|Ronald Breslow has been taking some flack from the bloggosphere (here,here,here) not to mention twitter, for him mentioning dinosaurs in his JACS article on homochirality (DOI). Apparently dinosaurs sell. According to Breslow somewhere in the universe a planet exists inhabited by a life form with reversed stereochemistry but Breslow should have written that it could well be an advanced breed of grumpy bloggists and not dino's. A more serious offence according to the twitter mob is that Breslow in 2010 published a very similar article in Tetrahedron Letters but since we are boycotting Elsevier publications in this blog we are going to be lenient on this one. Never mind the dinosaurs or self-plagiarism, what point is Breslow trying to make? Here is the quick summary. |
Starting point is the well-known Murchison meteorite which famously contains a collection of 5 alpha methyl amino acids enriched in the S enantiomer. Interstellar space has all the building blocks to make amino acids (ammonia, ketones, acetylenes etc.) and when concentrated on a meteorite the Strecker reaction could do the trick. Enantiomeric enrichment then could result from selective destruction of one enantiomer by circularly polarized light. The astronomers still argue what type of star could generate this type of radiation (neutron star? dwarf?) and with what energy.
In phase two, having arrived on planet earth the amino acid passengers disembark the meteorite and start an ambitious program of chirality transfer (make new chiral molecules) and chiral amplification (increasing enantiomeric excess).
With respect to the first objective Breslow explains his group has already demonstrated it is possible to remove the methyl group from an amino acid with some preservation of chirality by simply exposing it to an alpha keto-acid with a copper salt as catalyst. D-sugars such as ribose can be synthesised from formaldehyde in the Formose reaction and an intermediate step to glyceraldehyde catalysed by chiral amino acids is enantioselective.
With respect to amplification Breslow mentions that when amino acid solutions containing a small excess of one enantiomer are concentrated, racemic crystals form more easily than homochiral crystals and are thus removed from the system. Same story with glyceraldehyde where the racemic crystalline form has poor water solubility. Ribose on its own does not do this trick but when combined with a purine to a uridine (en route to RNA) again the racemic crystals stand out.
Is this a solid theory for the origin of homochirality? No, Breslow is quick to point out the theory is plausible at best but the hunt is on for the remaining missing links.