Imitating prebiotic homochirality on Earth.
We show how the amino acids needed on prebiotic earth in their homochiral L form can be produced by a reaction of L-alpha-methyl amino acids-that have been identified in the Murchison meteorite-with alpha-keto acids under credible prebiotic conditions. When they are simply heated together they perform a process of decarboxylative transamination but with almost no chiral transfer, and that in the wrong direction, producing D-amino acids from the L-alpha-methyl amino acids. With copper ion a square planar complex with two of the reaction intermediates is formed, and now there is the desired L to L transformation, producing small enantioexcesses of the normal L-amino acids. We also show how these can be amplified, not by making more of the L form but by increasing its concentration in water solution. The process can start with a miniscule excess and in one step generate water solutions with L/D ratios in the over 90% region. Kinetic processes can exceed the results from equilibria. We have also examined such amplifications with ribonucleosides, and have shown that initial modest excesses of the D-nucleosides can be amplified to afford water solutions with D to L ratios in the high 90's. We have shown that the homochiral compound has two effects on the solubility of the racemate. On one hand it decreases the solubility of the racemate by its role in the solubility product, as a theoretical equation predicts. On the other hand, it increases the solubility of the racemate by changing the nature of the solvent, acting as a cosolvent with the water. This explains why the amplification, while large, is not as large as the simple theoretical equation predicts. Thus when credible examples are produced where small enantioexcesses of D-ribose are created under credible prebiotic conditions, the prerequisites for the RNA world will have been exemplified.
Breslow
Ronald
R
Department of Chemistry, Columbia University, New York, NY 10027, USA. rb33@columbia.edu
Levine
Mindy
M
Cheng
Zhan-Ling
ZL
D000596:Amino Acids / Q000138:chemical synthesis / Q000737:chemistry; D019418:Evolution, Chemical; D005118:Extraterrestrial Environment / Q000737:chemistry; D019110:Meteoroids; D015394:Molecular Structure; D009705:Nucleosides / Q000138:chemical synthesis / Q000737:chemistry; D000069497:Origin of Life; D011732:Pyridoxal Phosphate / Q000737:chemistry; D011733:Pyridoxamine / Q000737:chemistry; D012266:Ribose / Q000737:chemistry; D013237:Stereoisomerism; D014867:Water / Q000737:chemistry
D000596:Amino Acids; D009705:Nucleosides; D014867:Water; D011732:Pyridoxal Phosphate; D011733:Pyridoxamine; D012266:Ribose