"The Origin of Prebiotic Homochirality on Earth"
Presented by Ronald Breslow
Columbia University
Abstract We show how the amino acids needed onprebiotic earth in their homochiral L form can be produced by a reaction of L-alpha-methyl amino acids—that have been identified in the Murchisonmeteorite—with alpha-keto acids under credible prebiotic conditions. Whenthey are simply heated together they perform a process of decarboxylativetransamination but with almost no chiral transfer, and that in the wrongdirection, producing D-amino acids from the L-alpha-methyl amino acids. With copper ion a square planar complex with two of the reactionintermediates is formed, and now there is the desired L to L transformation,producing small enantioexcesses of the normal L-amino acids. Wealso show how these can be amplified, not by making more of the L form but byincreasing its concentration in water solution. The process can startwith a miniscule excess and in one step generate water solutions with L/Dratios in the over 90% region. Kinetic processes can exceed the resultsfrom equilibria. We have also examined such amplifications withribonucleosides, and have shown that initial modest excesses of theD-nucleosides can be amplified to afford water solutions with D to L ratios inthe high 90’s. We have shown that the homochiral compound has two effects onthe solubility of the racemate. On one hand it decreases the solubilityof the racemate by its role in the solubility product, as a theoreticalequation predicts. On the other hand, it increases the solubilityof the racemate by changing the nature of the solvent, acting as a cosolventwith the water. This explains why the amplification, while large, is not aslarge as the simple theoretical equation predicts. Thus when credibleexamples are produced where small enantioexcesses of D-ribose are created undercredible prebiotic conditions, the prerequisites for the RNA world will havebeen exemplified.
Hosted by Prof. Sam Danishefsky
Thursday, April 22, 2010
Tea & cookies at 4:00pm in room 328 Havemeyer
Seminar at 4:30pm in room 209 Havemeyer
Reception to follow in the 7th Floor Lounge, Havemeyer
