Unidirectional circulation in a prebiotic photochemical cycle

Abstract

In this brief note, we suggest the possibility that a soliton-assisted unidirectional photochemical cycle has played a role in prebiotic evolution. This suggestion is based on a model calculation which is itself based on the detection of Davydov-type soliton overtones in acetanilide.Events, and in particular biochemical events involving reactions or changes of state of a macromolecule, may be supposed to occur one step at a time (1). Thus they may often be represented in terms of a network of closed pathways, or loops. At equilibrium, there will be no net circulation of the macromolecule round the network, the conditions of detailed balance being fulfilled. But equilibrium means death; a living system requires process, such as a one-way circulation round the network. This may be realized as a steady state or a transient approach to a steady state. Such behavior can only be produced and maintained at the expense of the environment, which together with the system itself constitutes a larger system undergoing, as a whole, a steady degeneration in accordance with the concept of entropy increase (thermodynamic time's arrow). Thus life becomes a local eddy in the stream of evolution. In the fully developed organism, this state of affairs, which provides for the transduction of energy (or free energy) from one chemical reaction to another, is made possible by a set of highly developed polyfunctional enzymes. It was to provide a mechanism for such one-way circulation that the concept of a trapped soliton as an energy packet, created at one point of the network and liquidated at another, was introduced in an earlier paper (2). For the special conditions of prebiotic life, before the development of chlorophyll (3), we suggest retention of a trapped soliton as the driving mechanism for maintaining the unidirectional circulation of a complex system round a closed network but suppose that the soliton results from the absorption of solar radiation, which replaces the chemical free energy that is supplied by the driving reaction under present, more complex conditions. The realism of this suggestion hinges on the relation between the rate of absorption of radiation of suitable wavelength by the primitive enzyme and the rate of the diffusion-limited binding of the substrate by the enzyme under the conditions of high dilution characteristic of the prebiotic world. Unless the two rates are of the same order of magnitude, there will be no appreciable transduction of free energy-either the radiation will be absorbed and dissipated without appreciable effect or the substrate will be bound and liberated unchanged. It is with estimates of these two rates that the present note is concerned; for this purpose, we shall consider a model system consisting of a hypothetical protein in which a Davydov-type soliton can be trapped and which is present at high dilution in aqueous solution.A possibility for vibrational soliton formation by infrared radiation in the backbone of a protein is provided by acetanilide. Recent studies of the in...