Similar Kinetic Features Research Articles

Kinetic Rationalization of Nonlinear Effects in Asymmetric Catalytic Cascade Reactions under Curtin-Hammett Conditions.

Observations of nonlinear effects of catalyst enantiopurity on product enantiomeric excess in asymmetric catalysis are often used to infer that more than one catalyst species is involved in one or more reaction steps. We demonstrate here, however, that in the case of asymmetric catalytic cascade reactions, a nonlinear effect may be observed in the absence of any higher order catalyst species or any reaction step involving two catalyst species. We illustrate this concept with an example from a recent report of an organocatalytic enantioselective [10 + 2] stepwise cyclization reaction. The disruption of pre-equilibria (Curtin–Hammett equilibrium) in reversible steps occurring prior to the final irreversible product formation step can result in an alteration of the final product ee from what would be expected based on a linear relationship with the enantiopure catalyst. The treatment accounts for either positive or negative nonlinear effects in systems over a wide range of conditions including “major-minor” kinetics or the more conventional “lock-and-key” kinetics. The mechanistic scenario proposed here may apply generally to other cascade reaction systems exhibiting similar kinetic features and should be considered as a viable alternative model whenever a nonlinear effect is observed in a cascade sequence of reactions.

Structure‐Making Effects of Metal Nanoparticles in Amyloid Peptide Fibrillation

There is growing concern that nanoparticles (NPs) may accelerate amyloid protein aggregation and thus cause amyloid‐related diseases. Here, the potential of silver and gold NPs is explored (diameter 20 nm) on the aggregation of the amyloid peptide sequences NNFGAIL from human islet amyloid polypeptide and the yeast prion protein sequence GNNQQNY, which are both the sequences of the full systems, which are able to aggregate into characteristic amyloid cross‐beta sheet fibrillar structures. Here, it is shown that silver and gold NPs in physiological aqueous solution at ambient temperatures accelerate the aggregation kinetics of both peptides significantly (in vitro). Scanning electron microscopy and X‐ray diffraction provide solid evidence for a “structure‐making” effect of the NPs. In particular, we are able to image the initial peptide corona and measure its structural reorganization in time‐resolved kinetic experiments. After a conversion time Δt, the coated NPs appear to act as templates or seeds for rapid fibrillation. Interestingly, cross‐fibrillation experiments with different peptide‐coated NPs (pcNPs) reveal that they can efficiently induce aggregation of similar peptides once the pcNPs are structurally converted. It is discussed that these structurally converted pcNPs may display similar kinetic features as toxic and aggregation inducing oligomers/protofibrils in normal amyloid aggregation, without being transient and very low‐concentration species. Finally, we suggest and discuss a simple mechanistic picture with the biomolecule corona of NPs being central to the function of the coated NPs in amyloid fibrillation.

Purification and characterization of cytosolic and cell wall β-galactosidases from Vigna unguiculata stems

Three β-galactosidase isoforms, β-gal I and β-gal II (cytosolic) and β-gal III (cell wall-associated), were isolated from stems of Vigna unguiculata (L.) Walp. cv. Pitiuba seedlings. Purification consisted of aμMonia sulfate fractionation followed by chromatography in DEAE-Sephadex and Lactosyl-Sepharose columns. The two cytosolic isoforms showed the same chromatography pattern, which differed from that of β-gal III. Electrophoresis revealed a single band of protein for β-gal II and β-gal III which also expressed β-galactosidase activity in gel. The apparent molecular mass of the β-gal I, II and III was 89, 146 and 124 kDa, respectively. The three isoforms revealed the same optimal pH (4.0) and the same optimal assay temperature (55oC) for enzyme activity. The three isoforms were stable at temperatures up to 50oC, and incubation with glucose and galactose expanded their thermal stability as well as inhibited their activities. Galactose was the most effective in promoting these effects and β-gal I and II were competitively inhibited by this sugar. Kinetic analysis using β-PNPG as substrate, revealed KM of 1.69, 1.76 and 1.43 for β-gal I, β-gal II and β-gal III, respectively. The β-gal I was able to hydrolyze all synthetic substrates tested, whereas β-gal II exhibited only β-fucosidase and α-arabinosidase activities, and β-gal III was limited to the α-galactosidase, β-fucosidase and α-arabinosidase activities. These results are consistent with three distinct β-galactosidases exhibiting quite similar kinetic features, but endowed with different functional specificities probably related to their specific roles in the plant cell physiology.

Production and characterization of recombinant PbPM4 and inhibition analysis of combinatorial chemistry derived compounds against PbPM4

Plasmepsins, a series of homologous aspartic proteinases, have been considered novel anti-malarial drug targets for battling the increasing drug-resistance in malaria parasites. Plasmepsin 4 (PbPM4) of the rodent malarial parasite Plasmodium beighei shares high sequence identity with its orthologs of the human malarial species. Therefore, PbPM4 is considered to be able to mimic the target of anti-malarial compounds in a parasite-infected mouse model. Here, a semi-proPbPM4 construct was over-expressed in E. coli and refolded in vitro. PbPM4 showed full catalytic activity at pH 5.5 in the presence of the N-terminal truncated prosegment. In contrast, specific self-processing of the prosegment between L117p and L118p at acidic conditions resulted in time-dependent loss of catalytic activity. In addition, the mature PbPM4 showed comparable kcat and kcat/Km values on chromogenic substrates with plasmepsins of the human malarial parasites, indicating that it shared similar kinetic features with its human orthologs. The inhibition constants were determined for a series of peptidomimetic compounds developed by combinatorial chemistry against mature PbPM4. These inhibitors exhibited nanomolar binding affinities, which were comparable with those of plasmepsins of human parasites. This evidence further suggested PbPM4 can serve as a potential drug target in a rodent model. Supported by NIH grant AI39211.

