In 1975, Buchwalter and Closs reported one of the first examples of heavy-atom quantum mechanical tunneling (QMT) by studying the ring closure of triplet cyclopentane-1,3-diyl to singlet ...bicyclo2.1.0pentane in cryogenic glasses. Since then, no clear theoretical evidence has been provided to elucidate how the intersystem crossing (ISC) and QMT are related in the reaction mechanism. In this work, we unequivocally show that at cryogenic temperatures, the ISC occurs solely in the quantum tunneling regime, with weak coupling non-adiabatic transition state theory rate constants predicting a spontaneous reaction in fair agreement with experimental observations. Despite its limitations, such an approach can be used to help unlock a comprehensive understanding of a variety of spin-forbidden chemical reactions in the low temperature regime.
The putative spin-forbidden heavy-atom tunneling process for the ring closure of cyclopentane-1,3-diyl at cryogenic temperatures is confirmed with calculations employing the weak-coupling formulation of nonadiabatic transition state theory.
Competitive Nitrogen versus Carbon Tunneling Nunes, Cláudio M; Eckhardt, André K; Reva, Igor ...
Journal of the American Chemical Society,
09/2019, Letnik:
141, Številka:
36
Journal Article
Recenzirano
Odprti dostop
Quantum mechanical tunneling (QMT) of heavy atoms like carbon or nitrogen has been considered very unlikely for the longest time, but recent evidence suggests that heavy-atom QMT does occur more ...frequently than typically assumed. Here we demonstrate that carbon vs nitrogen heavy-atom QMT can even be competitive leading to two different products originating from the same starting material. Amino-substituted benzazirine was generated in solid argon (3–18 K) and found to decay spontaneously in the dark, with a half-life of 210 min, to p-aminophenylnitrene and amino-substituted ketenimine. The reaction rate is independent of the cryogenic temperature, in contradiction to the rules inferred from classical transition state theory. Quantum chemical computations confirm the existence of two competitive carbon vs nitrogen QMT reaction pathways. This discovery emphasizes the quantum nature of atoms and molecules, thereby enabling a much higher level of control and a deeper understanding of the factors that govern chemical reactivity.
Synthesis of Chiral Cyclopentenones Simeonov, Svilen P; Nunes, João P. M; Guerra, Krassimira ...
Chemical reviews,
05/2016, Letnik:
116, Številka:
10
Journal Article
Recenzirano
The cyclopentenone unit is a very powerful synthon for the synthesis of a variety of bioactive target molecules. This is due to the broad diversity of chemical modifications available for the enone ...structural motif. In particular, chiral cyclopentenones are important precursors in the asymmetric synthesis of target chiral molecules. This Review provides an overview of reported methods for enantioselective and asymmetric syntheses of cyclopentenones, including chemical and enzymatic resolution, asymmetric synthesis via Pauson–Khand reaction, Nazarov cyclization and organocatalyzed reactions, asymmetric functionalization of the existing cyclopentenone unit, and functionalization of chiral building blocks.
Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided by fully ...anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm–1 in Ar, 6643 cm–1 in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm–1 in Ar, 6894 cm–1 in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed.
•Coffee silverskin is very rich in insoluble fibre (49%).•Potassium, magnesium and calcium are major macrominerals of silverskin.•The vitamin E profile of silverskin comprises seven ...vitamers.•Silverskin extracts protected erythrocytes from oxidative induced hemolysis,•At higher extract concentrations a pro-oxidant effect was observed.
Coffee silverskin (a coffee roasting by-product) contains high amounts of dietary fibre (49% insoluble and 7% soluble) and protein (19%). Potassium (∼5g/100g), magnesium (2g/100g) and calcium (0.6g/100g) are the major macrominerals. The vitamin E profile of silverskin comprises α-tocopherol, β-tocopherol, ɣ-tocopherol, δ-tocopherol, β-tocotrienol, ɣ-tocotrienol, and δ-tocotrienol. The fatty acid profile is mainly saturated (C16:0 and C22:0), but the total amount of fat is low (2.4%). Caffeine (1.25g/100g), chlorogenic acid (246mg/100g), and 5-hydroxymethylfurfural (5.68mg/100g) are also present in silverskin. Total phenolics and flavonoids are partially responsible for the in vitro antioxidant activity. Silverskin extracts protected erythrocytes from oxidative AAPH- and H2O2-induced hemolysis, but at high concentrations a pro-oxidant effect on erythrocyte morphology was observed.
