β-structures of polypeptides with L- and D-residues

The possible formation of beta-structures from polypeptide chains with L-and D-Residues randomly distributed was statistically analyzed within the frame of two hypotheses. Firstly, only those segments containing residues of identical chirality can associate to form antiparallel beta-structures, and secondly these segments must have a minimum length. The influence of different factors was examined: initial ratio of the L-and D-monomer, minimum length required for the segments to be incorporated into beta-sheets, average length of the peptide molecules, and stereoselectivity in the course of the polymerization process. The results show that in all cases nuclei of beta-sheets surrounded by random coil segments are formed, the optical activity of which very increases to purity when the initial ratio of monomers deviates from the racemic mixture. This suggests experiments to enrich the system in one enantiomer. Comparison is made with the corresponding behavior and properties of the alpha-helical structure.     

Computer experiment for selection of optical isomer during prebiotic polymerization

Computer experiments simulating prebiotic polymerization were carried out using the Monte Carlo method. A high degree of selection of single optical isomers from racemic monomers occurred by assuming a small asymmetry in their chemical natures, which was included to investigate an, amplification of the asymmetry due to parity non-conservation. The asymmetry developed much more slowly than the growth of polymers, and the degree of asymmetry in the polymers increased with the degree of polymerization. Competition between the preferential selection and the disturbance by the statistical fluctuation are discussed.     

Chirality of electrons from beta-decay and the left-handed asymmetry of proteins

A simplified mathematical model of the origin of the left-handed asymmetry of proteins in living matter is presented. The model is based on the hypothesis of Vester and Ulbricht that the chirality of (lefthanded) electrons from naturally beta-active elements, e.g.,¹⁴C,⁴⁰K, etc., was the specific source of the asymmetry; it requires for its application data on the interaction of electrons having non-zero chirality with racemic mixtures of amino acids. This interaction is here treated theoretically in an order-of-magnitude calculation. Our analysis yields a very approximate value of the induced steady-state asymmetry in the amino acids at the beginning of protein synthesis and indicates that this asymmetry, though small, may have been suffcient to account for the dominant left-handedness of proteins now observed.     

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.     

An Extended Model for the Evolution of Prebiotic Homochirality: A Bottom-Up Approach to the Origin of Life

A generalized autocatalytic model for chiral polymerization is investigated in detail. Apart from enantiomeric cross-inhibition, the model allows for the autogenic (non-catalytic) formation of left and right-handed monomers from a substrate with reaction rates epsilon L and epsilon R, respectively. The spatiotemporal evolution of the net chiral asymmetry is studied for models with several values of the maximum polymer length, N. For N = 2, we study the validity of the adiabatic approximation often cited in the literature. We show that the approximation obtains the correct equilibrium values of the net chirality, but fails to reproduce the short time behavior. We show also that the autogenic term in the full N = 2 model behaves as a control parameter in a chiral symmetry-breaking phase transition leading to full homochirality from racemic initial conditions. We study the dynamics of the N--> infinity model with symmetric (epsilon L = epsilon R) autogenic formation, showing that it only achieves homochirality for epsilon > epsilon c, where epsilon c is an N-dependent critical value. For epsilon 相似文献   

Chirality and life

The crucial role of homochirality and chiral homogeneity in the self-replication of contemporary biopolymers is emphasized, and the experimentally demonstrated advantages of these chirality attributes in simpler polymeric systems are summarized. The implausibility of life without chirality and hence of a biogenic scenario for the origin of chiral molecules is stressed, and chance and determinate abiotic mechanisms for the origin of chirality are reviewed briefly in the context of their potential viability on the primitive Earth. It is concluded that all such mechanisms would be non-viable, and that the turbulent prebiotic environment would require an ongoing extraterrestrial source for the accumulation of chiral molecules on the primitive Earth. A scenario is described wherein the circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae engenders asymmetric photolysis of the racemic constituents in the organic mantles on interstellar dust grains, whereupon these chiral constituents are transported repetitively to the primative Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.     

Life and chirality beyond the earth

An attempt is made to show that the phenomenon of chirality—of which optical activity is but one consequence—is by no means restricted to life on Earth, but is common throughout the universe. Several independent sources have been investigated including: statistical fluctuations; stereoselective physical factors; and energetic differences between enantiomeric molecules. It is emphasised that a search for chirality as an indicator for life elsewhere in space provides an excellent tool for the fascinating question of exobiology. Still one must be aware of the limitations of the experimental methods and their interpretations.     

Punctuated Chirality

Most biomolecules occur in mirror, or chiral, images of each other. However, life is homochiral: proteins contain almost exclusively L-amino acids, while only D-sugars appear in RNA and DNA. The mechanism behind this fundamental asymmetry of life remains an open problem. Coupling the spatiotemporal evolution of a general autocatalytic polymerization reaction network to external environmental effects, we show through a detailed statistical analysis that high intensity and long duration events may drive achiral initial conditions towards chirality. We argue that life's homochirality resulted from sequential chiral symmetry breaking triggered by environmental events, thus extending the theory of punctuated equilibrium to the prebiotic realm. Applying our arguments to other potentially life-bearing planetary platforms, we predict that a statistically representative sampling will be racemic on average.     

