Spontaneous Deracemizations Buhse, Thomas; Cruz, José-Manuel; Noble-Terán, María E ...
Chemical reviews,
02/2021, Letnik:
121, Številka:
4
Journal Article
Recenzirano
Spontaneous deracemizations is a challenging, multidisciplinary subject in current chirality research. In the absence of any chiral inductors, an achiral substance or a racemic composition is driven ...into an enantioenriched or even homochiral state through a selective energy input, e.g., chemical potential, photoirradiation, mechanical grinding, ultrasound waves, thermal gradients, etc. The most prominent examples of such transformations are the Soai reaction and the Viedma deracemization. In this review, we track the most recent developments in this topic and recall that many other deracemizations have been reported for solutions from mesophases to conglomerate crystallizations. A compiled set of simply available achiral organic, inorganic, organometallic, and MOF compounds, yielding conglomerate crystals, should give the impetus to realize new experiments on spontaneous deracemizations. Taking into account thermodynamic constraints, modeling efforts have shown that structural features alone are not sufficient to describe spontaneous deracemizations. As a guideline of this review, particular attention is paid to the physicochemical origin and symmetry requirements of such processes.
Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or ...supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions. A black hole was formally allowed as a solution for the companion to the Be star MWC 656 (ref. 5; also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe II profiles from the equatorial disk, and a refined Be classification (to that of a B1.5-B2 III star), indicates that a black hole of 3.8 to 6.9 solar masses orbits MWC 656, the candidate counterpart of the γ-ray source AGL J2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6 × 10(-7) times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.
Recent reports on both theoretical simulations and on the physical chemistry basis of spontaneous mirror symmetry breaking (SMSB), that is, asymmetric synthesis in the absence of any chiral ...polarizations other than those arising from the chiral recognition between enantiomers, strongly suggest that the same nonlinear dynamics acting during the crucial stages of abiotic chemical evolution leading to the formation and selection of instructed polymers and replicators, would have led to the homochirality of instructed polymers. We review, in the first instance, which reaction networks lead to the nonlinear kinetics necessary for SMSB, and the thermodynamic features of the systems where this potentiality may be realized. This could aid not only in the understanding of SMSB, but also the design of reliable scenarios in abiotic evolution where biological homochirality could have taken place. Furthermore, when the emergence of biological chirality is assumed to occur during the stages of chemical evolution leading to the selection of polymeric species, one may hypothesize on a tandem track of the decrease of symmetry order towards biological homochirality, and the transition from the simple chemistry of astrophysical scenarios to the complexity of systems chemistry yielding Darwinian evolution.
The pulsar/massive star binary system PSR B1259−63/LS 2883 is one of the best-studied gamma-ray binaries, a class of systems whose bright gamma-ray flaring can provide important insights into ...high-energy physics. Using the Australian Long Baseline Array, we have conducted very long baseline interferometric observations of PSR B1259−63 over 4.4 years, fully sampling the 3.4-year orbital period. From our measured parallax of 0.38 ± 0.05 mas- we use a Bayesian approach to infer a distance of |$2.6^{+0.4}_{-0.3}$| kpc. We find that the binary orbit is viewed at an angle of 154 ± 3° to the line of sight, implying that the pulsar moves clockwise around its orbit as viewed on the sky. Taking our findings together with previous results from pulsar timing observations, all seven orbital elements for the system are now fully determined. We use our measurement of the inclination angle to constrain the mass of the stellar companion to lie in the range 15–31 M_⊙. Our measured distance and proper motion are consistent with the system having originated in the Cen OB1 association and receiving a modest natal kick, causing it to have moved ∼8 pc from its birthplace over the past ∼3 × 10^5 years. The orientation of the orbit on the plane of the sky matches the direction of motion of the X-ray synchrotron-emitting knot observed by the Chandra X-ray Observatory to be moving away from the system.
Thermodynamic hypotheses and models are the touchstone for chemical results, but the actual models based on time-invariance, which have performed efficiently in the development of chemistry, are ...nowadays invalid for the interpretation of the behavior of complex systems exhibiting nonlinear kinetics and with matter and energy exchange flows with the surroundings. Such fields of research will necessarily foment and drive the use of thermodynamic models based on the description of irreversibility at the macroscopic level, instead of the current models which are strongly anchored in microreversibility.
Chiral hypercycle replicators (first-order autocatalysis together with mutual cross-catalysis) formed from achiral or racemizing resources may lead to spontaneous mirror symmetry breaking (SMSB) ...without the need for additional heterochiral inhibition reactions, such as those of the Frank-like models, which are an obstacle for the emergence of evolutionary selection properties. The results indicate that the chemical models for the emergence of primordial autocatalytic self-reproducing systems, of and by themselves, can also explain naturally the emergence of biological homochirality.
