Editorial of the PCCP themed issue

The elementary constituents of biomolecules present striking chiral asymmetry. Almost all DNA-coded aminoacids in proteins are left-handed isomers and ribose sugars of DNA and RNA are right-handed. The “homochiral” nature of elementary biomolecular building blocks is often considered as a requirement for the existence of living species since Pasteur some 150 years ago, and therefore understanding how the observed homochirality came into existence is viewed as a crucial doorway to tackle the problem of the origin of Life on Earth. Some think that prebiotic homochirality and process of Evolution are the results of pure chance (de facto) whereas others view them as necessity (de lege) resulting from a law of Nature working out the observed symmetry breaking.

This themed issue of PCCP illustrates some important progresses achieved in the last few years and the diverse sources of interest among scientists seeking to understand how homochirality came about and has been maintained. It contains a collection of invited and contributed papers describing experimental and theoretical work in connection with problems arising from a wide range of disciplines such as statistical thermodynamic, physical chemistry, geochemistry and evolutionary biology.

Parity violation induced by weak interactions has emerged as a plausible candidate responsible for a spontaneous symmetry breaking, in support of the necessity hypothesis. Evaluation of the parity violation energy differences (PVED) between enantiomers requires elaborate quantum chemistry. In order to provide guidance in the search for molecules that are most likely to be capable of leading to a first experimental observation of PVED, Saue et al. (DOI: 10.1039/C0CP01483D) presents a detailed decomposition of the molecular value into contributions of the different atoms. Thermal fluctuations can easily mask the existence of PVED that is evaluated between 10⁻¹⁵ and 10⁻¹² J mol⁻¹. Ultra-low temperature conditions reached in a Bose–Einstein condensate (BEC) have thus attracted attention. Low-temperatures may not seem biologically-relevant but it has recently been suggested that cold water ice may be a possible medium for apparition of life. Bargueño et al. (DOI: 10.1039/C0CP00907E) elaborates a simplified model of Bose–Einstein condensation of non-interacting chiral molecules. Taking into account the internal structure of the molecules and the presence of parity violation, detection of a small optical activity of the condensate and of a subcritical temperature in the heat capacity are proposed as alternative routes for observing PVED.

The cooperative effect between a weak external driving signal and noise in a non-linear system leads to a maximum response to the periodic force termed stochastic resonance. Coherent superposition of different quantum states and induced tunnelling effects intervene in quantum stochastic resonance (QSR). Gonzalo et al. (DOI: 10.1039/C0CP01319F) proposes using QSR in the population difference between the two enantiomers of a chiral molecule. Observation of PVED through a QSR signal would then require an ultra-low temperature such as that prevailing in a BEC. Optically active enantiomorphs and their racemic mixtures have non-identical structures in crystals due to their different hydrogen-bonding networks. Differences in fractional sublimation rates leading to chiral self-purification have recently been observed in α-(trifluoromethyl) lactic acid crystals. Schwerdtfeger et al. (DOI: 10.1039/C0CP01155J) provide the first explanation of this phenomenon at the molecular level. A new scheme for detection of PVED using Bose–Einstein condensation of chiral molecules is suggested.

In case ultra-low temperature is not warranted, a direct scheme for observing parity violation in molecules consists of measuring differences of infrared spectral absorption line frequencies. First attempts conducted on molecules containing only light atoms failed. More promising candidates are molecules containing heavy atoms leading to a larger predicted PVED due to increased interaction between electrons and nucleons. Darquié et al. (DOI: 10.1039/C0CP01806F) present a first such experimental step through ultra-high resolution infrared spectroscopy of methyltrioxorhenium, a member of a promising molecular family.

The occurrence of homochirality has been suggested to first take place in elementary building blocks and be possibly further amplified in polymers (RNA- or protein-world scenarios) within elementary prebiotic cells or even only concomitant with apparition of the genetic code. Homochirality may have been transferred from initial building blocks acting as asymmetric catalysts or more simply incorporated into asymmetric diastereoisomer complexes with different stabilities to other chiral molecules. In this context, Piccirillo et al. (DOI: 10.1039/C0CP01401J) use combined UV and infrared spectroscopy in order to investigate chiral recognition by comparing homochiral and heterochiral complex structures. The mass spectrometric discovery of magic numbers of serine octamers by Cooks has opened new avenues. In this issue, Cooks et al. (DOI: 10.1039/C0CP01402H) investigate the subtle hydrogen-bonding network of the serine tetramer, the smallest cluster displaying homochiral preference, providing clues to deciphering why enantiomeric enrichment is observed when serine octamers are formed from non-racemic solutions.

Templating of organic precursors of biomolecules has also been proposed as a possible mechanism for the origin of homochirality. Clays, in particular vermiculite, appear to be an appealing candidate due to their natural abundance, high surface area, and adsorption properties that may turn them into non-centrosymmetric minerals. Fraser et al. (DOI: 10.1039/C0CP01388A) investigate the adsorption of alanine, lysine, and histidine by chiral HPLC by vermiculite clay gels. Their results demonstrate that, during reaction with fresh vermiculite interlayers, a significant chiral enrichment, depending on the amino acid, of either L- and D-enantiomers occurs. Fraser et al. (DOI: 10.1039/C0CP01387K) also use in situ neutron scattering experiments designed to follow amino acid adsorption on one-dimensionally ordered vermiculite clay. Chiral specificity of the clay is highlighted by the observation of swelling that leads to a different amount of shift in the interlayer spacing upon addition of D- vs.L-histidine solutions.

Maintenance of an initial enantiomeric enrichment achieved in the early stage of the elaboration of life had to face the problem of the presence of racemization processes. Hochberg et al. (DOI: 10.1039/C0CP00992J) examine the problem of chiral polymerization and its ability to amplify a possible small initial enantiomeric excess facing unavoidable statistical fluctuations. They show that chiral asymmetric states can persist for a long time in long homopolymer chains before racemization takes place.

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