THE ORIGIN OF OPTICAL ACTIVITY

No other chemical characteristic is as distinctive of living organisms as is optical activity. Outside of organisms, all syntheses of disymmetric molecules produce equal numbers of optical antipodes (racemic mixtures) unless deliberate means are employed to bias the result by the use of asymmetric reagents or forces. Inside living organisms, however, all syntheses and degradations of such molecules involve one enantiomorph alone. Only the fact that chemistry is learned from the plane surfaces of paper and blackboard makes such selectivity seem strange. We tend to think of optical isomers as very much alike, but in fact they represent profound differences in shape; and, in the types of reaction upon which life depends, involving the ceaseless, intimate fitting together of molecular surfaces, shape is all-important. Organisms made the choice between optical antipodes long ago. To tamper with that choice now would be like trying to draw a left glove on a right hand. A start in either direction might be self perpetuating, but how was the original choice made? I n the past I think discussions of this problem have been misdirected, in the sense that an attempt was made to propose ways in which inorganic devices might have produced populations of organic molecules of predominantly one configuration or the other which, on their later incorporation into living organisms, conferred their optical activity on the latter. It is enormously more probable, of course, that all geochemical syntheses of organic molecules produced racemic mixtures. I think that organisms acquired optical activity, not as a gift from the inorganic world, but through processes of selection out of originally racemic mixtures. In what follows I shall try to outline the nature of such processes. How was this choice made initially?