Parity violation and chiral symmetry breaking of a racemic mixture

The chiral symmetry breaking of a racemic mixture by the parity violating weak interaction is considered. Particular attention is given to a mechanism recently proposed by Mason and Tranter whereby the weak neutral current interaction in chiral molecules leads to the differential absorption of unpolarized light by D vs. L enantiomers. After extending the usual theory of optical activity to include weak neutral currents, it is found that for spin-allowed transitions in typical organic molecules the weak photoabsorption asymmetry is much smaller than the value obtained using the reasoning of Mason and Tranter. Upon making a comparison with other mechanisms, it is concluded that differential radiolysis by beta electrons is likely to produce the largest symmetry breaking effect by the weak interaction.     

Superweak interactions and the biological time direction

A hypothesis is proposed in qualitative terms, according to which left helical electrons in chiral molecules, and right helical electrons in oppositely chiral molecules have different internal timings. The basis of this hypothesis stems from two facts: (1) It has been observed that the decay processes caused by superweak interactions go slower if left helical electrons are produced compared to decay processes in which right helical positrons are produced. (2) The excited states of D and L molecules are not only parity transformed but time reversed pairs as well. Re-evaluation of many data makes the hypothesis likely. The significance of this hypothesis in the origin and evolution of life is apparent since life and evolution as a whole have a special time direction.     

Can biological homochirality result from a phase transition?

The problem of chiral purity in living organisms is still one of the prominent difficulties in the study of the origins of life. In particular the parity non-conservation known to occur in weak interactions could not be related to this lack of symmetry: these physical forces, though universal, are very weak and up to now no amplification process had been proposed.In 1991, A. Salam remarked that, due to the attractive character of the parity violating force in electro-weak interactions, a phase transition at low temperature should exist, leading eventually to enantiomeric purity.We undertook then a series of experimental tests, looking for a sizeable change in the optical activity of cystine molecules. We found no evidence for the phase transition down to 0.01 K. The interpretation of these negative results will be discussed, and future experiments proposed.     

Cosmic Chirality both True and False

The discrete symmetries of parity P, time reversal T, and charge conjugation C may be used to characterize the properties of chiral systems. It is well known that parity violation infiltrates into ordinary matter via an interaction between the nucleons and electrons, mediated by the Z⁰ particle, that lifts the degeneracy of the mirror‐image enantiomers of a chiral molecule. Being odd under P but even under T, this P‐violating interaction exhibits true chirality and so may induce absolute enantioselection under all circumstances. It has been suggested that CP violation may also infiltrate into ordinary matter via a P‐odd, T‐odd interaction mediated by the (as yet undetected) axion. This CP‐violating interaction exhibits false chirality and so may induce absolute enantioselection in processes far from equilibrium. Both true and false cosmic chirality should be considered together as possible sources of homochirality in the molecules of life. Chirality 24:957‐958, 2012.© 2012 Wiley Periodicals, Inc.     

On the physical origin of biological handedness.

In the racemic conglomerate crystallization of over 1,000 samples of D,L-sodium-ammonium tartrate the effect of 32P beta irradiation on the weight, optical activity, and crystallite size was measured. Both weight and optical activity showed a statistical dependence on the intensity of beta irradiation. The crystallite size is also affected by the presence of 32P. Asymmetric crystals are suggested to have been potential mediators between asymmetric parity violating forces and molecular asymmetry so that stereo-selective prebiotic chemical reactions involving crystals need not be considered 'chance' processes. No measurable difference in the energy content of optical isomers was found. An upper limit for the direct contribution of weak interactions to electromagnetic ones has been calculated. The mechanism of stereoselective crystal seeding by beta particles is discussed.     

Parity nonconservation and the origin of biological chirality: Theoretical calculations

Recent theoretical calculations concerning the hypothesized connection between the parity violating weak interactions and the origin of biological chirality are extended in order to assess the relative importance of differential beta radiolysis of enantiomers and the energy difference between enantiomers. It is found that the largest chiral, polarizations are produced at lower temperatures where beta radiolysis dominates. At higher temperatures the energy difference between enantiomers dominates but the chiral polarization is appreciably smaller.     

The Role of Supernova Neutrinos on Molecular Homochirality

Electroweak parity violating interaction between supernova (SN) neutrinos and electrons of a simple chiral molecule is studied related to the origin of molecular homochirality. Appearance of supernova remnants inside molecular clouds favours the interaction of SN-neutrinos with interstellar molecules, leading to a energetic difference between the two enantiomers of the order of 10^–5 eV. This energetic difference is closer to the thermic energy of the interstellar medium, so molecular homochirality could be enhanced in molecular clouds containing supernova remnants inside it due to neutrino interaction.     