Until recently, uncertainty quantification in low energy nuclear theory was typically performed using frequentist approaches. However in the last few years, the field has shifted toward Bayesian ...statistics for evaluating confidence intervals. Although there are statistical arguments to prefer the Bayesian approach, no direct comparison is available. In this work, we compare, directly and systematically, the frequentist and Bayesian approaches to quantifying uncertainties in direct nuclear reactions. Starting from identical initial assumptions, we determine confidence intervals associated with the elastic and the transfer process for both methods, which are evaluated against data via a comparison of the empirical coverage probabilities. Expectedly, the frequentist approach is not as flexible as the Bayesian approach in exploring parameter space and often ends up in a different minimum. We also show that the two methods produce significantly different correlations. In the end, the frequentist approach produces significantly narrower uncertainties on the considered observables than the Bayesian. Our study demonstrates that the uncertainties on the reaction observables considered here within the Bayesian approach represent reality more accurately than the much narrower uncertainties obtained using the standard frequentist approach.
Olive pomace is a major output of olive oil processing. This by-product is a valuable source of bioactive compounds with well-recognized benefits for human health and well-being.
In this work, the ...proximate composition and the profiles of vitamin E (HPLC-DAD-FLD), fatty acids (GC-FID) and phenolics (HPLC-DAD-FL/MSn) were determined. Additionally, a sustainable process for antioxidants extraction - Multi-frequency Multimode Modulated (MMM) ultrasonic technique – was compared to a conventional solid-liquid extraction. The total phenolics content and antioxidant activity (ferric reducing antioxidant power and DPPH scavenging ability) of the extracts were analysed to assess the efficacy of both extraction methodologies.
The vitamin E profile of the olive pomace comprised the vitamers α-tocopherol, β-tocopherol, α-tocotrienol and γ-tocopherol. α-Tocopherol was the major form (2.63 mg/100 g), while the other vitamers were present in amounts lower than 0.1 mg/100 g. The lipid fraction was especially rich in oleic acid (75%), followed by palmitic (10%), linoleic (9%), and stearic (3%) acids. Hydroxytyrosol and comsegoloside represented ≈79% of the total phenolics present in olive pomace. Hydroxytyrosol content was 83.6 mg/100 g, while tyrosol was present in lower amounts (3.4 mg/100 g).
Concerning the antioxidants extraction, the MMM technique allowed a faster and higher recovery (p < 0.05) of the compounds, compared to the conventional solid-liquid extraction. By this way, it seems to be a very promising eco-friendly and effective methodology to extract antioxidants from this and other matrices.
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•As olive oil production increases worldwide, huge amounts of wastes are generated.•Olive pomace (the major waste produced) is an environmental burden.•The chemical composition of the olive pomace was assessed.•Olive pomace is a source of bioactive compounds with human health benefits.•A sustainable method for antioxidants extraction is presented.
Freshwater crayfish, the world's third largest crustacean species, has been reported to accumulate high levels of metals, while the current knowledge of potential risk associated with crayfish ...consumption lags behind that of finfish. We provide the first estimate of human health risk associated with crayfish (Procambarus clarkii) consumption in China, the world's largest producer and consumer of crayfish. We performed Monte Carlo Simulation on a standard risk model parameterized with local data on metal concentrations, bioaccessibility (φ), crayfish consumption rate, and consumer body mass. Bioaccessibility of metals in crayfish was found to be variable (68–95%) and metal-specific, suggesting a potential influence of metal bioaccessibility on effective metal intake. However, sensitivity analysis suggested risk of metals via crayfish consumption was predominantly explained by consumption rate (explaining >92% of total risk estimate variability), rather than metals concentration, bioaccessibility, or body mass. Mean metal concentrations (As, Cd, Cu, Ni, Pb, Se and Zn) in surveyed crayfish samples from 12 provinces in China conformed to national safety standards. However, risk calculation of φ-modified hazard quotient (HQ) and hazard index (HI) suggested that crayfish metals may pose a health risk for very high rate consumers, with a HI of over 24 for the highest rate consumers. Additionally, the φ-modified increased lifetime risk (ILTR) for carcinogenic effects due to the presence of As was above the acceptable level (10−5) for both the median (ILTR=2.5×10−5) and 90th percentile (ILTR=1.8×10−4), highlighting the relatively high risk of As in crayfish. Our results suggest a need to consider crayfish when assessing human dietary exposure to metals and associated health risks, especially for high crayfish-consuming populations, such as in China, USA and Sweden.
