1,721,616 research outputs found
CONTROL OF CYTOCHROME-OXIDASE ACTIVITY - A TRANSIENT SPECTROSCOPY STUDY
No full textThe kinetics of cytochrome oxidase reconstituted into small phospholipid vesicles (COV) has been followed by transient optical spectroscopy under steady-state and pre-steady-state conditions, in the presence and absence of ionophores. The effect of valinomycin on the activity of reconstituted cytochrome oxidase is shown to depend on the absolute concentration of the ionophore and on the number of turnovers elapsed by the enzyme; this novel observation, which escaped previous investigations, may account for important differences in results and therefore in interpretation of the mechanism of control of the enzyme activity as between Brunori et al. (Brunori, M., Sarti, P., Colosimo, A., Antonini, G., Malatesta, F., Jones, M. G., and Wilson, M. T. (1985) EMBO J. 4, 2365-2368), Gregory and Ferguson-Miller (Gregory, L., and Ferguson-Miller, S. (1989) Biochemistry 28, 2655-2662) and Capitanio et al. (Capitanio, N., De Nitto, E., Villani, G., Capitanio, G., and Papa, S. (1990) Biochemistry 29, 2939-2944). Quantitative analysis of the optical spectra acquired within 10 ms over a large wavelength and time range (500-650 nm and 5 ms to 60 s) under different experimental conditions, indicates that the electrical component of the transmembrane electro-chemical gradient controls the rate of the internal electron transfer from cytochrome alpha-Cu(A) to cytochrome alpa-3-Cu(B) as well as the cytochrome c to cytochrome alpha-electron transfer. The slow down of cytochrome oxidase activity observed in the presence of valinomycin after several (> 10) turnovers is attributed to alkalinization of the vesicle interior, which affects the internal electron transfer rate. These two mechanisms of control act most likely independently
CONTROL OF ELECTRON-TRANSFER BY THE ELECTROCHEMICAL POTENTIAL GRADIENT IN CYTOCHROME-C OXIDASE RECONSTITUTED INTO PHOSPHOLIPID-VESICLES
No full textThe kinetics of electron transfer between cytochrome-c oxidase and ruthenium hexamine has been characterized using the native enzyme or its cyanide complex either solubilized by detergent (soluble cytochrome oxidase) or reconstituted into artificial phospholipid vesicles (cytochrome oxidase-containing vesicles). Ru(NH3)2+6 (Ru(II] reduces oxidized cytochrome a, following (by-and-large) bimolecular kinetics; the second order rate constant using the cyanide complex of the enzyme is 1.5 x 10(6) M-1 s-1, for the enzyme in detergent, and slightly higher for COV. In the case of COV the kinetics are not affected by the addition of ionophores. Upon mixing fully reduced cytochrome oxidase with oxygen (in the presence of excess reductants), the oxidation leading to the pulsed enzyme is followed by a steady state phase and (eventually) by complete re-reduction. When the concentrations of dioxygen and oxidase are sufficiently low (micromolar range), the time course of oxidation can be resolved by stopped flow at room temperature, yielding an apparent bimolecular rate constant of 5 x 10(7) M-1 s-1. After exhaustion of oxygen and end of steady state, re-reduction of the pulsed enzyme by the excess Ru(II) is observed; the concentration dependence shows that the rate of re-reduction is limited at 3 s-1 in detergent; this limiting value is assigned to the intramolecular electron transfer process from cytochrome a-Cua to the binuclear center. Using the reconstituted enzyme, the internal electron transfer step is sensitive to ionophores, increasing from 2-3 to 7-8 s-1 upon addition of valinomycin and carbonyl cyanide m-chlorophenylhydrazone. This finding indicates for the first time an effect of the electrochemical potential across the membrane on the internal electron transfer rate; the results are compared with expectations based on the hypothesis formulated by Brunori et al. (Brunori, M., Sarti, P., Colosimo, A., Antonini, G., Malatesta, F., Jones, M.G., and Wilson, M.T. (1985) EMBO J. 4, 2365-2368), and their bioenergetic relevance is discussed with reference to the proton pumping activity of the enzyme
Electromagnetic Field Coupling between a Horizontal Loop Antenna and a Transmission Line: An Analytical Time-Domain Model
No full textThe time-domain electromagnetic (EM) reciprocity theorem and the Cagniard-DeHoop technique are applied to analyze the pulsed EM coupling between a horizontal small loop antenna and a finite transmission line above the perfect ground. It is demonstrated that this approach yields closed-form analytical formulas describing the impact of transmission, EM propagation, and reception on the exciting pulse signature. Moreover, approximate expressions applying to relatively short transmission lines are given and discussed, thus revealing similarities with the loop-to-loop EM coupling. Numerical results are presented and validated using an alternative analytical solution and an EM computational tool
