in certain amino acids extracted from primitive meteorites. According to this scenario, these amino acids were synthesized non-racemically in interstellar space and delivered to Earth by cometary grains and meteorites. To lend more weight to this hypothesis, the researchers first reproduced analogs of interstellar and cometary ices in the laboratory. The novel aspect of their experiment was that, using the DESIRS beamline at the SOLEIL synchrotron facility, the ices were subjected to circularly polarized ultraviolet radiation (UV-CPL), which is supposed to mimic the conditions encountered in some space environments. When the ices were warmed up, an organic residue was produced. A detailed analysis of this mixture revealed that it contained a significant enantiomeric excess in one chiral amino acid, alanine. The excess, which was over 1.3%, is comparable to that measured in primitive meteorites. The researchers thus succeeded in producing, under interstellar conditions, asymmetrical molecules of life from a mixture that did not contain chiral substances. This is the first time that a scenario that explains the origin of this asymmetry has been demonstrated using an experiment that reproduces an entirely natural synthesis.

This result reinforces the hypothesis that the origin of homochirality is prebiotic and cosmic, in other words genuinely interstellar. According to this scenario, the delivery of extraterrestrial organic material containing an enantiomeric excess synthesized by an asymmetrical astrophysical process (in this case, UV-CPL radiation) is the cause of the asymmetry of life's molecules on Earth. This material may even have formed outside the solar system. Finally, the may have formed in regions of massive star formation. In such regions, infrared radiation circularly polarized in the same direction has been observed. These findings imply that the selection of a single enantiomer for the molecules of life observed on Earth is not the result of chance but rather of a deterministic physical mechanism.

Explore further: Full lunar eclipse delights Americas, first of year

More information: Non-racemic amino acid production by ultraviolet irradiation of achiral interstellar ice analogs with circularly polarized light. Pierre de Marcellus, et al. The Astrophysical Journal Letters, vol. 727 (issue 2), L27 (2011). iopscience.iop.org/2041-8205/727/2/L27

add to favorites email to friend print save as pdf

Related Stories

Space Hand-Me-Downs

Sep 18, 2009

Molecules vital to life have been detected in outer space and isolated in meteorites and comets. Some of this material that rained down on Earth may have jump-started biology. If so, these space seeds also ...

Scientists Find Clues to a Secret of Life

Mar 17, 2009

(PhysOrg.com) -- NASA scientists analyzing the dust of meteorites have discovered new clues to a long-standing mystery about how life works on its most basic, molecular level.

Recommended for you

Vegetables on Mars within ten years?

19 hours ago

The soil on Mars may be suitable for cultivating food crops – this is the prognosis of a study by plant ecologist Wieger Wamelink of Wageningen UR. This would prove highly practical if we ever decide to ...

NASA Cassini images may reveal birth of a Saturn moon

20 hours ago

(Phys.org) —NASA's Cassini spacecraft has documented the formation of a small icy object within the rings of Saturn that may be a new moon, and may also provide clues to the formation of the planet's known ...

Meteorite studies suggest hidden water on Mars

21 hours ago

Geochemical calculations by researchers at Tokyo Institute of Technology to determine how the water content of Mars has changed over the past 4.5 billion years suggest as yet unidentified reservoirs of water ...

User comments : 0

More news stories

Astronomers: 'Tilt-a-worlds' could harbor life

A fluctuating tilt in a planet's orbit does not preclude the possibility of life, according to new research by astronomers at the University of Washington, Utah's Weber State University and NASA. In fact, ...

NASA Cassini images may reveal birth of a Saturn moon

(Phys.org) —NASA's Cassini spacecraft has documented the formation of a small icy object within the rings of Saturn that may be a new moon, and may also provide clues to the formation of the planet's known ...

Vegetables on Mars within ten years?

The soil on Mars may be suitable for cultivating food crops – this is the prognosis of a study by plant ecologist Wieger Wamelink of Wageningen UR. This would prove highly practical if we ever decide to ...

Patent talk: Google sharpens contact lens vision

(Phys.org) —A report from Patent Bolt brings us one step closer to what Google may have in mind in developing smart contact lenses. According to the discussion Google is interested in the concept of contact ...

Tech giants look to skies to spread Internet

The shortest path to the Internet for some remote corners of the world may be through the skies. That is the message from US tech giants seeking to spread the online gospel to hard-to-reach regions.

Wireless industry makes anti-theft commitment

A trade group for wireless providers said Tuesday that the biggest mobile device manufacturers and carriers will soon put anti-theft tools on the gadgets to try to deter rampant smartphone theft.