The extraterrestrial origin of the homochirality of biomolecules--rebuttal to a critique

Ronald C.D. Breslow (1931-2017): A career in review

Reviewed herein are key research accomplishments of Professor Ronald Charles D. Breslow (1931-2017) throughout his more than 60 year research career. These accomplishments span a wide range of topics, most notably physical organic chemistry, medicinal chemistry, and bioorganic chemistry. These topics are reviewed, as are topics of molecular electronics and origin of chirality, which combine to make up the bulk of this review. Also reviewed briefly are Breslow's contributions to the broader chemistry profession, including his work for the American Chemical Society and his work promoting gender equity. Throughout the article, efforts are made to put Breslow's accomplishments in the context of other work being done at the time, as well as to include subsequent iterations and elaborations of the research.

Data-Driven Astrochemistry: One Step Further within the Origin of Life Puzzle

Astrochemistry, meteoritics and chemical analytics represent a manifold scientific field, including various disciplines. In this review, clarifications on astrochemistry, comet chemistry, laboratory astrophysics and meteoritic research with respect to organic and metalorganic chemistry will be given. The seemingly large number of observed astrochemical molecules necessarily requires explanations on molecular complexity and chemical evolution, which will be discussed. Special emphasis should be placed on data-driven analytical methods including ultrahigh-resolving instruments and their interplay with quantum chemical computations. These methods enable remarkable insights into the complex chemical spaces that exist in meteorites and maximize the level of information on the huge astrochemical molecular diversity. In addition, they allow one to study even yet undescribed chemistry as the one involving organomagnesium compounds in meteorites. Both targeted and non-targeted analytical strategies will be explained and may touch upon epistemological problems. In addition, implications of (metal)organic matter toward prebiotic chemistry leading to the emergence of life will be discussed. The precise description of astrochemical organic and metalorganic matter as seeds for life and their interactions within various astrophysical environments may appear essential to further study questions regarding the emergence of life on a most fundamental level that is within the molecular world and its self-organization properties.

Imitating prebiotic homochirality on Earth

We show how the amino acids needed on prebiotic earth in their homochiral L form can be produced by a reaction of L-alpha-methyl amino acids-that have been identified in the Murchison meteorite-with alpha-keto acids under credible prebiotic conditions. When they are simply heated together they perform a process of decarboxylative transamination but with almost no chiral transfer, and that in the wrong direction, producing D-amino acids from the L-alpha-methyl amino acids. With copper ion a square planar complex with two of the reaction intermediates is formed, and now there is the desired L to L transformation, producing small enantioexcesses of the normal L-amino acids. We also show how these can be amplified, not by making more of the L form but by increasing its concentration in water solution. The process can start with a miniscule excess and in one step generate water solutions with L/D ratios in the over 90% region. Kinetic processes can exceed the results from equilibria. We have also examined such amplifications with ribonucleosides, and have shown that initial modest excesses of the D-nucleosides can be amplified to afford water solutions with D to L ratios in the high 90's. We have shown that the homochiral compound has two effects on the solubility of the racemate. On one hand it decreases the solubility of the racemate by its role in the solubility product, as a theoretical equation predicts. On the other hand, it increases the solubility of the racemate by changing the nature of the solvent, acting as a cosolvent with the water. This explains why the amplification, while large, is not as large as the simple theoretical equation predicts. Thus when credible examples are produced where small enantioexcesses of D-ribose are created under credible prebiotic conditions, the prerequisites for the RNA world will have been exemplified.

On the origin of terrestrial homochirality for nucleosides and amino acids

Before life could start on earth, it was important that the amino acid building blocks be present in a predominant handedness called the L configuration and that the ribose of RNA be predominantly in the D configuration. Because ordinary chemical processes would produce them in equal L and D amounts, it has long been a puzzle how the needed selectivities could have arisen. Carbonaceous chondrites such as the Murchison meteorite, which landed in Australia in 1969, brought some unusual amino acids with a methyl group replacing their alpha hydrogen. They cannot racemize and have a small but real excess of those with the L configuration. We have shown that they can partake in a synthesis of normal L amino acids under credible prebiotic conditions. We and others showed that small preferences can be amplified into solutions with very high dominance of the L amino acids because of the higher solubility of the pure L form than of the more stable DL racemic compound crystal. Here, we show that such solubility-based amplification of small excesses of three D nucleosides, uridine, adenosine, and cytidine, can also occur to form solutions with very high D dominance under credible prebiotic conditions. Guanosine crystallizes as a conglomerate and does not amplify in this way. However, under prebiotic conditions it could have been formed from homochiral D ribose from the hydrolysis of amplified adenosine or cytidine.

Unexpected differences between D- and L- tyrosine lead to chiral enhancement in racemic mixtures

We report here an unexpected difference in the solubilities of D- and L-tyrosine in water, which could be discerned by their rate of crystallization and the resulting concentrations of their saturated solutions. A supersaturated solution of 10 mM L-tyrosine at 20 degrees C crystallized much more slowly than that of D-tyrosine under the same conditions, and the saturated solution of L-tyrosine was more concentrated than that of D-tyrosine. Supersaturated solutions of 10 mM DL-tyrosine in water formed precipitates of predominantly D-tyrosine and DL-tyrosine, resulting in an excess of L-tyrosine in the saturated solution. The experimental setups were monitored independently by UV-absorption, radioactivity tracing, optical rotation and X-ray diffraction. The process of nucleation and crystallization of D- and L-tyrosine is characterized by an exceptionally high cooperativity. It is possible that minute energy differences between D- and L-tyrosine, originating from parity violation or other non-conservative chiral discriminatory rules, could account for the observations. The physical process that initiated chiral selection in biological systems remains a challenging problem in understanding the origin of life, and it is possible that chiral compounds were concentrated from supersaturated racemic mixtures by preferential crystallization.

