J- and H-aggregates of 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin and interconversion in PEG-b-P4VP micelles

Micellization of poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG114-b-P4VP61) induced by 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (TPPS) in acidic solutions were studied by dynamic and static light scattering, atomic force microscope, and UV-vis spectroscopy. The resultant complex micelles had a core-shell structure with the electrostatically complex TPPS/P4VP as the core and the soluble PEG as the shell. The anionic TPPS in the micellar core formed J-aggregates at pH 1.5-2.5 and H-aggregates at pH 3.0-4.0, respectively. Interconversion between the J-aggregates and the H-aggregates was carried out by adjusting the pH value of the micelle solutions. It is worth noting that the micelles showed strong split Cotton effect in the circular dichroism spectra although TPPS and the copolymer were all achiral. The resulting chirality sign could be selected by the hydrodynamic forces of a stirring vortex. Positive or negative chiral signals appeared when stirring clockwise or anticlockwise.     

Chirality as a primary switch of hierarchical levels in molecular biological systems

A synergetic law, being of common physicochemical and biological sense, is formulated: any evolving system that possesses an excess of free energy and elements with chiral asymmetry, while being within one hierarchical level, is able to change the type of symmetry in the process of self-organization increasing its complexity but preserving the sign of prevailing chirality (left — L or right — D twist). The same system tends to form spontaneously a sequence of hierarchical levels with alternating chirality signs of de novo formed structures and with an increase of the structures’ relative scales. In living systems, the hierarchy of conjugated levels of macromolecular structures that begins from the “lowest” asymmetric carbon serves as an anti-entropic factor as well as the structural basis of “selected mechanical degrees of freedom” in molecular machines. During transition of DNA to a higher level of structural and functional organization, regular alterations of the chirality sign D-L-D-L and L-D-L-D for DNA and protein structures, respectively, are observed. Sign-alternating chiral hierarchies of DNA and protein structure, in turn, form a complementary conjugated chiral pair that represents an achiral invariant that “consummates” the molecular-biological block of living systems. The ability of a carbon atom to form chiral compounds is an important factor that determined the carbon basis of living systems on the Earth as well as their development though a series of chiral bifurcations. The hierarchy of macromolecular structures demarcated by the chirality sign predetermined the possibility of the “block” character of biological evolution.     

Asymmetric Autocatalysis Induced by Chiral Crystals of Achiral Tetraphenylethylenes

The achiral hydrocarbon tetraphenylethylene crystallizes in enantiomorphous forms (chiral space group: P2₁) to afford right- and left-handed hemihedral crystals, which can be recognized by solid-state circular dichroism spectroscopic analysis. Chiral organic crystals of tetraphenylethylene mediated enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde to give, in conjunction with asymmetric autocatalysis with amplification of chirality, almost enantiomerically pure (S)- and (R)-5-pyrimidyl alkanols whose absolute configurations were controlled efficiently by the crystalline chirality of the tetraphenylethylene substrate. Tetrakis(p-chlorophenyl)ethylene and tetrakis(p-bromophenyl)ethylene also show chirality in the crystalline state, which can also act as a chiral substrate and induce enantioselectivity of diisopropylzinc addition to pyrimidine-5-carbaldehyde in asymmetric autocatalysis to give enantiomerically enriched 5-pyrimidyl alkanols with the absolute configuration correlated with that of the chiral crystals. Highly enantioselective synthesis has been achieved using chiral crystals composed of achiral hydrocarbons, tetraphenylethylenes, as chiral inducers. This chemical system enables significant amplification of the amount of chirality using spontaneously formed chiral crystals of achiral organic compounds as the seed for the chirality of asymmetric autocatalysis.     

On chirality transfer in electron donor-acceptor complexes. A prediction for the sulfinimine···BF3 system

Stabilization energies of the electron donor-acceptor sulfinimine···BF(3) complexes calculated at either the B3LYP/aug-cc-pVTZ or the MP2/aug-cc-pVTZ level do not allow to judge, whether the N- or O-atom in sulfinimine is stronger electron-donor to BF(3) . The problem seems to be solvable because chirality transfer phenomenon between chiral sulfinimine and achiral BF(3) is expected to be vibrational circular dichroism (VCD) active. Moreover, the bands associated with the achiral BF(3) molecule are predicted to be the most intense in the entire spectrum. However, the VCD band robustness analyses show that most of the chirality transfer modes of BF(3) are unreliable. Conversely, variation of VCD intensity with change of intermolecular distance, angle, and selected dihedrals between the complex partners shows that to establish the robustness of chirality transfer mode. It is also necessary to determine the influence of the potential energy surface (PES) shape on the VCD intensity. At the moment, there is still no universal criterion for the chirality transfer mode robustness and the conclusions formulated based on one system cannot be directly transferred even to a quite similar one. However, it is certain that more attention should be focused on relation of PES shape and the VCD mode robustness problem.     

