Challenging the concept of "recycling" as a mechanism for the evolution of homochirality in chemical reactions.

The concept that "recycling" of reactants may be key to the spontaneous generation of a homochiral state in closed autocatalytic reaction networks has recently been introduced and has been supported by computer simulations of such reaction networks. It has been suggested that unidirectional cycles maintained away from equilibrium may avoid the inevitable establishment of a racemic state, and under such conditions the explicit reverse reactions dictated by microscopic reversibility may be all be treated as having negligible rates. We show here that because the equilibrium constants in a recycled network are interdependent, it is not valid to neglect all reverse reactions simultaneously; a very low value for the rate constant of one reverse reaction in the network dictates that another reverse reaction in the same network will exhibit a large rate constant. This conclusion is general and applies to any closed mass system where the energy input is subject to microscopic reversibility. Therefore, chemical reversibility cannot be invoked as a mechanism for the evolution of a single chiral molecular state in thermally activated reactions. (c) 2008 Wiley-Liss, Inc.