Enantiomer separation of hydrocarbons in preparation for ROSETTA's "chirality-experiment"

Until now the favored method for separating racemic pairs of underivatized alcohols, diols, and phenylsubstituted amines has been gas chromatography on cyclodextrin phases. However, certain enantiomers of saturated chiral hydrocarbons could not be resolved in this way because they lack the functional groups necessary to undergo "intensive" diastereomeric interactions with the cyclodextrins. The present study describes a gas-chromatographic technique for resolution of saturated aliphatic hydrocarbons into their enantiomers and presents a brief discussion of the possible applications. The (enantiomer) separations were performed in preparation for the Cometary Sampling and Composition Experiment on board the cometary lander RoLand, part of ESA's cornerstone mission ROSETTA. This experiment has been designed to investigate the hypotheses that biomolecular asymmetry has an interstellar origin and to separate and identify a wide range of organic enantiomers in situ on the surface of a comet's nucleus.

The emergence of life on Earth

Combined top-down and bottom-up research strategies and the principle of biological continuity were employed in an attempt to reconstruct a comprehensive origin of life theory, which is an extension of the coevolution theory (Lahav and Nir, Origins of Life Evol. Biosphere (1997) 27, 377-395). The resulting theory of emergence of templated-information and functionality (ETIF) addresses the emergence of living entities from inanimate matter, and that of the central mechanisms of their further evolution. It proposes the emergence of short organic catalysts (peptides and proto-ribozymes) and feedback-loop systems, plus their template-and-sequence-directed (TSD) reactions, encompassing catalyzed replication and translation of populations of molecules organized as chemical-informational feedback loop entities, in a fluctuating (wetting-drying) environment, functioning as simplified extant molecular-biological systems. The feedback loops with their TSD systems are chemically and functionally continuous with extant living organisms and their emergence in an inanimate environment may be defined as the beginning of life. The ETIF theory considers the emergence of bio-homochirality, a primordial genetic code, information and the incorporation of primordial metabolic cycles and compartmentation into the emerging living entities. This theory helps to establish a novel measure of biological information, which focuses on its physical effects rather than on the structure of the message, and makes it possible to estimate the time needed for the transition from the inanimate state to the closure of the first feedback-loop systems. Moreover, it forms the basis for novel laboratory experiments and computer modeling, encompassing catalytic activity of short peptides and proto-RNAs and the emergence of bio-homochirality and feedback-loop systems.

Homochirality of biomolecules: counter-arguments against critical notes

Three recently published critical papers by Bonner and coworkers on the extraterrestrial origin of the homochirality of biomolecules and the amplification of tiny enantiomeric excess are discussed. The presented arguments show the difficulties involved in circularly polarized u.v. and visible synchrotron radiation from neutron stars, in their ratio to non circularly polarized light and in racemization of the products. Attention is called upon another mechanism for production of extraterrestrial handedness based on Salam's condensation theory and on the recent experimental demonstration of the enantioselective magnetochiral photochemistry. Arguments, as well as experiments, point out that tiny enantiomeric excesses might be amplified via the Yamagata accumulation principle questioned by Bonner.

Conception of the 'Chirality-Experiment' on ESA's mission ROSETTA to comet P46/Wirtanen

In the last years extraterrestrial scenarios for the origin of homochirality in biological structures received considerable attention in the topical literature: Rubenstein and Bonner postulated a rapidly rotating neutron star emitting circularly polarised synchrotron radiation responsible for the first asymmetric synthesis; the group of Bailey published the observation of circular polarisation caused by Mie scattering from aligned dust grains in the Orion OMC-1 star-formation region that might provide an enantioselective effect on prochiral or racemic organic molecules. Rikken and Raupach observed a magnetochiral effect and considered extraterrestrial magnetic fields of sufficient strengths to introduce biomoleculars parity violation. With the aim to investigate these hypotheses among other theories describing the origin of biological asymmetry, our laboratory participates in the conception and development of ROSETTA's COSAC Experiment, that is designed to identify organic molecules in the cometary matter in situ. Within COSAC's 'Chirality Module' enantiomers will be separated gas chromatographically with the help of capillary columns coated with chirally active liquid films. This technique will allow the separation of specific chiral organic compounds out of the analysed cometary matter into their enantiomeric constituents. Both thermo conductivity and mass spectrometric detectors will be used to determine each enantiomer's amount and therefore the corresponding enantiomeric excesses. As a consequence of COSAC's 'Chirality-Experiment' far-reaching results are expected to investigate the various hypotheses about the first asymmetric synthesis.

Can enantiomorphic crystals like quartz play a role in the origin of homochirality on earth?

This communication reviews the possible actions of enantiomorphic crystals on the surface of Earth as sources of homochirality of organic compounds. The discovery of asymmetric adsorption and asymmetric catalysis involving optically active quartz crystals has led some authors to conclude that this source of asymmetry played an important role as a source of homochirality in nature, a concept that later proved erroneous. Moreover, data regarding the preponderance in nature of l-quartz crystals have been used to confirm calculations of the parity violation energy difference (PVED) for l-quartz and, hence, to explain the prevalence of L-amino acids and D-sugars in living matter. As discussed here, quartz and other enantiomorphs such as sodium chlorate can produce chiral intermediates active in autocatalytical processes. Our most recent compilation of the literature, however, reveals that the distribution of d- and l-quartz crystals at the surface of the Earth when all possible locations are included is quite random. Although quartz can serve as an effective asymmetric inductor in autocatalytic processes, it cannot be the source of homochirality of living matter because of the random occurrence of the two types of enantiomorphs. The calculations of PVED values for quartz therefore lack a sound physical basis.