Morphological and chiral symmetry breaking in reaction-diffusion systems

Morphological and chiral symmetry breaking in reaction-diffusion systems is considered on the basis of the theory of imperfect codimension-two bifurcations. A new type of pattern selection with two triggers is elucidated: (1) morphologically asymmetric structures displaying optical activity can probably be originated from initially racemic and homogeneous conditions when chiral interaction, having the characteristic strength delta (such as electroweak interaction and circularly polarized light) as well as external field, having the characteristic strength eta (such as gravitational field and electrostatic field) are considered; (2) the selective sensitivity of molecular chirality and morphological asymmetry is omicron(delta 1/3) and omicron(eta 1/3), respectively; the sensitivity of mode-mode interaction between chiral polarization and concentration vector is omicron(delta 2/3) or omicron(eta 2/3), respectively. The relation of these conclusions to the life problem is discussed briefly.     

Primate polyspecific associations: Do they occur by chance?

Contrary to subjective impressions, the frequency of mixed-species associations of African forest primates is often no greater than would be expected by ‘chance’. Two null hypotheses predicting association frequency are developed and compared with data on mangabeys in the Kibale forest, Uganda; association frequencies imply behavioural attraction in the the case of only one of six species present. For other species, the simpler possibilities that associations reflect high population density or use of mutual feeding sites cannot be rejected. Comparative data from West African associations suggest more ‘significant’ association tendencies there. It is suggested that, even where the evidence indicates that tendencies to associate between species have been favoured by selection, association need not benefit both species.     

Thermal effects in guest–host systems: [Zn2(bdc)(S‐lac)(dmf)]•PhEtOH: A DSC and NMR study

Differential scanning calorimetry and nuclear magnetic resonance were used to investigate thermal effects in the guest–host systems where homochiral metal–organic sorbent [Zn₂(bdc)(S ‐lac)(dmf)] is considered as a host while 1‐phenylethanol enantiomers and their racemic mixture serve as guest molecules. A maximum energy gain from the guest–host interaction was observed in the system with the racemic mixture. The effect of host–guest recognition was revealed for the case of the host and guest having a similar type of chirality in the presence of antipode guest molecules.     

Analysis of asymmetries in the African fruit bats Eidolon helvum and Rousettus egyptiacus (Mammalia: Megachiroptera) from the islands of the Gulf of Guinea. II. Integration and levels of multivariate fluctuating asymmetry across a geographical range

Using a set of cranial morphometric characters, trends of variation in multivariate fluctuating asymmetry were evaluated and compared in populations of African fruit bats Rousettus egyptiacus and Eidolon helvum from the Gulf of Guinea islands, and the adjacent mainland. Levels of asymmetry were compared across populations and species, and significant differences were found in both comparisons. Differences coincided with species‐specific patterns of morphological and genetic differentiation. Concordance of correlation matrices of asymmetry was also compared. Results were significant; concordance is hypothesized to be a by‐product of developmental processes that produce the ‘fox‐like’ morphology shared by these species. Consistency of asymmetry patterns suggests that the developmental pathway producing it is highly canalized. A prediction of the above hypothesis is that a radical change in the ‘fox‐like’ structural pattern would result in breakage of the asymmetry parameter associated with it.     

Meso and racemic organisms and the transition to asymmetry

In this paper we analyse extensive but previously unappreciated and scattered data on anomalous enzyme modulation by metabolite enantiomers for enzymes in the principle metabolic cycles and develop a coherent theory for their derivation from reciprocally coupled antipodal metabolisms in antecedent meso and racemic organisms. Structures for such chirally symmetric organisms are outlined, as is the gradual decoupling of their integrated antipodal metabolisms upon an evolutionary progression from meso organism to racemic organism to racemate of organisms. The driving forces and the dynamics for a transition to asymmetry are developed.     

Chemoenzymatic Preparation of Optically Active Long-Chain 3-Hydroxyalkanoates

The synthesis of optically active 3-hydroxyalkanoates of varying chain length (C_lo, C₁₄ and C₁₈) was investigated using chemoenzymatic methods. While bakers' yeast mediated reduction of 3-oxoalkanoates gave (R)-3-hydroxyalkanoates (e.e. >80%, yield —10%), both enantiomers were obtained by enzymatic resolution of racemic methyl 3-butanoyloxyalkanoates using Geotrichum candidum lipase with moderate to good optical purity (e.e. 32–92%, yield 40–60% for both enantiomers). Lipase-catalysed interesterification, however, was found to proceed with similar enantioselectivity but with slow rate of conversion.     