Nonhanded chirality in octahedral metal complexes

Chiral molecules can either be handed (i.e., "shoes") or nonhanded ("potatoes"). The only chiral ligand partition for tetrahedral metal complexes (or for a tetrahedral carbon atom such as that found in amino acids and other chiral biological molecules) is the fully unsymmetrical degree 6 partition (1(4)), which leads to handed metal complexes of the type MABCD with a lowest-degree chirality polynomial consisting of the product of all six possible linear factors of the type (s(i)-s(j)) where 1 < or = i,j < or = 4. The lowest-degree chiral ligand partitions for octahedral metal complexes are the degree 6 partitions (31(3)) and (2(3)) leading to handed chiral metal complexes of the types fac-MA(3)BCD and cis-MA(2)B(2)C(2). The form of the lowest-degree chirality polynomial for the (31(3)) chiral ligand partition of the octahedron resembles that of the (1(4)) chiral ligand partition of the tetrahedron, likewise with four different ligands. However, the form of the lowest-degree chirality polynomial for the (2(3)) chiral ligand partition of the octahedron corresponds to the square of the chirality polynomial of the (1(3)) chiral ligand partition of the polarized triangle, which likewise has three different ligands. Ligand partitions for octahedral metal complexes such as (2(2)1(2)), (21(4)), and (1(6)), which are less symmetrical than the lowest-degree chiral ligand partitions (31(3)) and (2(3)), lead to chiral octahedral metal complexes which are nonhanded. In such complexes, pairs of enantiomers can be interconverted by simple ligand interchanges without ever going through an achiral intermediate.     

Crystallization and solid-state reaction as a route to asymmetric synthesis from achiral starting materials

Many molecules which are achiral can crystallize in chiral (enantiomorphic) crystals and, under suitable conditions, crystals of only one chirality may be obtained. The formation of right- or left-handed crystals in excess is equally probable. Lattice-controlled (topochemical) photochemical or thermal solid-state reactions may then afford stable, optically active products. In the presence of the chiral products, achiral reactants may preferentially produce crystals of one chirality, leading to a feedback mechanism for the generation and amplification of optical activity. Amplification of optical activity can also be achieved by solid-state reactions. The optical synthesis of biologically relevant compounds by such routes may be envisaged.     

Asymmetric autocatalysis and its application to chiral discrimination

Chiral pyrimidyl, quinolyl, and pyridyl alkanols act as asymmetric autocatalysts with significant amplification of enantiomeric excess (ee) in the enantioselective addition of diisopropylzinc to pyrimidine-5-, quinoline-3-, and pyridine-3-carbaldehydes, respectively. 2-Alkynyl-5-pyrimidyl alkanol with as low as 0.6% ee automultiplies during the consecutive asymmetric autocatalysis with increasing ee to as high as >99.5%. Asymmetric autocatalysis is applied to chiral discrimination of organic compounds. In the presence of methyl mandelate or 2-butanol with very low ee's (0.05-0.1%) as chiral initiators, the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc affords pyrimidyl alkanol with higher ee's with the correlated absolute configurations to those of the chiral initiators. Chirality of amino acids (such as leucine) and helicenes with very low ee's are also discriminated by asymmetric autocatalysis, affording pyrimidyl alkanol with very high ee's. Asymmetric autocatalysis also discriminates the chirality of primary alcohols-alpha-d, monosubstituted [2.2]paracyclophanes and octahedral cobalt complex with achiral ligands of which the chirality is due to the topology of coordination of the achiral ligand. Even the chirality of inorganic crystals such as quartz and sodium chlorate is discriminated by asymmetric autocatalysis of pyrimidyl alkanol. Thus, asymmetric autocatalysis provides a unique method for the discrimination of chiral compounds and crystals.     

Space Asymmetry as a Possible Global Feature

A series of reports in the literature indicated symmetry breaking in assemblies of chiral molecules of opposite handedness. These unexpected observations could be accounted for as being generated by the “parity violation” of the nuclear weak force, combined with an autocatalytic amplification process. However, in many such cases, in particular of chiral fluids, this putative mechanism is far from providing a reasonable explanation for such discrimination. In this article it is suggested that space may have deviated a priori from absolute symmetry, a possibility which complies with observations in atoms and molecules and may even be implicated in the asymmetrical configuration of spiral galaxies. Space asymmetry can be extrapolated to a difference between the relative statistical weights of the “right” versus the “left” directions with respect to Euclidian coordinates or, analogously, to a difference between the clockwise and anticlockwise orientations in polar coordinates. The difference in weights of these directions in space is estimated to be around 1%, based on the differences observed in density values of chiral fluids and chiral crystals of NaClO₃. The implied asymmetry of time, as the conjugated fourth dimension, suggests a similar difference in magnitude of the time coordinate in a right‐handed versus left‐handed space, which is feasible for experimental verification. Chirality 25:308–311, 2013. © 2013 Wiley Periodicals, Inc.     

