Life Born Left-Handed

Icy patch in southern hemisphere on Titan. Scientists hope for ways to detect chirality on future missions to Titan. Chirality, or "handedness," is a hallmark of terrestrial biomolecules: It’s what makes living chemistry work, from assembling DNA to digesting food.
Image Credit: NASA/JPL

A chemical reaction that demonstrates how key molecules in the biological world might have come to be predominately left or right handed has been reported by scientists at Imperial College London.

Ever since discovering that the building blocks of the biological world, such as amino acids and sugars, are distinctively left or right handed – possessing a quality known as chirality – scientists have been puzzling to answer how and why.

They believe that at the dawn of biological life there were even numbers of molecules in each form, but through hitherto unknown processes, one particular form came to completely dominate over the others (for example left-handed amino acids and right-handed sugars), a feature known as homochirality.

Now, using simple organic molecules, the Imperial researchers have demonstrated that an amino acid itself can amplify the concentration of one particular chiral form of reaction product. Importantly, the experiment works in similar conditions to those expected around pre-biotic life and displays all the signs to suggest it may be a model for how biological homochirality evolved.

When a molecule comes in two mirror-image forms, it is termed chiral. The majority of amino acids are chiral molecules (shown above). Amino acids of biological origin are exclusively homochiral which in turn are left-handed. All proteins on Earth are composed of amino acids of the L type, allowing a chain of them to fold up nicely into a compact protein. When scientists synthesize amino acids from nonchiral precursors, the result is always a "racemic" mixture – equal numbers of right- and left-handed forms.
Credit: Bernhard Rupp

The research is published this week in the journal Angewandte Chemie International Edition.

Today, chemists regularly make catalysts that will steer a reaction towards products in one particular left or right form. Known as asymmetric reactions, they are commonly used in the pharmaceutical industry to make drugs which are active in the body in only one of the chiral forms.

"Chemists today can make templates to steer towards one form or another, but what happened at the beginning of the world when no such templates existed?" asks Professor Donna Blackmond, Professor of Catalysis at Imperial and senior author on the paper.

Over 50 years ago, a theorist explained how domination of one chiral form may arise. F.C. Frank suggested in 1953 that a tiny amount of one particular chiral form may become amplified into an excess over the other, through a process known as autocatalysis where the substance acts as a catalyst for producing more of itself. His paper concluded with the teasing words: "A laboratory demonstration may not be impossible."

Until 1995, researchers searched in vain to conquer what had been further ‘dubbed a challenge to all red-blooded chemists’. That year Kenso Soai and colleagues from Tokyo University demonstrated the first reaction to meet the Frank criteria, using an organozinc compound as catalyst.

In 2003, Professor Blackmond first heard reports by chemists at the Scripps Research Institute of a new reaction catalysed by proline, an amino acid.

What caught her eye was that the reaction appeared to be much faster than other proline-catalysed reactions.

Back in her laboratories she began to run the reaction, analysing it with sensitive calorimetry equipment that continuously monitors its rate. Proline indeed catalysed the reaction, exhibiting an unexpectedly high, accelerating reaction rate and an amplification of product excess in one particular chiral form; both tell-tale signs of a reaction that can rationalize the evolution of biological homochirality.

"This work may offer the first purely organic complement to the Soai reaction in the search for the chemical origin of life," says Professor Blackmond.

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