A Double-ended Queue with Catastrophes and Repairs, and a Jump-diffusion Approximation

Consider a system performing a continuous-time random walk on the integers, subject to catastrophes occurring at constant rate, and followed by exponentially-distributed repair times. After any repair the system starts anew from state zero. We study both the transient and steady-state probability laws of the stochastic process that describes the state of the system. We then derive a heavy-traffic approximation to the model that yields a jump-diffusion process. The latter is equivalent to a Wiener process subject to randomly occurring jumps, whose probability law is obtained. The goodness of the approximation is finally discussed.     

The crystalline state as a dynamic system: IR microspectroscopy under electrochemical control for a [NiFe] hydrogenase

Controlled formation of catalytically-relevant states within crystals of complex metalloenzymes represents a significant challenge to structure–function studies. Here we show how electrochemical control over single crystals of [NiFe] hydrogenase 1 (Hyd1) from Escherichia coli makes it possible to navigate through the full array of active site states previously observed in solution. Electrochemical control is combined with synchrotron infrared microspectroscopy, which enables us to measure high signal-to-noise IR spectra in situ from a small area of crystal. The output reports on active site speciation via the vibrational stretching band positions of the endogenous CO and CN⁻ ligands at the hydrogenase active site. Variation of pH further demonstrates how equilibria between catalytically-relevant protonation states can be deliberately perturbed in the crystals, generating a map of electrochemical potential and pH conditions which lead to enrichment of specific states. Comparison of in crystallo redox titrations with measurements in solution or of electrode-immobilised Hyd1 confirms the integrity of the proton transfer and redox environment around the active site of the enzyme in crystals. Slowed proton-transfer equilibria in the hydrogenase in crystallo reveals transitions which are only usually observable by ultrafast methods in solution. This study therefore demonstrates the possibilities of electrochemical control over single metalloenzyme crystals in stabilising specific states for further study, and extends mechanistic understanding of proton transfer during the [NiFe] hydrogenase catalytic cycle.

Electrochemically-coupled IR microspectroscopy of single crystals provides insight into proton-coupled electron transfer in [NiFe] hydrogenase.     

Characterization of the Trans-Epithelial Transport of Green Tea (C. sinensis) Catechin Extracts with In Vitro Inhibitory Effect against the SARS-CoV-2 Papain-like Protease Activity

This work describes an untargeted analytical approach for the screening, identification, and characterization of the trans-epithelial transport of green tea (Camellia sinensis) catechin extracts with in vitro inhibitory effect against the SARS-CoV-2 papain-like protease (PLpro) activity. After specific catechin extraction, a chromatographic separation obtained six fractions were carried out. The fractions were assessed in vitro against the PLpro target. Fraction 5 showed the highest inhibitory activity against the SARS-CoV-2 PLpro (IC₅₀ of 0.125 μg mL⁻¹). The untargeted characterization revealed that (−)-epicatechin-3-gallate (ECG) was the most abundant compound in the fraction and the primary molecule absorbed by differentiated Caco-2 cells. Results indicated that fraction 5 was approximately 10 times more active than ECG (IC₅₀ value equal to 11.62 ± 0.47 μg mL⁻¹) to inhibit the PLpro target. Overall, our findings highlight the synergistic effects of the various components of the crude extract compared to isolated ECG.     

Absolute infrared intensities in some metal carbonyls and the charge flux model of the dipole derivatives

The absolute intensities of the four infrared active modes of certain metal carbonyls, M(CO)₄ or M(CO)₆, are analyzed from the point of view of the equilibrium charge-charge flux model of the dipole derivatives. It is shown that the charges obtained from the bending modes, assuming negligible flux for these modes, are apparently identical with those reported by previous authors. However, such charges are much too sensitive to the choice of L matrix (which transforms the normal coordinates to symmetry coordinates) to permit any conclusions. The carbonyl stretching mode implies values of the dipole derivative three- to sixfold greater than in gaseous CO. It seems most reasonable to attribute most of this enhancement to charge flux between the metal and carbon atoms.     

Diffusion approximation to a queueing system with time-dependent arrival and service rates

A time nonhomogeneous diffusion approximation to a single server-single queue service system is obtained. Under various assumptions on the time-dependent functions appearing in the infinitesimal moments, transient and steady-state behaviour are analyzed. In particular, a diffusion approximation characterized by space-linear and time-varying moments is studied. The density of the busy period and the probability for the busy period to terminate are obtained. Finally, estimates of the goodness of the diffusion approximation are given.     

The alkaloid brachycerine is induced by ultraviolet radiation and is a singlet oxygen quencher

The effects of ultraviolet (UV) radiation on chlorophyll content and accumulation of the anti-inflammatory monoterpene-indole alkaloid brachycerine in plants and calli of Psychotria brachyceras (Rubiaceae) were investigated. In this study, we also investigated a protective role for brachycerine against stress conditions. Calli and tip cuttings incubated in nutrient media were daily supplemented with 4 or 16 h of UV. High-performance liquid chromatography analyses of methanolic extracts showed only traces of brachycerine in irradiated aseptic cultures, with no alkaloid being observed in control calli. In cuttings, a 10-fold increase in brachycerine content was seen after exposure for 16 h to UV-C, whereas a 4 h daily supplementation doubled the amount of the alkaloid in leaves. Exposure to a UV-B source also doubled the alkaloid yield. In vitro brachycerine was able to quench singlet oxygen. The data indicate a potential protective role for brachycerine against UV radiation, acting as a UV filter (absorption peaks are within the UV range) and a reactive oxygen species scavenger. In addition, UV radiation may be used to increase yields of this compound of pharmaceutical interest.     

Chemical characterization of gorse pyrolytic oils by capillary gas chromatography

Thermochemical conversion of biomass produces solid charcoal, gaseous fuels, and condensable oils. Because these oils vary widely in composition depending on the cellulosic feedstock and the operating conditions used, analysis of the oils is necessary in order to determine the best ways of utilizing or disposing of them. The major components of the organic oil from pyrolysis of gorse have been characterized by capillary gas chromatography – mass spectrometry. A qualitative study of the aqueous oil from the same source has also been attempted.     

Exploring the solid-phase synthesis of 3,4-disubstituted beta-lactams: scope and limitations

This work describes a comprehensive study on the solid-phase synthesis of 3,4-disubstituted beta-lactams. In situ generated ketenes react with immobilized aldimines under mild conditions to generate libraries of beta-lactams in good to very good overall isolated yields. Different commercially available solid supports were studied, with the cost-effective Wang resin proving to be the most effective. The utility of the protocol was also demonstrated by the highly efficient asymmetric versions when homochiral ketenes or homochiral aldimines were used. A practical technique for the preparation of manual solid-phase parallel libraries of biologically interesting beta-lactam compounds, using Mukaiyama's salt as dehydrating agent, is also presented. Reactions were easily monitored by FT-IR and gel-phase 13C NMR using conventional equipment.