Chapter 14 - Possible Mechanisms for Production of Large Enantiomeric Excess

Abstract

Several asymmetries in nature that might have led to excess production of a pair of enantiomers have been suggested, but they could produce only a very small enantiomeric excess. There are two conditions that must exist for a small initial enantiomeric excess to survive and increase. There must be a source of free energy that keeps the system far from equilibrium. There must also be some mechanism that amplifies the initial enantiomeric excess. Also, there are several mechanisms that can cause enantiomeric excess to increase in a system that is initially racemic or achiral. None of these can increase the enantiomeric excess to unity, and some of them produce only a transient increase that decays to a steady state having zero or very small enantiomeric excess. Mutual destruction is capable of producing enantiomeric excess that is close to unity, but mutual destruction causes the steady state concentrations of both S and R to be small. One of the most effective mechanisms for the production of enantiomeric excess close to unity is the random formation of a single homochiral oligomer molecule that catalyzes the production of its own monomers. Two or more mechanisms acting together are more effective than any one of them alone. Even a small or transient enantiomeric excess can lead to a large, permanent one because the chiral species can undergo favorable mutations, and the more abundant enantiomers will have greater probability of mutating. The relative importance of the several amplification mechanisms depends on the values of the rate constants. These values can be determined by experiments for specific reactions.