On the physical origin of biological handedness

In the racemic conglomerate crystallization of over 1000 samples of D, L-sodium-ammonium tartrate the effect of³²P beta irradiation on the weight, optical activity, and crystallite size was measured. Both weight and optical activity showed a statistical dependence on the intensity of beta irradiation. The crystallite size is also affected by the presence of³²P. Asymmetric crystals are suggested to have been potential mediators between asymmetric parity violating forces and molecular asymmetry so that stereo-selective prebiotic chemical reactions involving crystals need not be considered chance processes.No measurable difference in the energy content of optical isomers was found. An upper limit for the direct contribution of weak interactions to electromagnetic ones has been calculated. The mechanism of stereoselective crystal seeding by beta particles is discussed.     

Chirality and Gravitational Parity Violation

In this review, parity‐violating gravitational potentials are presented as possible sources of both true and false chirality. In particular, whereas phenomenological long‐range spin‐dependent gravitational potentials contain both truly and falsely chiral terms, it is shown that there are models that extend general relativity including also coupling of fermionic degrees of freedom to gravity in the presence of torsion, which give place to short‐range truly chiral interactions similar to that usually considered in molecular physics. Physical mechanisms which give place to gravitational parity violation together with the expected size of the effects and their experimental constraints are discussed. Finally, the possible role of parity‐violating gravity in the origin of homochirality and a road map for future research works in quantum chemistry is presented. Chirality 27:375–381, 2015.© 2015 Wiley Periodicals, Inc.     

The origin of optical asymmetry on earth.

The nature of optical isomerism, and the problem of the origin of optical asymmetry in relation to the origin of life are defined. Developments in particle physics, such as the discovery of parity non-conservation in weak interactions and more recently, of neutral currents, are described. Their significance is that there are a number of possible mechanisms whereby the fundamental asymmetry of matter could be reflected in a preference for one enantiomer over the other, and that, contrary to long-established views, optical isomers do not have identical energy contents: the difference, however, is estimated to be very small. Theories regarding the origin of optical asymmetry are classified in a two-dimensional matrix (origin by chance or due to already existing order; susceptible or not susceptible to experimental test). Recent experimental results and theoretical speculations are reviewed, and proposals are made for further experimental work.     

Weak selection and protein evolution

The "nearly neutral" theory of molecular evolution proposes that many features of genomes arise from the interaction of three weak evolutionary forces: mutation, genetic drift, and natural selection acting at its limit of efficacy. Such forces generally have little impact on allele frequencies within populations from generation to generation but can have substantial effects on long-term evolution. The evolutionary dynamics of weakly selected mutations are highly sensitive to population size, and near neutrality was initially proposed as an adjustment to the neutral theory to account for general patterns in available protein and DNA variation data. Here, we review the motivation for the nearly neutral theory, discuss the structure of the model and its predictions, and evaluate current empirical support for interactions among weak evolutionary forces in protein evolution. Near neutrality may be a prevalent mode of evolution across a range of functional categories of mutations and taxa. However, multiple evolutionary mechanisms (including adaptive evolution, linked selection, changes in fitness-effect distributions, and weak selection) can often explain the same patterns of genome variation. Strong parameter sensitivity remains a limitation of the nearly neutral model, and we discuss concave fitness functions as a plausible underlying basis for weak selection.     

Traction force microscopy of migrating normal and H-ras transformed 3T3 fibroblasts

Mechanical interactions between cell and substrate are involved in vital cellular functions from migration to signal transduction. A newly developed technique, traction force microscopy, makes it possible to visualize the dynamic characteristics of mechanical forces exerted by fibroblasts, including the magnitude, direction, and shear. In the present study such analysis is applied to migrating normal and transformed 3T3 cells. For normal cells, the lamellipodium provides almost all the forces for forward locomotion. A zone of high shear separates the lamellipodium from the cell body, suggesting that they are mechanically distinct entities. Timing and distribution of tractions at the leading edge bear no apparent relationship to local protrusive activities. However, changes in the pattern of traction forces often precede changes in the direction of migration. These observations suggest a frontal towing mechanism for cell migration, where dynamic traction forces at the leading edge actively pull the cell body forward. For H-ras transformed cells, pockets of weak, transient traction scatter among small pseudopods and appear to act against one another. The shear pattern suggests multiple disorganized mechanical domains. The weak, poorly coordinated traction forces, coupled with weak cell-substrate adhesions, are likely responsible for the abnormal motile behavior of H-ras transformed cells.     

