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hubie [soylentnews.org] writes:

Chirality is the property whereby an object is not superposable with its mirror image, meaning that if you took an object and its mirror image and stacked them on top of each other, they would not match. A nice example of this [ucla.edu] are your hands; your hands are mirror images of each other, but if you turned one so that the palms pointed in the same direction, then they don't look the same (one thumb points up and the other points down). The chiral object and its mirror image, taken as a pair, are called enantiomers. In the language of organic chemistry, an enantiomeric pair is made up of one L-form and one D-form.

Whenever enantiomeric molecules are made in the lab, they are always produced in a 50/50 proportion of L- and D- forms. However, it turns out that the active organic compounds in a living being consist of only one or the other. Amino acids are only found in L-form while sugars are only found in D-form. One of the grand mysteries of life is why did this preference arise?

Some scientists at the NASA Ames Research Center [nasa.gov] analysed samples from 4.5 billion-year-old carbonaceous meteorites and found a large excess of D-form over L-form in the sugars on the meteorites [rsc.org]. This gives credence to idea that life developed from material carried by meteorites. Their work was published (DOI: 10.1073/pnas.1603030113 [pnas.org]) in the Proceedings of the National Academy of Science [pnas.org]

The kinds of molecules Cooper and Rios identified are thought to be precursors to some of the earliest forms of life. They reason that if molecules delivered to the early Earth were one particular handedness, any primitive life forms that eventually developed would favour the prevalent form. The results also fit with previous studies showing some meteorites contain excesses of the left handed version of some amino acids.

[BREAK]

Paper abstract:

Biological polymers such as nucleic acids and proteins are constructed of only one—the D or L—of the two possible nonsuperimposable mirror images (enantiomers) of selected organic compounds. However, before the advent of life, it is generally assumed that chemical reactions produced 50:50 (racemic) mixtures of enantiomers, as evidenced by common abiotic laboratory syntheses. Carbonaceous meteorites contain clues to prebiotic chemistry because they preserve a record of some of the Solar System’s earliest (∼4.5 Gy) chemical and physical processes. In multiple carbonaceous meteorites, we show that both rare and common sugar monoacids (aldonic acids) contain significant excesses of the D enantiomer, whereas other (comparable) sugar acids and sugar alcohols are racemic. Although the proposed origins of such excesses are still tentative, the findings imply that meteoritic compounds and/or the processes that operated on meteoritic precursors may have played an ancient role in the enantiomer composition of life’s carbohydrate-related biopolymers.

Original Submission