Stereodesign of chiral para-substituted calix[4]arenes

According to the hypothesis that the chirality of molecule hosts is a cause of their enantioselectivity, the chirality of para-substituted calix[4]arenes was analyzed quantitatively. The relationship between types of para-substituents and the dissymmetry of the 2D (two-dimensional) entrance into the cavity and the whole 3D (three-dimensional) cavity of calix[4]arenes was studied by means of the enantiomer dissimilarity factor (EDF) method for quantitative evaluation of molecular chirality. The design of the most chiral, and probably enantioselective, para-substituted calix[4]arenes was planned such that all four substituents should be different and the two largest should be near each other (adjacent). It was, on the other hand, shown that the 2D chiral entrance determines chirality of the whole 3D structures of these molecules. This phenomenon is interpreted as an example of the chirality transition from 2D into 3D space.Electronic Supplementary Material available.     

Proline autocatalysis in the origin of biological enantioenriched chirality

Biological enantioenriched chirality is a phenomenon that in living organisms, amino acids and carbohydrates typically have the same absolute configuration. Perhaps one of the earliest attempts to delineate the origins of this phenomenon was a theory known as asymmetric autocatalysis, a reaction in which the structures of the chiral catalyst and the product are the same, and in which the chiral product acts as a chiral catalyst for its own production. In theory, this would mean that small asymmetries in the product will propagate rapidly. However, autocatalysis also relies on the cross‐inhibition of chiral states, something that would not likely be possible on primordial Earth. But recently, theories on asymmetric autocatalysis have begun to resurface as more recent findings indicate that other mechanisms exist to stabilize the homochiral states. In this study, I propose an autocatalytic cycle, and using density functional theory, prove that (1) it is plausible on primordial Earth, and (2) it propagates arbitrary asymmetries in proline. Thus, facilitating asymmetry in proline and allowing access to a wide variety of asymmetric proline‐catalyzed reactions, including those involved in the synthesis of amino acids and carbohydrates from achiral precursors.     

Probing the "additive effect" in the proline and proline hydroxamic acid catalyzed asymmetric addition of nitroalkanes to cyclic enones

The effect of chirality and steric bulk of 2,5-disubstituted piperazines as additives in the conjugate addition of 2-nitropropane to cyclohexenone, catalyzed by l-proline, was investigated. Neither chirality nor steric bulk affects the enantioselectivity of addition, which gives 86-93% ee in the presence of achiral and chiral nonracemic 2,5-disubstituted piperazines. Proline hydroxamic acid is shown for the first time to be an effective organocatalyst in the same Michael reaction.     

First Results of the RAMBAS Experiment on Investigation of the Radiation Mechanism of Chiral Influence

The first results of the RAdiation Mechanism of Biomolecular ASymmetry (RAMBAS) experiment on investigation of the radiation mechanism of the influence on chiral molecules, as a factor leading to origination of chiral asymmetry are presented. It was found that irradiation of simple achiral materials by a flux of electrons from radioactive source initiated the synthesis of amino acids, and it resulted in asymmetric degradation and chiral asymmetry in a racemic mixture of amino acids. The results obtained can be important for the solution of the origin-of-life and biological homochirality problems.     

Chiral beta-particles interact differentially with enantiomers--theory to solve a long-existing debate.

The crucial organic molecules associated with life are chiral. Beta irradiation on the causal origin for the great preference of biomolecules in life over their corresponding enantiomers has been extensively studied. The left helical beta electrons, with spin and momentum antiparallel, should propagate with different velocities in the two enantiomers. Much effort has been done to investigate possible mechanism of inducing asymmetry, using longitudinally polarized beta rays to decompose asymmetrically racemic mixtures of biomolecules, but the results were inconclusive and contradictory. In this paper we present our viewpoint that the direct inelastic scattering between polarized electrons and chiral molecules is the dominant one in producing the asymmetry of cross sections for both beta+ and beta- irradiation. The inelastic cross section sigma (0 --> n) is dependent on the dipole strength Dn, rotatory strength Rn of the molecule. According to the theoretical study, the asymmetry of cross section F is approximately 10(-6) and it depends on the sign of Rn+(L type). When Rn+ > 0, sigma D > sigma L, it favors L-amino acid surviving. When Rn+ < 0, sigma D < sigma L, it favours D-amino acid surviving. Our results show that helical electrons do distinguish between molecules of opposite chirality, and the disputation and suspicion about the controversial experimental results between Garay and Darge & Thiemann can be explained.