Proton-coupled electron transfer of flavodoxin immobilized on nanostructured tin dioxide electrodes: thermodynamics versus kinetics control of protein redox function

In this paper, we report a spectroelectrochemical investigation of proton-coupled electron transfer in flavodoxin D. vulgaris Hildenborough (Fld). Poly-L-lysine is used to promote the binding of Fld to the nanocrystalline, mesoporous SnO(2) electrodes. Two reversible redox couples of the immobilized Fld are observed electrochemically and are assigned by spectroelectrochemistry to the quinone/semiquinone and semiquinone/hydroquinone couples of the protein's flavin mononucleotide (FMN) redox cofactor. Comparison with control data for free FMN indicates no contamination of the Fld data by dissociated FMN. The quinone/semiquinone and semiquinone/hydroquinone midpoint potentials (E(q/sq) and E(sq/hq)) at pH 7 were determined to be -340 and -585 mV vs Ag/AgCl, in good agreement with the literature. E(q/sq) exhibited a pH dependence of 51 mV/pH. The kinetics of these redox couples were studied using cyclic voltammetry, cyclic voltabsorptometry, and chronoabsorptometry. The semiquinone/quinone reoxidation is found to exhibit slow, potential-independent but pH-sensitive kinetics with a reoxidation rate constant varying from 1.56 s(-)(1) at pH 10 to 0.0074 s(-)(1) at pH 5. The slow kinetics are discussed in terms of a simple kinetics model and are assigned to the reoxidation process being rate limited by semiquinone deprotonation. It is proposed that this slow deprotonation step has the physiological benefit of preventing the undesirable loss of reducing equivalents which results from semiquinone oxidation to quinone.     

An overview of techniques and strategies for isolation of flavonoids from the genus Erythrina

Plants in the genus Erythrina is a potential source of chemical constituents, one of which is flavonoids, which have diverse bioactivities. To date, literature on the flavonoids from the genus Erythrina has only highlighted the phytochemical aspects, so this review article will discuss isolation techniques and strategies for the first time. More than 420 flavonoids have been reported in the Erythrina genus, which are grouped into 17 categories. These flavonoid compounds were obtained through isolation techniques and strategies using polar, semi-polar, and non-polar solvents. Various chromatographic techniques have been developed to isolate flavonoids using column flash chromatography, quick column chromatography, centrifugally accelerated thin-layer chromatography, radial chromatography, medium-pressure column chromatography, semi-preparative high-performance liquid chromatography, and preparative high-performance liquid chromatography. Chromatographic processes for isolating flavonoids can be optimized using multivariate statistical applications such as response surface methodology with central composite design, Box–Behnken design, Doehlert design, and mixture design.     

Cytotoxic effect,enzyme inhibition,and in silico studies of some novel N-substituted sulfonyl amides incorporating 1,3,4-oxadiazol structural motif

Molecular Diversity - The acetylcholinesterase and carbonic anhydrase inhibitors (AChEIs and hCAIs) remain key therapeutic agents for many bioactivities such as anti-Alzheimer and antiobesity...     

The Physical Properties of P2O5–Sm2O3–MnO2 Glass System

Phosphate glasses doped with samarium oxide have been made using the melt quenching technique. The density of the glass was determined using the Archimedes method while the Vickers hardness was measured using a micro-vicker tester. Both density and hardness have shown an increasing trend with the addition of a small amount of Sm₂O₃. The refractive index was characterized using UV-visible spectroscopy and the Sellmeier fitting equation was used to verify the result. The refractive index was found to vary from 1.972 to 1.807 depending on the wavelength and well agreed with the fitting curve     

Studying Systematic Errors on Estimation Decision, Detection, and Quantification Limit on Micro-TLC

The decision limit (CCα), capability of detection (CCβ) and quantification limit (QL) are importance performance characteristics in method validation. The TLC-Scanner 3 from Camag provides the possibility to choose the slit dimension of light to determine the peak chromatogram of a substance. The influence of the slit dimension for determination of CCα, CCβ and QL of paracetamol has been carried out. Paracetamol was spotted onto plates of AL-TLC Si G 60 F254 by linomat 4 in the range of 50–400 ng/spot and 10–400 ng/band, then on twin chambers eluted with TAEA (toluene:acetone-ethanol:conc.ammonia, 45 + 45 + 7 + 3 v/v) for 45 mm. Eluted spots were scanned in different slit dimensions at 248 nm. The CCα, CCβ and QL of paracetamol were estimated through the linear regression (LRM) and signal-to-noise (S/N) methods. Slit lengths between 50 and 133 % of the band width of the spots, and with the noise factor of the slit under 2.6, produced good precision measurements of TLC-densitometry between plates, while slit lengths between 50 and 83 % of the band width of the spots introduced a higher sensitivity response of the detector. The estimated CCα, CCβ and QL were determined by how the data were collected, the analytical optical setting, and the usage method for the estimation of both validation parameters.     

On exact solutions to epidemic dynamic models

In this study, we address an SIR (susceptible-infected-recovered) model that is given as a system of first order differential equations and propose the SIR model on time scales which unifies and extends continuous and discrete models. More precisely, we derive the exact solution to the SIR model and discuss the asymptotic behavior of the number of susceptibles and infectives. Next, we introduce an SIS (susceptible-infected-susceptible) model on time scales and find the exact solution. We solve the models by using the Bernoulli equation on time scales which provides an alternative method to the existing methods. Having the models on time scales also leads to new discrete models. We illustrate our results with examples where the number of infectives in the population is obtained on different time scales.     

Oscillation Criteria for Four-Dimensional Time-Scale Systems

In this paper, we obtain oscillation and nonoscillation criteria for solutions to four-dimensional systems of first-order dynamic equations on time scales. Especially, we are interested in the conditions which insure that every solution is oscillatory in the sub-linear, half-linear, and super-linear cases. Our approach is based on the sign of the components of nonoscillatory solutions. Several examples are included to highlight our main results.     

Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy

We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt % 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a nonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (approximately 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt % PCBM resulted in 70-90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer+ and PCBM- polarons, assayed by the appearance of a long-lived, power-law decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over 2 orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find an excellent correlation between the free energy difference for charge separation (deltaG(CS)rel) and yield of the long-lived charge generation, with efficient charge generation requiring a much larger deltaG(CS)rel than that required to achieve efficient PL quenching. We suggest that this observation is consistent with a model where the excess thermal energy of the initially formed polaron pairs is necessary to overcome their Coulombic binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large deltaG(CS)rel (or LUMO level offset) is required to achieve efficient charge dissociation.     

Fucose Migration Pathways Identified Using Infrared Spectroscopy**

Fucose is a ubiquitous monosaccharide associated to major classes of glycans. A main obstacle to the sequencing of fucosylated glycans is the migration of fucose, which leads to misinterpretations in mass spectrometry analysis. Here, using ion vibrational spectroscopy, we resolve the structure of fucosylated fragments of Lewis and blood group H antigen trisaccharides and we unveil the position and linkage of the fucose after migration. Our findings demonstrate that the structure of fragment ions resulting from fucose migration can be characterized. Additionally, we report a new type of fucose migration, which does not feature any change of mass and therefore had not been previously reported: it consists of a local migration where the fucose changes its position remaining on the initial residue. Our approach allows the characterization of glycans, an essential step to interpret glycomics data, as well as to understand underlying processes at play in mass spectrometry.