This week’s ASAP is literally out of this world. The Meierhenrich group presents a review discussing the origin of molecular chirality in meteorites and interstellar ices.

We all know that life depends exclusively upon L-amino acid and D-sugar enantiomers for the molecular architecture of proteins and nucleic acids. This review collects the current thinking as to why! It also discusses the instrument in the chirality module on board the cometary probe Philae.

Now I was aware that meteorites contain organic material but the spectrum of organics is very large, which is amazing, from the review: “alkyl-substituted bicyclic and tricyclic aromatic compounds, aliphatic compounds ranging from C1 to C7, including both saturated and unsaturated hydrocarbons, more than 80 amino acids, including diamino acids, N-alkylated amino acids[16] and iminodiacids, small amounts of aldehydes and ketones up to C5, a wide spectrum of carboxylic acids and hydroxycarboxylic acids, several nucleobases, and sugar acids”. Wow, that is certainly a great supply of starting materials for organic chemistry however somewhat scarce with a long delivery time, sounds like Aldrich.

The amino acid with the highest L-%ee is isovaline ranging between -1 and +18%. The amino acids are thought to proceed via a Strecker reaction, although exactly how the chirality is generated is still a question to be answered, perhaps organocatalysis or quantum mechanical dynamic kinetic resolution.

Moving onto comets: These objects can be modelled in the lab by vacuum deposition of compounds known to be present in space onto a cold surface and irradiating the whole lot with either UV or charged particles. Apparently 26 amino acids have been identified using this simulation, no chirality though. But the chirality is observable in the model system. Irradiation of an ice mixture of H2O, CH3OH, and NH3 with ultraviolet circularly polarized light (UV-CPL) produced amino acids with an ee of around -2.5%, the rotation being dependant upon the direction of polarisation. So there may be several mechanistic interpretations for this observation which are difficult to distinguish, asymmetric photosynthesis and absolute asymmetric photolysis being two of them.

As we all know crystallisation can lead to optical enhancement. An effect known as Viedma ripening was demonstrated for sodium chlorate. A single chiral solid state was formed under “near-equilibrium” conditions, under which no new crystals nucleate. This was extended to proteinogenic amino acids under prebiotic conditions and resulted in a  in a solid-phase enantiomeric enrichment of up to 99% for aspartic acid! Of course aspartic acid meets all the requirements for this process i.e. formation of a conglomerate and rapid racemisation in solution. This turns out to be no so efficient for the rest of the amino acids. But I suppose anything is possible in outer space, especially under the harsh conditions out there.

In any case this review is worth a read and makes me even more interested in the upcoming results from the Rosetta mission. Organic molecules have been detected, but they have not yet announced which and if they are chiral. At least we know the answer to the ultimate question, it is 42, but how we got there is still to be determined.

2,649 total views, 1 views today