We have studied in detail the 0.15–15 GHz radio spectrum of the gamma-ray binary LS 5039 to look for a possible turnover and absorption mechanisms at low frequencies, and to constrain the physical ...properties of its emission. We have analysed two archival Very Large Array monitorings, all the available archival Giant Metrewave Radio Telescope (GMRT) data and a coordinated quasi-simultaneous observational campaign conducted in 2013 with Giant Metrewave Radio Telescope and Westerbork Synthesis Radio Telescope. The data show that the radio emission of LS 5039 is persistent on day, week and year time-scales, with a variability ≲ 25 per cent at all frequencies, and no signature of orbital modulation. The obtained spectra reveal a power-law shape with a curvature below 5 GHz and a turnover at ∼0.5GHz, which can be reproduced by a one-zone model with synchrotron self-absorption plus Razin effect. We obtain a coherent picture for the size of the emitting region of ∼0.85 mas, setting a magnetic field of B ∼ 20 mG, an electron density of n
e ∼ 4 × 105 cm−3 and a mass-loss rate of
$\dot{M}\sim 5\times 10^{-8} \,\mathrm{M_{\odot }\, yr^{-1}}$
. These values imply a significant mixing of the stellar wind with the relativistic plasma outflow from the compact companion. At particular epochs the Razin effect is negligible, implying changes in the injection and the electron density or magnetic field. The Razin effect is reported for the first time in a gamma-ray binary, giving further support to the young non-accreting pulsar scenario.
Achiral diprotonated porphyrins, forming homoassociates in aqueous solution, lead to spontaneous chiral symmetry breaking. The unexpected result is that the chirality sign of these homoassociates can ...be selected by vortex motion during the aggregation process. This result is confirmed by means of circular dichroism spectra. These experimental findings are rationalized in terms of the asymmetric influence of macroscopic forces on bifurcation scenarios and by considering the specific binding characteristics of the porphyrin units to form the homoassociates.
Summary Background The REVASCAT trial and other studies have shown that the neurovascular thrombectomy improves outcomes at 90 days post stroke. However, whether the observed benefit is sustained in ...the long term remains unknown. We report the results of the prespecified 12-month analysis of the REVASCAT trial. Methods Patients with acute ischaemic stroke who could be treated within 8 h of symptom onset were randomly assigned to medical therapy (including intravenous alteplase when eligible) and neurovascular thrombectomy with Solitaire FR or medical therapy alone. The main secondary outcome measure at 1 year follow-up was disability, measured using the modified Rankin Scale (mRS), ranging from 0 (no symptoms) to 6 (death) with categories 5 (severe disability) and 6 (death) collapsed into one category (severe disability or death), analysed as the distribution of the mRS. Additional prespecified secondary outcome measures included health-related quality of life measured with the EuroQol five dimensions questionnaire (EQ-5D) utility index (ranging from −0·3 to 1, higher values indicate better quality of life), the rate of functional independence (mRS 0–2), and cognitive function measured with the Trail Making Test (reported elsewhere). Treatment allocation was open label but endpoints at 12 months were assessed by masked investigators. The trial was registered at ClinicalTrials.gov , number NCT01692379. Findings From Nov 24, 2012, to Dec 12, 2014, 206 patients were randomly assigned to medical therapy plus endovascular treatment (n=103) or medical treatment alone (n=103), at four centres in Catalonia, Spain. At 12 months post randomisation, based on 205 of 206 outcomes available at 12 months, thrombectomy reduced disability over the range of the mRS (common adjusted odds ratio aOR 1·80, 95% CI 1·09–2·99), and improved functional independence (mRS=0–2; 45 44% of 103 patients vs 31 30% of 103 patients; aOR 1·86, 95% CI 1·01–3·44). Health-related quality of life was superior in the thrombectomy group (mean EQ-5D utility index score, 0·46 SD 0·38 in the thrombectomy group vs 0·33 0·33 in the control group, difference 0·12 95% CI 0·03–0·22; p=0·01). 1-year mortality was 23% (24 of 103 patients) in the thrombectomy group versus 24% (25 of 103 patients) in the control group. Interpretation At 12 months follow-up, neurovascular thrombectomy reduced post-stroke disability and improved health-related quality of life, indicating sustained benefit. These findings have important clinical and public health implications for evaluating the cost-effectiveness of the intervention in the long term. Funding Fundació Ictus Malaltia Vascular through an unrestricted grant from Medtronic.
The aldol reaction between acetone and 4‐nitrobenzaldehyde run in the nominal absence of any enantioselective catalyst was monitored by chiral HPLC with the aid of an internal standard. The collected ...data show the presence of a detectable initial enantiomeric excess of the aldol product in the early stages of the reaction in about 50 % of the experiments. Only a small fraction of the reaction contained the non‐racemic aldol product after 24 h. This temporary emergence of natural optical activity could be the signature of a coupled reaction network that leads to a spontaneous mirror‐symmetry‐breaking process, which originates at very low conversions (i.e., strongly depends on events taking place at the very first stages of the process). The reaction is not autocatalytic in the aldol product, which rules out a simple Frank‐type reaction network as the source of the observed symmetry breaking. On the other hand, the isolation and characterisation of a double‐aldol adduct suggested a reaction network that involved both indirect autocatalysis and indirect mutual inhibition between the enantiomers of the reaction product.
Mirror, mirror on the wall! HPLC monitoring of the aldol reaction between 4‐nitrobenzaldehyde and acetone (see figure) in the absence of a chiral catalyst reveals that spontaneous mirror symmetry breaking takes place initially in about 50 % of the reactions. A novel coupled reaction network that involves indirect autocatalysis and does not require heterochiral inhibition in the aldol product can explain this behavior.
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