Biological Implications of Organic Compounds in Comets. In: Newburn RL, Neugebauer M, Rahe J, editors. Comets in the Post-Halley Era. Dordrecht: Springer Netherlands; 1991. pp. 439-62. [DOI: 10.1007/978-94-011-3378-4_19]

Comets and the formation of biochemical compounds on the primitive Earth--a review

Thirty years ago it was suggested that comets impacting on the primitive Earth may have represented a significant source of terrestrial volatiles, including some important precursors for prebiotic synthesis (Oró, 1961, Nature 190: 389). This possibility is strongly supported not only by models of the collisional history of the early Earth, but also by astronomical evidence that suggests that frequent collisions of comet-like bodies from the circumstellar disk around the star beta Pictoris are taking place. Although a significant fraction of the complex organic compounds that appear to be present in cometary nuclei were probably destroyed during impact, it is argued that cometary collisions with the primitive Earth represented an important source of both free-energy and volatiles, and may have created transient, gaseous environments in which prebiotic synthesis may have taken place.

Homochirality and life

After clarifying the frequently misused term homochirality, the crucial importance of homochirality and chiral purity in the development and maintenance of the essential biopolymers of life--proteins and nucleic acids--is discussed. The harsh and forbidding prebiotic environment during the era of cometary impact after formation of the Earth approximately 4.5 Gyr ago is described, after which the most important abiotic mechanisms proposed historically for the genesis of chiral molecules on the primitive Earth are enumerated. Random and determinate terrestrial mechanisms are each evaluated with regard to the environmental restraints imposed during the impact era, and it is concluded that all such mechanisms would be inapplicable and implausible in the realistic prebiotic environment. To circumvent these limitations, an extended hypothesis is presented describing an extraterrestrial source of homochiral terrestrial molecules. Illustrated in Figure 2, this scenario involves the partial asymmetric photolysis of the racemic constituents of organic mantles on interstellar dust grains by the circularly polarized ultraviolet components of the synchrotron radiation emanating from the neutron star remnants of super-novae. The resulting homochiral constituents with low enanantiomeric excesses (e.e.s) so produced in the organic mantles are subsequently conveyed to Earth either by direct accumulation or, more likely, after coalescence into comets or asteroids, followed by repetitive impingement during the impact era. Finally, the low e.e.s of the extraterrestrial homochiral molecules so introduced are amplified by terrestrial autocatalytic or polymerization mechanisms into a state of chiral purity, then are ultimately concentrated and protected by sequestration in the interiors of spontaneously formed protocellular vesicles--there to await further chemical evolution toward the biomolecules of life. Recent observations of the excess of L-over D-amino acids in the Murchison meteorite are cited as validation for the early stages of the proposed hypothesis.

Origin of the homochirality of biomolecules

Molecules built up from a given set of atoms may differ in their three-dimensional structure. They may have one or more asymmetric centres that serve as reference points for the steric distribution of the atoms. Carbon atoms, common to all biomolecules, are often such centres. For example, the Cα atom between the carboxyl and amino groups in amino acids is an asymmetric centre: looking ON ward (i.e. from the carbOxyl to the amiNo group, with the Cα oriented so that it is above the carboxyl and amino groups) the radical characterizing the amino acid may be to the right (D-molecules) or to the left (L-molecules). Nineteen of the 20 amino acids occurring in proteins have such a structure (the exception is glycine, where the radical is a hydrogen atom). These pairs of molecules cannot be brought into coincidence with their own mirror image, as is the situation with our hands. The phenomenon has therefore been named handedness, or chirality, from the Greek word cheir, meaning hand. The two forms of the chiral molecules are called enantiomers or antipodes. They differ in rotating the plane of the polarized light either to the right or to the left. The sense of rotation depends on the wavelength of the measuring light, but at a given wavelength it is always opposite for a pair of enantiomers. Chirality may also occur when achiral molecules form chiral substances during crystallization (for example, quartz forms D-quartz or Lquartz). A detailed theoretical treatment of molecular chirality is given by Barron (1991).

Chirality and life

The crucial role of homochirality and chiral homogeneity in the self-replication of contemporary biopolymers is emphasized, and the experimentally demonstrated advantages of these chirality attributes in simpler polymeric systems are summarized. The implausibility of life without chirality and hence of a biogenic scenario for the origin of chiral molecules is stressed, and chance and determinate abiotic mechanisms for the origin of chirality are reviewed briefly in the context of their potential viability on the primitive Earth. It is concluded that all such mechanisms would be nonviable, and that the turbulent prebiotic environment would require an ongoing extraterrestrial source for the accumulation of chiral molecules on the primitive Earth. A scenario is described wherein the circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae engenders asymmetric photolysis of the racemic constituents in the organic mantles on interstellar dust grains, whereupon these chiral constituents are transported repetitively to the primitive Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.