Experimental Attempts for the Study of the Origin of Optical Activity on Earth

Abstract

Optical activity of natural compounds is a characteristic of our living world which is based on the asymmetry of the molecular set-up. It is hard to realize a biological cell which would be constructed from racemic compounds alone. Yet it seems attractive to ask why nature preferred only one of two possible enantiomers, e.g. the L-amino-acids and D-sugars. Was there or is there a chance for an antipodic biosphere constructed on the basis of the ‘unnatural’ enantiomers like D-amino-acids and L-sugars on Earth or elsewhere? — The paper presents in its first part a review about hypotheses that would be able to explain the apparent discrepancy between the expectation from laboratory experience and the observation that biological matter consists of extremely asymmetric molecules. The speculations found in literature are divided mainly into two categories: The first one interprets the appearance of optical activity by a chance process and its amplification by suitable means, the second one postulates a cogency leading to the chirality of the biosphere observed today. The discovery of the non-conservation of parity in nuclear physics stimulated a search for related ‘asymmetry effects’ in chemistry. Experiments were undertaken by some workers to construct possible laboratory models for the evolution of optical activity, but many of them failed due to different causes. On the other hand a number of papers has been published that were not directed specifically to the problem discussed, but could be interpreted on the basis of the various hypotheses. It is particularly interesting in this context to look into papers describing the crystallisation of race-mates from solutions, that were published as early as 70 yr ago. — In its second part the paper deals with the study of the polymerization of racemic amino-acids as a model that would possibly allow a decision between the hypotheses for the origin of optical activity, — mere chance or a physical driving force determining the chirality of evolution. Since great care was taken to eliminate all sources of systematical errors, one expected — from the classical standpoint — racemic poly-peptides of absolute zero optical activity. — The monomer amino-acids (α-alanine, α-anino-butyric acid, and lysine) were racemized before the polymerization in order to guarantee ‘ideally racemic’ substrates. Polymerization was achieved via the N-carboxyanhydrides of the amino-acids. Reaction vessels and measuring cells were thoroughly cleaned with boiling chromic sulfuric acid and kept sealed from the laboratory atmosphere to prevent any contamination. The optical activity was determined in a Cary 60 spectropolarimeter calibrated to detect angles of rotations in the range of 0.5 mdeg with a maximum error of ± 50%. All the poly-amino-acids investigated showed negative angles of rotation at 310 nm between o.25 and 0.84 mdeg that would correspond to an hypothetical asymmetry effect — i.e. the relative difference of the polymerization constants of L- and D-amino-acids — in the order of 8 × 10⁻⁶. We believe that this result emphasises the existence of a physical force that enables a slight accumulation of the L-amino-acids within the high molecular weight polymers in excess to the D-amino-acids and could be of significance for the evolution of the biomass. At this point the experiments do not allow any conclusion about the nature of the observed ‘asymmetry effect’.

Partially extracted from the thesis of the latter author, RWTH Aachen, 1972.