Formation of chiral CO polyhedral crystals on icy interstellar grains

The crystallinity and morphology of solid CO on icy interstellar grains were examined by observing the deposition, crystallisation, and UV and electrons irradiation of solid CO using transmission electron microscopy. Herein, we found that solid CO deposited in molecular clouds was crystalline, and that even if amorphous CO was deposited amorphous CO crystallised within 103 years at 10 K. Conversely, crystalline CO was not amorphised by UV rays or electron beam at 10 K. These results indicated the occurrence of chiral crystalline CO instead of amorphous CO in space. Furthermore, the large surface diffusion coefficients of CO on amorphous H2O and crystalline CO at 10 K facilitated the morphological equilibration of crystalline CO. Bad wetting of crystalline CO with amorphous H2O proved that the morphology of the ice grains was not spherical with an onion-like structure, as hitherto assumed, but rather it was a polyhedral crystalline CO attached to amorphous H2O. This has important implications for phenomena associated with the collision and subsequent sticking between ice grains, surface chemical reactions, non-thermal desorption of molecules and the origin of homochirality in interstellar biomolecules.

Homochirality Is Originated from Handedness of Helices

Homochirality is a common feature of amino acids and carbohydrates, whose origin is still unknown. For example, 19 of 20 natural amino acids are L-chiral but deoxyribose sugars in DNA are always D-chiral. Meanwhile, right-handed helices are ubiquitous in nature. Are these two phenomena intrinsically correlated? Here, we propose that homochirality of amino acids and nucleotide sugars is originated from the handedness of helices. We show that right-handed 3_10-helix and alpha-helix favor the L-chiral form for amino acids, but for deoxyribose sugars right-handed helices prefer the D-chiral form instead. Our analyses unveil that there exist strong cooperativity effects dominated by electrostatic interactions. This work not only resolves the mystery of homochirality by providing a unified explanation for the origin of homochirality in proteins and DNA using helical secondary structures as the root cause, but also ratifies the Principle of Chirality Hierarchy, where chirality of a higher hierarchy dictates that of lower ones. Possible applications of the present work to asymmetric synthesis and macromolecular assembly are discussed.

Homochirality Is Originated from Handedness of Helices

Homochirality is a common feature of amino acids and carbohydrates, whose origin is still unknown. For example, 19 of 20 natural amino acids are L-chiral but deoxyribose sugars in DNA are always D-chiral. Meanwhile, right-handed helices are ubiquitous in nature. Are these two phenomena intrinsically correlated? Here, we propose that homochirality of amino acids and nucleotide sugars is originated from the handedness of helices. We show that right-handed 3_10-helix and alpha-helix favor the L-chiral form for amino acids, but for deoxyribose sugars right-handed helices prefer the D-chiral form instead. Our analyses unveil that there exist strong cooperativity effects dominated by electrostatic interactions. This work not only resolves the mystery of homochirality by providing a unified explanation for the origin of homochirality in proteins and DNA using helical secondary structures as the root cause, but also ratifies the Principle of Chirality Hierarchy, where chirality of a higher hierarchy dictates that of lower ones. Possible applications of the present work to asymmetric synthesis and macromolecular assembly are discussed.

The Chirality Induction and Modulation of Polymers by Circularly Polarized Light

Chirality is a natural attribute nature of living matter and plays an important role in maintaining the metabolism, evolution and functional activities of living organisms. Asymmetric conformation represents the chiral structure of biomacromolecules in living organisms on earth, such as the L-amino acids of proteins and enzymes, and the D-sugars of DNA or RNA, which exist preferentially as one enantiomer. Circularly polarized light (CPL), observed in the formation regions of the Orion constellation, has long been proposed as one of the origins of single chirality. Herein, the CPL triggered asymmetric polymerization, photo-modulation of chirality based on polymers are described. The mechanisms between CPL and polymers (including polydiacetylene, azobenzene polymers, chiral coordination polymers, and polyfluorene) are described in detail. This minireview provides a promising flexible asymmetric synthesis method for the fabrication of chiral polymer via CPL irradiation, with the hope of obtaining a better understanding of the origin of homochirality on earth.

Entropic Analysis of Mirror Symmetry Breaking in Chiral Hypercycles

Replicators are fundamental to the origin of life and evolvability. Biology exhibits homochirality: only one of two enantiomers is used in proteins and nucleic acids. Thermodynamic studies of chemical replicators able to lead to homochirality shed valuable light on the origin of homochirality and life in conformity with the underlying mechanisms and constraints. In line with this framework, enantioselective hypercyclic replicators may lead to spontaneous mirror symmetry breaking (SMSB) without the need for additional heterochiral inhibition reactions, which can be an obstacle for the emergence of evolutionary selection properties. We analyze the entropy production of a two-replicator system subject to homochiral cross-catalysis which can undergo SMSB in an open-flow reactor. The entropy exchange with the environment is provided by the input and output matter flows, and is essential for balancing the entropy production at the non-equilibrium stationary states. The partial entropy contributions, associated with the individual elementary flux modes, as defined by stoichiometric network analysis (SNA), describe how the system’s internal currents evolve, maintaining the balance between entropy production and exchange, while minimizing the entropy production after the symmetry breaking transition. We validate the General Evolution Criterion, stating that the change in the chemical affinities proceeds in a way as to lower the value of the entropy production.