Possible resolution of racemic mixtures by bistability in “bioids”, open systems which can exist in several steady states

Abstract

The kinetics of an open system are discussed in which by a new effect of kinetic bistability the two antipodes of a racemate can react at different rates without participation of an enzyme or another external asymmetric agent. For this end it is required i) that each antipode A is transformed to a chiral product B by a stereoselective autocatalytic reaction: A1 + B1 → 2B1; A2 + B2 → 2B2, ii) the enantiomeric products B consume each other by a stereospecific reaction: B1 + B2 → (B1B2), iii) the racemic substrate mixture is supplied continuously into the system and substrates and products are withdrawn from the system by first order kinetics. No absolute but only gradual stereospecifity is required. The reciprocal consumption of the two autocatalysts leads to a cross inhibition. An analysis of the differential equations describing this system shows that under certain defined conditions the intermediary steady state (with both antipodes reacting at equal rates) is instable, and that every minute imbalance in the amounts of the autocatalysts will be amplified leading eventually to the dominance of one antipode and to the suppression of the other one. Such a system represents a chemical model of a flip-flop circuit. The generation of asymmetry by kinetic bistability in open systems allows an explanation of the origin of molecular asymmetry in biological evolution at the “prenucleoprotic” stage. It does not require the assumption of macromolecules like enzymes, linear matrices or external asymmetric agents like optically active minerals, circularly polarized light or a hypothetical cosmic asymmetry.