Numerical study of a DRAKON-type closed magnetic configuration with a spatial axis

Results are presented from numerical studies of the magnetic field lines and the charged-particle trajectories in the magnetic system of the DRAKON device and of its curvilinear element—the KREL with magnetic mirrors. For the KREL, mirror ratio values are found that do not worsen the particle-drift compensation. The dimensions of the input region for the electrons injected into the KREL to create the beam-plasma discharge are calculated. Calculations show that, in the paraxial approximation, after multiple passes around the device, the magnetic field lines and trajectories of individual transit particles form a system of embedded toroidal surfaces with circular cross sections. When symmetrically changing the current distributions in the coils of the device, these surfaces shift with respect to their previous positions, but their shape remains the same. For the DRAKON device with helical KRELs, the shift of the drift surfaces with respect to the magnetic surfaces, as well as the flow of the longitudinal diamagnetic currents from the KRELs into the rectilinear regions, is found as a function of the pitch angle θ of the KREL helix.     

Structure and interactions of aggrecans: statistical thermodynamic approach

Weak polyelectrolytes tethered to cylindrical surfaces are investigated using a molecular theory. These polymers form a model system to describe the properties of aggrecan molecules, which is one of the main components of cartilage. We have studied the structural and thermodynamical properties of two interacting aggrecans with a molecular density functional theory that incorporates the acid-base equilibrium as well as the molecular properties: including conformations, size, shape, and charge distribution of all molecular species. The effect of acidity and salt concentration on the behavior is explored in detail. The repulsive interactions between two cylindrical-shaped aggrecans are strongly influenced by both the salt concentration and the pH. With increasing acidity, the polyelectrolytes of the aggrecan acquire charge and with decreasing salt concentration those charges become less screened. Consequently the interactions increase in size and range with increasing acidity and decreasing salt concentration. The size and range of the forces offers a possible explanation to the aggregation behavior of aggrecans and for their ability to resist compressive forces in cartilage. Likewise, the interdigitation of two aggrecan molecules is strongly affected by the salt concentration as well as the pH. With increasing pH, the number of charges increases, causing the repulsions between the polymers to increase, leading to a lower interdigitation of the two cylindrical polymer layers of the aggrecan molecules. The low interdigitation in charged polyelectrolytes layers provides an explanation for the good lubrication properties of polyelectrolyte layers in general and cartilage in particular.     

Weak trophic interactions and the balance of enriched metacommunities

Weak trophic interactions have been shown to promote the stability of ecological food webs characterized by perfect mixing. However, their importance at the landscape level and response to enrichment has not been extensively examined. In this paper we examine the food-web model explored by McCann et al. [1998. Weak trophic interactions and the balance of nature. Nature 395, 794-798]. The model is expanded into a metacommunity construct where local communities are coupled through global or local dispersal. We analyze global and local stability, as well as spatial synchrony in relation to trophic interaction strength and dispersal regimes. Results reveal that weak interactions can operate through two scale-dependent mechanisms: (i) under low local dispersal regimes, local stabilization of each community under weak interactions directly scales-up to global stability. (ii) Under high local dispersal, asynchronous local destabilization associated with weak interactions proves the driver behind global stability. In the face of enrichment, weak trophic interactions are shown to be instrumental in promoting global stability when dispersal is local. These results demonstrate how the importance of weak trophic interactions can be generalized at the landscape level despite contrary local predictions.     

Prebiotic sources of biomolecular handedness

Fischer demonstrated (1890–1919) that functional biomolecules are composed specifically of the D-sugars and the L-amino acids, and that in the laboratory synthetic reactions of such molecules propagate with chiral stereoselectivity. Given a primordial enantiomer, biomolecular homochirality followed without the intervention of the chiral natural force conjectured by Pasteur (1860), except prebiotically. Polarized solar radiation and other classical chiral forces were proposed as agencies generating a prebiotic enantiomeric excess, but the forces then known were found to be evenhanded on a time and space average, exemplifying parity conservation (1927). The weak nuclear force, shown to violate parity (1956), was unified with electromagnetism in the electroweak force (1970). Ab initio estimations including the chiral electroweak force indicate that the L-amino acids and the D-sugars are more stable than the corresponding enantiomers. The small energy difference between these enantiomer pairs, with Darwinian reaction kinetics in a flow reactor, account for the choice of biomolecular handedness made when life began.     