Uniform carbon-covered titania and its photocatalytic property

A series of carbon-covered titania (CCT) were prepared via pyrolysis of sucrose highly dispersed on the surface of titania in flowing N 2. The samples were characterized by XRD, BET, DTA–TG, UV–vis, and their photocatalytic properties were evaluated with a model pollutant, methylene blue (MB), at room temperature. It is found that carbon coverage inhibits the phase transformation of TiO 2 and makes the absorption edge of titania shift to the visible region. Photocatalytic tests show that under UV illumination, the kinetics of the dyes’ elimination in the suspension of CCT powders follows an apparent first-order rate as in pure titania suspension, but the rate constant is much higher than that of pure titania, even commercial Degussa P-25 TiO 2. In addition, as a photocatalyst responsive to visible light, CCT shows similar kinetic features for the photodegradation of the dyes under visible light illumination to that under UV illumination. From the detection of the products of MB degradation, and the absorption spectra of MB solution after the photocatalytic reaction, it is proved that MB is decomposed over CCT samples under visible light illumination, whereas it is simply bleached over pure titania at the same condition.

Gating currents associated with intramembrane charge displacement in HERG potassium channels.

HERG (human ether-a-go-go-related gene) encodes a delayed rectifier K+ channel vital to normal repolarization of cardiac action potentials. Attenuation of repolarizing K+ current caused by mutations in HERG or channel block by common medications prolongs ventricular action potentials and increases the risk of arrhythmia and sudden death. The critical role of HERG in maintenance of normal cardiac electrical activity derives from its unusual gating properties. Opposite to other voltage-gated K+ channels, the rate of HERG channel inactivation is faster than activation and appears to be intrinsically voltage dependent. To investigate voltage sensor movement associated with slow activation and fast inactivation, we characterized HERG gating currents. When the cut-open oocyte voltage clamp technique was used, membrane depolarization elicited gating current with fast and slow components that differed 100-fold in their kinetics. Unlike previously studied voltage-gated K+ channels, the bulk of charge movement in HERG was protracted, consistent with the slow rate of ionic current activation. Despite similar kinetic features, fast inactivation was not derived from the fast gating component. Analysis of an inactivation-deficient mutant HERG channel and a Markov kinetic model suggest that HERG inactivation is coupled to activation.

Kinematic and Kinetic Relationships Between an Olympic-Style Lift and the Vertical Jump

ABSTRACT This study was done to determine the kinematic and kinetic relationships between the squatting vertical jump and the Olympic hang snatch lift. Kinematic data were analyzed via the Peak 3-D system; kinetic data were analyzed via the AMTI force plate system. Two genlocked video cameras recorded performance. The subjects were 7 male varsity athletes from an NCAA Div. I school. Ground reaction force data of the lower extremities and angular displacements of the left hip, knee, and ankle joints were collected. The moments of power and force and the angular displacements were analyzed. Results revealed similar kinetic features between the squatting vertical jump and the hang snatch lift during the propulsive phase. However, angular displacements of the left hip, knee, and ankle were statistically dissimilar between both exercises during the propulsive phase. On the basis of the similar kinetic features, Olympic-style lifting may be beneficial in improving power.

A kinetic study of the formation of carbene complexes from the reaction between hexakismethylisonitriloferrate(II) and some nitrogenous bases in aqueous solution

The formation of the product containing a chelating dicarbene, formed by the attack of hydrazine on the hexakismethylisonitriloferrate(II) complex, is first order in each reagent, independent of ionic strength but varies with hydrogen ion concentration. While attack by methylhydrazine shows similar kinetic features, the behaviour when methylamine is used as the nucleophile is different. Rate data are presented and a mechanism suggested for the reactions involved.

Synthesis of Procoagulant Factor VIII, Factor VIII Related Antigen and other Coagulation Factors by the Isolated Perfused Rat Liver

The synthesis of factor VIII and other coagulation factors has been studied using an isolated, perfused rat liver. Synthetic function of the liver was validated by adding [35S]L-methionine to the perfusion medium and performing two-dimensional crossed immunoelectrophoresis and autoradiography on samples obtained during perfusion. Progressive incorporation of radioactivity into plasma proteins was demonstrated. This was inhibited by cycloheximide. Coagulation factor assays demonstrated synthesis of factors II, IX and X and of factor V and procoagulant factor VIII (VIIIC). Synthesis of factor VIII related antigen (VIIIRAg), measured in an immunoradiometric assay, was not significantly demonstrated. Addition of warfarin to the perfusion medium inhibited the synthesis of factors II, IX and X but not of factors V and VIII. Cycloheximide completely inhibited synthesis of all coagulation factors but actinomycin acted only after a latent period. Reticuloendothelial cell blockade was attempted by adding ethionine to the perfusion medium or by administration of Indian ink to the donor animals prior to removal of the livers. In these instances synthesis of factor V and factor VIIIC was inhibited but not that of factors II, IX and X. The results confirmed the functional capacity of the isolated liver for synthesizing proteins and the vitamin K dependent coagulation factors, and suggested similar kinetic features for the synthesis of factors V and VIIIC. Failure to detect significant VIIIRAg synthesis in these experiments is consistent with the hypothesis that this protein is released by vascular endothelial cells throughout the body and is activated or joined to VIIIC or stimulates its production in the liver.