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RE: Chirality
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  UCLA life scientists unlock mystery of how 'handedness' arises

Why many of the important functional molecules in our bodies almost always occur in just one chiral form when they could potentially exist in either is a mystery that has confounded researchers for years.
In addressing this question, Mason and his team sought to discover how chirality occurs in the first place. Their findings offer new insights into how the phenomenon can arise spontaneously, even with achiral building-blocks.
Mason and his colleagues used a manufacturing technique called lithography, which is the basis for making computer chips, to make millions of microscale particles in the shape of achiral triangles. In the past, Mason has used this technique to "print" particles in a wide variety of shapes, and even in the form of letters of the alphabet.
Using optical microscopy, the researchers then studied very dense systems of these lithographic triangular particles. To their surprise, they discovered that the achiral triangles spontaneously arranged themselves to form two-triangle "super-structures," with each super-structure exhibiting a particular chirality.

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Homochirality in Amino Acids
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Evidence for the Likely Origin of Homochirality in Amino Acids, Sugars, and Nucleosides on Prebiotic Earth

Over the past century the origin of terrestrial prebiotic homochirality has been the subject of many speculations. For life to start on earth and elsewhere, it is critical that the building blocks of amino acids, sugars, and nucleosides be created in predominant homochiral form. Recent findings of a modest excess L chirality of -methylamino acids in some meteorites that landed on earth have furnished an important piece of evidence. We have shown how these meteoritic components can furnish normal L amino acids, and therefrom D sugars and nucleosides, in high chiral excess under sensible prebiotic conditions. Some important remaining goals are also described.
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Title: Life's Chirality From Prebiotic Environments
Authors: Marcelo Gleiser, Sara Imari Walker

A key open question in the study of life is the origin of biomolecular homochirality: almost every life-form on Earth has exclusively levorotary amino acids and dextrorotary sugars. Will the same handedness be preferred if life is found elsewhere? We review some of the pertinent literature and discuss recent results suggesting that life's homochirality resulted from sequential chiral symmetry breaking triggered by environmental events. In one scenario, autocatalytic prebiotic reactions undergo stochastic fluctuations due to environmental disturbances. In another, chiral-selective polymerisation reaction rates influenced by environmental effects lead to substantial chiral excess even in the absence of autocatalysis. Applying these arguments to other potentially life-bearing platforms has implications to the search for extraterrestrial life: we predict that a statistically representative sampling of extraterrestrial stereochemistry will be racemic (chirally neutral) on average.

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Origine de la vie sur Terre : l'asymétrie « naturelle » des molécules biologiques viendrait de l'espace

Certaines molécules existent sous deux formes qui sont l'image symétrique l'une de l'autre dans un miroir : elles sont dites chirales. Or, sur Terre, les molécules chirales du vivant, notamment les acides aminés et les sucres, ne sont présentes que sous une seule forme : gauche ou droite. Comment la vie a-t-elle privilégié l'une de ces deux formes au détriment de l'autre ? Un consortium réunissant plusieurs équipes françaises piloté par Louis d'Hendecourt (1), directeur de recherche CNRS à l'Institut d'astrophysique spatiale (Université Paris-Sud 11 / CNRS), a pour la première fois obtenu un excès de molécules de forme gauche (puis un excès de forme droite) dans des conditions reproduisant celles de l'espace interstellaire. Ce résultat rend donc possible une origine « cosmique » de l'asymétrie des molécules biologiques sur Terre. Les chercheurs suggèrent également que la nébuleuse solaire s'est formée dans une zone d'étoiles massives. Ces travaux viennent d'être publiés en ligne sur le site de The Astrophysical Journal Letters. Réalisée au synchrotron SOLEIL, cette expérience a été menée en collaboration avec le Laboratoire de chimie des molécules bioactives et des arômes (Université de Nice/CNRS) et a bénéficié du soutien du CNES.
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Shedding light on the origin of chirality on Earth

An enantioenrichment of the amino acid valine, which could shed light on the origin of chirality on Earth, has been achieved by scientists in Spain. Reporting in Chemical Communications, Cristóbal Viedma and colleagues showed that they have been able to amplify the enantiomeric excess of valine, one of life's building blocks. Valine has been found in meteorite samples.
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Enriching the origin of life theory

An enantioenrichment of the amino acid valine, which could shed light on the origin of chirality on Earth, has been achieved by scientists in Spain.
Chirality is important as amino acids and sugars are present in one single enantiomer in all organisms, and enzymes and receptors are chiral too. As organisms don't have the same response to different enantiomers and many active components of medicines are chiral, understanding how pure enantiomers form can help us understand how we evolved and help develop the medicines of the future.

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Title: Amino Acids in Comets and Meteorites: Stability under Gamma Radiation and Preservation of Chirality
Authors: Susana Iglesias-Groth, Franco Cataldo, Ornella Ursini, Arturo Manchado

Amino acids in solar system bodies may have played a key role in the chemistry that led to the origin of life on Earth. We present laboratory studies testing the stability of amino acids against gamma radiation photolysis. All the 20 chiral amino acids in the levo form used in the proteins of the current terrestrial biochemistry have been irradiated in the solid state with gamma radiation to a dose of 3.2 MGy which is the dose equivalent to that derived by radionuclide decay in comets and asteroids in 1.05x109 years. For each amino acid the radiolysis degree and the radioracemization degree was measured by differential scanning calorimetry (DSC) and by optical rotatory dispersion (ORD) spectroscopy. From these measurements a radiolysis rate constant kdsc and a radioracemization rate constant krac have been determined for each amino acid and extrapolated to a dose of 14 MGy which corresponds to the expected total dose delivered by the natural radionuclides decay to all the organic molecules present in comets and asteroids in 4.6x109 years, the age of the Solar System. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy in significant quantity although part of them are lost in radiolytic processes. Similarly, also the radioracemization process accompanying the radiolysis does not extinguish the chirality. The knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentration of the amino acids at the times of the formation of the Solar System starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the presolar nebula could have been up to 6 times higher than currently observed in meteorites.

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Did exploding stars shatter life's mirror?

Mr Spock is dying. Fortunately for the crew of the USS Enterprise, the Spock in question is not the real one, but an evil mirror-image version created in a freak transporter malfunction. This Spock's back-to-front body can digest only right-handed amino acids; meanwhile, like all organic matter, the food around him is made of left-handed amino acids. He is starving in the midst of plenty.
This plot line from the 1970 novel Spock Must Die! - the first literary spin-off from the Star Trek TV series - highlights one of life's fundamental mysteries. Why does biology use only one of two mirror-image forms in which most complex molecules can occur? The latest pop at an answer weaves astrophysics, particle physics and biochemistry into a startling proposal: that the stellar explosions known as supernovae are to blame.

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A simple and reliable method for converting one of the simplest chemical entities into one of the most difficult-to-make molecular building blocks of life, with complete control over its shape, is reported by scientists at the University of Bristol in this weeks Nature.
It will have major implications for the synthesis of drugs and agrochemicals.
Many important molecules required for life exist in two forms that are mirror images of each other like our left and right hands. This property is called 'chirality', from the Greek word for hand, and the two forms are called 'enantiomers', from the Greek word for opposite.

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