The Electrodynamics of Current-Current Interaction in a Double Helix

General expressions for the tangential and radial forces for two current elements moving contra-wise to each other in a helical manner are derived. The two repulsive forces are calculated as a function of velocity, with explicit values given for parameters, appropriate to DNA. The calculated electrodynamic respulsive forces oppose the stabilizing standard radial and tangential forces in the helix and, if large enough, can result in strand separation. The currents required are of the same order of magnitude as observed intra-cellular currents and as passed recently through short segments of duplex DNA. A speculative mechanism involving a resistance-capacitance membrane network in series with a resistance DNA network is formulated. Strand separation in DNA may well originate in such an electrodynamic system.     

Structural changes that occur in scallop myosin filaments upon activation

Myosin filaments isolated from scallop striated muscle have been activated by calcium-containing solutions, and their structure has been examined by electron microscopy after negative staining. The orderly helical arrangement of myosin projections characteristic of the relaxed state is largely lost upon activation. The oblique striping that arises from alignment of elongated projections along the long-pitched helical tracks is greatly weakened, although a 145 A axial periodicity is sometimes partially retained. The edges of the filaments become rough, and the myosin heads move outwards as their helical arrangement becomes disordered. Crossbridges at various angles appear to link thick and thin filaments after activation. The transition from order to disorder is reversible and occurs over a narrow range of free calcium concentration near pCa 5.7. Removal of nucleotide, as well as dissociation of regulatory light chains, also disrupts the ordered helical arrangement of projections. We suggest that the relaxed arrangement of the projections is probably maintained by intermolecular interactions between myosin molecules, which depend on the regulatory light chains. Calcium binding changes the interactions between light chains and the rest of the head, activating the myosin molecule. Intermolecular contacts between molecules may thus be altered and may propagate activation cooperatively throughout the thick filament.     

Synthesis and properties of mirror-image DNA.

We have investigated the conformations of the hexadeoxyribonucleotide, L-d(CGCGCG) composed of L-deoxyribose, the mirror image molecule of natural D-deoxyribose. In this paper, we report the synthesis of four L-deoxynucleosides and the L-oligonucleotide-ethidium bromide interactions. The L-deoxyribose synthon 9 was synthesized from L-arabinose with an over all yield of 28.5% via the Barton-McCombie reaction. The L-deoxynucleosides were obtained by a glycosylation of appropriate nucleobase derivatives with the 1-chloro sugar 9. After derivatization to nucleoside phosphoramidites, L-deoxycytidine and L-deoxyguanosine were incorporated into a hexadeoxynucleotide, L-d(CGCGCG) by a solid-phase beta-cyanoethylphosphoramidite method. This L-hexanucleotide was resistant to digestion with nuclease P1. The conformations of L-d(CGCGCG) were an exact mirror image of that of the corresponding natural one as described previously, and the conformations of the L-d(CGCGCG)-ethidium bromide complex were also the mirror images of those of the D-d(CGCGCG)-ethidium bromide complex under both low and high salt conditions. These results suggest that ethidium bromide prefers not a right-handed helical sense, but the base-base stacking geometry of the B-form rather than that of the Z-form. Thus, L-DNA would be a useful tool for studying DNA-drug interactions.     

Electrodynamics of Lipid Membrane Interactions in the Presence of Zwitterionic Buffers

Due to thermal motion and molecular polarizability, electrical interactions in biological systems have a dynamic character. Zwitterions are dipolar molecules that typically are highly polarizable and exhibit both a positive and a negative charge depending on the pH of the solution. We use multilamellar structures of common lipids to identify and quantify the effects of zwitterionic buffers that go beyond the control of pH. We use the fact that the repeat spacing of multilamellar lipid bilayers is a sensitive and accurate indicator of the force balance between membranes. We show that common buffers can in fact charge up neutral membranes. However, this electrostatic effect is not immediately recognized because of the concomitant modification of dispersion (van der Waals) forces. We show that although surface charging can be weak, electrostatic forces are significant even at large distances because of reduced ionic screening and reduced van der Waals attraction. The zwitterionic interactions that we identify are expected to be relevant for interfacial biological processes involving lipid bilayers, and for a wide range of biomaterials, including amino acids, detergents, and pharmaceutical drugs. An appreciation of zwitterionic electrodynamic character can lead to a better understanding of molecular interactions in biological systems and in soft materials in general.