The possibility of spontaneous resolution of enantiomers on a catalyst surface

Hyperchirality: a mathematically convenient and biochemically possible model for the kinetics of morphogenesis

Two general problems in morphogenesis are discussed: (a) scaling of the repeating unit of a pattern to the overall size of the organism; (b) biochemically realistic mechanisms for highly localized autocatalysis to produce pattern on a wall or membrane of a single cell. These problems are approached by comparing the Mills (1932) model for spontaneous resolution by bimolecular autocatalysis in formation of a pair of enantiomers (D and L) with the Turing (1952) model for morphogenesis by catalytic and inhibitory interactions of two morphogens (X and Y) very unsymmetrically matched in kinetic properties. A model for a morphogen M is proposed in the form of a pair of enantiomers M D and M L , with the out-of-equilibrium morphogen concentration variable in the Turing equations being the optical asymmetry M = M D — M L . Formation of M out of a precursor (pro-morphogen A), with two reactions A → M D and A → M L to control the single variable M , allows the Turing theory to encompass a variety of ways in which pattern unit scales with overall size. To this end, formation of morphogens by parallel reactions exceeding in number the morphogen concentration variables is a general principle independent of the reality of this specific model. More tentatively, the model is put forward as a possible real structure for morphogens on a cell wall or membrane. Both M D and M L must be present. It is suggested that the lowest level of organization at which structures of two chiralities might be found, when their molecules are of one chirality only, is the attachment of an enzyme polymer, with definite quaternary structure, to the cell surface. The enantiomers may then be the same polymeric assembly facing inwards or outwards. The word hyperchirality is suggested for this kind of asymmetry. Some morphogenetic features of the single-celled desmid alga Micrasterias are discussed, to illustrate the geometrical problems of translating morphogen kinetics into development of shape, and to show that the number of active morphogens and the dimensionality of the space in which they act may decrease during development.

Mirror symmetry breaking at the molecular level

Reasoning from two basic principles of molecular physics, P invariance of electromagnetic interaction and the second law of thermodynamics, one would conclude that mirror symmetry retained in the world of chiral molecules. This inference is fully consistent with what is observed in inorganic nature. However, in the bioorganic world, the reverse is true. Mirror symmetry there is definitely broken. Is it possible to account for this phenomenon without going beyond conventional concepts of the kinetics of enantioselective processes? This study is an attempt to survey all existing hypotheses containing this phenomenon.

What is the status of reaction-diffusion theory thirty-four years after turing?

Physicochemical explanations of phenomena are divisible into three classes: structure, equilibrium and kinetics. For the phenomena of biological development, many physical scientists have the preconception that the explanations must turn out to be principally kinetic. In this class of theory, reaction-diffusion is by far the most extensively developed, and is worthy of attention both for its own sake and because many of its features well exemplify the nature of the broader field of kinetic theory. Reaction-diffusion should be thought of as a class, rather than a species, of theory. This review addresses three aspects: first, the general nature of the two-morphogen interaction as first proposed by Turing and incorporated in many later models; second, the specifics of these later models and their probable relative scope; third, the current state of attempts to identify the chemical nature of morphogens. It is concluded that reaction-diffusion in particular, and kinetic theory in general, are now slowly emerging from the almost total neglect by biologists which reaction-diffusion suffered for its first 20 years.

Mirror symmetry breaking in biochemical evolution

The problem of origination of molecular asymmetry in biochemical evolution is discussed. The theoretical analysis shows that chiral purity of biomolecules has the biological significance for self-reproduction of organisms. The models of spontaneous symmetry-breaking in molecular systems are given. The aspects of various stages of biochemical evolution associated with the development of chiral polarization are analysed.

On the physical origin of biological handedness

In the racemic conglomerate crystallization of over 1,000 samples of D,L-sodium-ammonium tartrate the effect of 32P beta irradiation on the weight, optical activity, and crystallite size was measured. Both weight and optical activity showed a statistical dependence on the intensity of beta irradiation. The crystallite size is also affected by the presence of 32P. Asymmetric crystals are suggested to have been potential mediators between asymmetric parity violating forces and molecular asymmetry so that stereo-selective prebiotic chemical reactions involving crystals need not be considered 'chance' processes. No measurable difference in the energy content of optical isomers was found. An upper limit for the direct contribution of weak interactions to electromagnetic ones has been calculated. The mechanism of stereoselective crystal seeding by beta particles is discussed.

Stereoselective crystallization induced by traces of dissolved optically active impurities

Isothermal crystallization of D,L-sodium-ammonium tartrate with traces of different impurities admixed shows that the added chiral contaminations produce a preferential crystallization of the tartrate isomer of same handedness. The critical lowest concentration of effective seeding agents is 0.1-0.5%. 1% optically active excess material induces 1.0-3.6% optical purity in the deposited crystals. An analysis of the relevant data reported so far gives similar figures in different crystallization systems. The relation of the results to the suggested lattice energy difference between enantiomers is discussed.

Life and chirality beyond the earth

An attempt is made to show that the phenomenon of chirality- of which optical activity is but one consequence- is by no means restricted to life on Earth, but is common throughout the universe. Several independent sources have been investigated including: statistical fluctuations; stereoselective physical factors; and energetic differences between enantiomeric molecules. It is emphasised that a search for chirality as an indicator for life elsewhere in space provides an excellent tool for the fascinating question of exobiology. Still one must be aware of the limitations of the experimental methods and their interpretations.