Electromagnetic Field Coupling to a Transmission Line - A Reciprocity-Based Approach
No full textThe electromagnetic (EM) reciprocity theorem of the time-convolution type is systematically applied to describe an EM-field-to-line coupling model interrelating the (actual) induced voltage and current quantities at the ends of a transmission line with the (testing) ones pertaining to the situation when the line operates as a transmitter. It is demonstrated that under certain conditions, the reciprocity-based coupling model is fully equivalent to the classic models of Agrawal, Taylor, and Rachidi. Furthermore, it is shown for the case of plane-wave incidence that the far-field amplitude radiated in the testing state can be used to replace the forcing functions that are traditionally expressed in terms of the excitation-field distribution along the transmission line. The reciprocity-based formulation is finally validated both analytically and numerically
Lightning-Induced Voltages on Transmission Lines over a Lossy Ground-An Analytical Coupling Model Based on the Cooray-Rubinstein Formula
No full textSpace-time closed-form expressions for the vertical-electric-dipole-induced Thévenin-voltage responses on a transmission line above a lossy ground are derived analytically. The problem is formulated via the electromagnetic reciprocity theorem of the time-convolution type and subsequently solved with the aid of the Cooray-Rubinstein formula and the Cagniard-de Hoop method. It is demonstrated that the obtained results are readily applicable to calculating lightning-induced voltages on overhead transmission lines over a finitely conducting ground
REVERSED PHASE HIGH PERFORMANCE LIQUID CHROMATOGRAPHY OF ADRENOCORTICOTROPIN 1 39 AND ITS FRAGMENTS IN THE NATIVE AND OXIDIZED FORMS
No full textTransient Close-Range Electromagnetic Field Coupling between Loop Antennas at the Interface of Dissipative Half-Spaces
No full textThe problem of electromagnetic (EM) field short-range coupling between coplanar loop antennas located at the interface of two half-spaces with dielectric and conductive properties is solved analytically in the time domain (TD) via the Cagniard-DeHoop (CdH) technique under the diffusive approximation. It is demonstrated that the thus obtained closed-form TD analytical expressions can be applied to evaluate the close-range EM-field signal transfer between two loops lying on the surface of a dissipative half-space
Editorial Introduction to the Special Issue on Advances of Computational Electromagnetics for EMC
No full textPartial element equivalent circuit formulation for moving objects
No full textThis paper presents the partial element equivalent circuit formulation of electromagnetic systems with moving objects. The motion of portion of the system generates two effects: additional specific electromotive forces related to the speed and magnetic flux density and time-varying partial elements. The proposed formulation is presented in a general context including conductors, dielectrics, and magnetic materials. Exploiting Galerkin's testing procedure, additional contributions to the electric field integral equation that are proportional to electrical currents and magnetization appear. Three numerical examples are presented to support the theoretical analysis
Multiscale decomposition based analysis of PEEC models
No full textThe high level of integration has made the analysis and design of integrated circuits and packages increasingly challenging. Hence, there exists an urgent need to reduce the computational complexity of existing numerical methods. The integral equation based method known as the Partial Element Equivalent Circuit (PEEC) method naturally generates an equivalent circuit which can be analyzed in both the time and frequency domains. The enforcement of Kirchoff laws to the equivalent circuit can easily result into a very large set of equations whose solution can be extremely time consuming. In this paper, a new frequency-domain nodal analysis PEEC solver is proposed which is based on the adaptive cross approximation and recursive partitioned matrix inverse formula. The proposed approach provides a significant computational speedup, while preserving the accuracy. The efficiency of the proposed method is demonstrated through its application to a relevant interconnect problem. © 2013 IEEE
- …
