Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

36/484 (page 12)

12 THE ORIGIN OF LIFE in the earth's history, and with this in mind attention may be directed to one interesting type of process in the field of chemical kinetics—the dynamic equilibrium in which active molecules and free radicals exist in certain oxidation reactions, because the significance of the similarity of this equilibrium to certain biological equilibria does not seem to have been appreciated (at any rate in published articles) by those physical chemists who have described the reactions. V. DEGENERATE BRANCHING IN CHAIN REACTIONS The low-temperature oxidation of hydrocarbons and alcohols, both in the gas phase and in solution, is characterized by a chain process, the free radicals formed as intermediate compounds being regenerated in the course of the reaction from the substrates which are undergoing oxidation, pro¬ ducing reaction chains of considerable length (Laidler, 1950). This type of behaviour is also found in other chemical reactions, but low-temperature oxidations appear to be peculiar in that the chains show what has been called 'degenerate branching' (Semenov, 1935). At the start of the reaction the velocity of oxidation increases in an approximately exponential manner for a considerable period of time, during which the active intermediate compound or compounds increase in concentration by a branching or 'reproduction' process. In some cases (Bardwell & Hinshelwood, 1950, 1951) the intermediate reaction responsible for the branching can be identified with fair certainty as the fission of a peroxide bond between two oxygen atoms. As the reaction proceeds, the velocity of oxidation (and the concentration of intermediates) does not continue to increase indefinitely but comes into a dynamic equilibrium which may be maintained for a long time by continuous flow techniques (Newitt & Gardner, 1936). This behaviour is remarkably similar to that shown by living organisms in culture, if the active intermediate compound is taken to represent the analogue of the organism. For example, the velocity of oxidation rapidly reverts to the original rate if it is temporarily accelerated by the introduction of an excess of an intermediate (Bone & Gardner, 1936), a behaviour similar to the stability of numbers of a species in a given ecological situation. Bardwell & Hinshelwood (1951) derive a mathematical expression for this kinetic behaviour of the form , + (3) where x is the concentration of intermediate, Vq the rate of initiation, k and k' the velocity constants of the regenerative and destructive reactions of the intermediate and v the multiplication factor (in biological terms, the size of the litter) of the regenerative reaction. As these authors point out, this