Defining null expectations for animal site fidelity

Site fidelity—the tendency to return to previously visited locations—is widespread across taxa. Returns may be driven by several mechanisms, including memory, habitat selection, or chance; however, pattern-based definitions group different generating mechanisms under the same label of ‘site fidelity’, often assuming memory as the main driver. We propose an operational definition of site fidelity as patterns of return that deviate from a null expectation derived from a memory-free movement model. First, using agent-based simulations, we show that without memory, intrinsic movement characteristics and extrinsic landscape characteristics are key determinants of return patterns and that even random movements may generate substantial probabilities of return. Second, we illustrate how to implement our framework empirically to establish ecologically meaningful, system-specific null expectations for site fidelity. Our approach provides a conceptual and operational framework to test hypotheses on site fidelity across systems and scales.     

Synthesis and enantiomeric analysis of cyclotriphosphazene derivatives with one centre of chirality

A series of chiral cyclotriphosphazene compounds 2-9 in which the spiro 3-amino-1-propanoxy moiety provides the one centre of chirality have been synthesised and characterised by elemental analysis, MS, ¹H and ³¹P NMR spectroscopies. The enantiomers of newly synthesised compounds have been analysed by the changes in the ³¹P NMR spectra on addition of a Chiral Solvating Agent (CSA), (S)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. HPLC methods have been developed for the enantiomeric separations of chiral cyclotriphosphazenes containing one centre of chirality. It is found that chiral HPLC gave a good resolution of enantiomers of the racemic compounds 2-9 with resolution factors between 2.49 and 7.50 making them good candidates for enantiomeric separations and determination of absolute configuration.     

Exploitation of asymmetric predator–prey interactions by trophically transmitted parasites

The directionality of asymmetric interactions between predators (definitive hosts) and prey (intermediate hosts) should impact trophic transmission in parasites. This study tests the prediction that trophically transmitted parasites are funneled towards asymmetric predator–prey interactions where intermediate hosts have few predators and definitive hosts feed upon many prey (‘downward asymmetry’). The distribution of trophically transmitted parasites was examined in four published food webs in relation to mismatch asymmetry of predator–prey interactions. We found that trophically transmitted parasites exploit downwardly asymmetric interactions in a nonrandom manner, and particular predator–prey pairs contain more trophically transmitted parasites than would be expected by random chance alone. These findings suggest that food web topology has great bearing on the ecology of trophically transmitted parasites, and that consideration of parasite life cycles in the context of food web organization can provide insights into the forces affecting the evolution of trophic transmission.     

Beta-agonist-induced alterations in organ weights and protein content: comparison of racemic clenbuterol and its enantiomers

Clenbuterol is a relatively selective beta2-adrenergic partial agonist that has bronchodilator activity. This drug has been investigated as a potential countermeasure to microgravity- or disuse-induced skeletal muscle atrophy because of presumed anabolic effects. The purpose of this study was to: 1) analyze the anabolic effect of clenbuterol's (-)-R and (+)-S enantiomers (0.2 mg/kg) on muscles (cardiac and skeletal) and other organs; and 2) compare responses of enantiomers to the racemate (0.4 mg/kg and 1.0 mg/kg). Male Sprague Dawley rats were treated with: a) racemic clenbuterol (rac-clenbuterol, 0.4 or 1.0 mg/kg); b) enantiomers [clenbuterol (-)-R or (+)-S]; or c) vehicle (1.0 mL/kg buffered saline). Anabolic activity was determined by measuring tissue mass and protein content. HPLC teicoplanin chiral stationary phase was used to directly resolve racemic clenbuterol to its individual enantiomers. In skeletal muscle, both enantiomers had equal anabolic activity, and the effects were muscle- and anatomic region-specific in magnitude. Although the enantiomers did not affect the ventricular mass to body weight ratio, clenbuterol (+)-S induced a small but significant increase in ventricular mass. Both clenbuterol enantiomers produced significant increases in skeletal muscle mass, while being less active in producing cardiac ventricular muscle hypertrophy than the racemic mixture.     

Elementary Analysis of Multidimensional Contingency Tables. Application to a Medical Example1/2

By the aid of analysing a medical example a three—step procedure for analysing multi—dimensional contingency tables is introduced. This procedure has some good properties. Step one is due to catch the relationship structure between the variables connected by the contingency table. Hereby only so—called graphical models, a subclass of hierarchical models in regard of the parameters of the log—linear model, are admitted. The models can be generated by combination of hypotheses of pairwise conditional independence. Hereby a so-called Extended Combination Procedure is proposed using the position of the Chain of (hierarchical) Hypotheses. A useful symbolic notation for ‘Dependence Models’ in addition to that in form of ‘Independence Models’ and ‘Minimal Sets’ is proposed. Step two analyses the significant conditional pairs in regard to the question for what attribute level combinations of the condition complexes the relations remain significant. Step three investigates those tables recognised as significant in step two more closely to get ideas about the ‘sources’ of dependencies and possibilities of collapsing parts of the table. The procedure is mostly used in explorative data analysis although the simple steps can be used to test hypotheses, too.