On the origin of terrestrial homochirality for nucleosides and amino acids.
Before life could start on earth, it was important that the amino acid building blocks be present in a predominant handedness called the L configuration and that the ribose of RNA be predominantly in the D configuration. Because ordinary chemical processes would produce them in equal L and D amounts, it has long been a puzzle how the needed selectivities could have arisen. Carbonaceous chondrites such as the Murchison meteorite, which landed in Australia in 1969, brought some unusual amino acids with a methyl group replacing their alpha hydrogen. They cannot racemize and have a small but real excess of those with the L configuration. We have shown that they can partake in a synthesis of normal L amino acids under credible prebiotic conditions. We and others showed that small preferences can be amplified into solutions with very high dominance of the L amino acids because of the higher solubility of the pure L form than of the more stable DL racemic compound crystal. Here, we show that such solubility-based amplification of small excesses of three D nucleosides, uridine, adenosine, and cytidine, can also occur to form solutions with very high D dominance under credible prebiotic conditions. Guanosine crystallizes as a conglomerate and does not amplify in this way. However, under prebiotic conditions it could have been formed from homochiral D ribose from the hydrolysis of amplified adenosine or cytidine.
Breslow
Ronald
R
Department of Chemistry, Columbia University, New York, NY 10027, USA. rb33@columbia.edu
Cheng
Zhan-Ling
ZL
D000596:Amino Acids / Q000737:chemistry*; D001315:Australia; D019418:Evolution, Chemical; D019110:Meteoroids; D009705:Nucleosides / Q000737:chemistry*; D012266:Ribose / Q000737:chemistry; D012995:Solubility; D013237:Stereoisomerism
D000596:Amino Acids; D009705:Nucleosides; D012266:Ribose