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•Metals in crayfish may pose a risk to high consumption rate individuals in China.•Arsenic contributes most to the risk (hazard index) of metals in crayfish.•Metal bioaccessibilities in crayfish are variable and metal-specific.•Consumption rate is the most important in controlling risk of crayfish metals.•Crayfish should be considered when assessing human dietary exposure to metals.
We report detailed peptide‐binding affinities between 438 HLA Class I and Class II proteins and complete proteomes of seven pandemic human viruses, including coronaviruses, influenza viruses and ...HIV‐1. We contrast these affinities with HLA allele frequencies across hundreds of human populations worldwide. Statistical modelling shows that peptide‐binding affinities classified into four distinct categories depend on the HLA locus but that the type of virus is only a weak predictor, except in the case of HIV‐1. Among the strong HLA binders (IC50 ≤ 50), we uncovered 16 alleles (the top ones being A*02:02, B*15:03 and DRB1*01:02) binding more than 1% of peptides derived from all viruses, 9 (top ones including HLA‐A*68:01, B*15:25, C*03:02 and DRB1*07:01) binding all viruses except HIV‐1, and 15 (top ones A*02:01 and C*14:02) only binding coronaviruses. The frequencies of strongest and weakest HLA peptide binders differ significantly among populations from different geographic regions. In particular, Indigenous peoples of America show both higher frequencies of strongest and lower frequencies of weakest HLA binders. As many HLA proteins are found to be strong binders of peptides derived from distinct viral families, and are hence promiscuous (or generalist), we discuss this result in relation to possible signatures of natural selection on HLA promiscuous alleles due to past pathogenic infections. Our findings are highly relevant for both evolutionary genetics and the development of vaccine therapies. However they should not lead to forget that individual resistance and vulnerability to diseases go beyond the sole HLA allelic affinity and depend on multiple, complex and often unknown biological, environmental and other variables.
An imino group was used for the first time as a vibrational antenna to manipulate molecular conformations. Imino-thiol isomers of thioacetamide were generated upon UV-irradiation of its amino-thione ...tautomer isolated in argon matrices at 11 K. Selective and reversible conformational isomerizations were induced by narrowband near-IR irradiation tuned at the frequencies of the 2
ν
(NH) first stretching overtone of each imino-thiol isomer. The conformational isomerization concerns the change in the orientation of a remote -SH group, while the orientation of the imino (C&z.dbd;NH) group remains the same. Supported by quantum chemical anharmonic computations, this allowed for a reliable, isomer-selective vibrational assignment of the four imino-thiol isomers extending now over the full mid-IR and near-IR ranges. It was found that the experimental IR intensities of the 2
ν
(NH) first stretching overtones (computed 4-5 km mol
−1
) of the imino-thiol forms are comparable to those of the
ν
(NH) stretching fundamentals (computed 2-4 km mol
−1
). This is the first time such a phenomenon is reported for an imine molecule. The kinetics of conformational isomerization was monitored
in situ
, indicating that the irradiation-induced processes are significantly faster than the tunneling-driven spontaneous
cis
-
trans
rotamerization of the -SH group. Quantum yields for the rotamerizations of the -SH group resulting from the vibrational excitation of a remote -NH group were estimated and found to be comparable to those observed for matrix-isolated carboxylic acids and amino acids, where conformational changes of the -OH group were induced by the direct vibrational excitation of 2
ν
(OH) first stretching overtones.
We provide experimental evidence for the occurrence of selective and reversible conformational control over the SH group by vibrational excitation of remote NH groups. Using an imino group that acts as a molecular antenna has no precedents.
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