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Rate Dependence (rate + dependence)
Selected AbstractsGlucose-6-phosphate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima: expression of the g6pd gene and characterization of an extremely thermophilic enzymeFEMS MICROBIOLOGY LETTERS, Issue 2 2002Thomas Hansen Abstract The gene (open reading frame Tm1155, g6pd) encoding glucose-6-phosphate dehydrogenase (G6PD, EC 1.1.1.49) of the hyperthermophilic bacterium Thermotoga maritima was cloned and functionally expressed in Escherichia coli. The purified recombinant enzyme is a homodimer with an apparent molecular mass of 95 kDa composed of 60-kDa subunits. Rate dependence (at 80°C) on glucose-6-phosphate and NADP+ followed Michaelis,Menten kinetics with apparent Km values of 0.15 mM and 0.03 mM, respectively; apparent Vmax values were about 20 U mg,1. The enzyme also reduced NAD+ (apparent Km 12 mM, Vmax 12 U mg,1). The 1000-fold higher catalytic activity (kcat/Km) with NADP+ over NAD+ defines the G6PD as NADP+ specific in vivo. G6PD activity was competitively inhibited by NADPH with a Ki value of 0.11 mM. With a temperature optimum of 92°C the enzyme is the most thermoactive G6PD described. [source] A damage mechanics model for power-law creep and earthquake aftershock and foreshock sequencesGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Ian G. Main It is common practice to refer to three independent stages of creep under static loading conditions in the laboratory: namely transient, steady-state, and accelerating. Here we suggest a simple damage mechanics model for the apparently trimodal behaviour of the strain and event rate dependence, by invoking two local mechanisms of positive and negative feedback applied to constitutive rules for time-dependent subcritical crack growth. In both phases, the individual constitutive rule for measured strain , takes the form ,(t),=,,0,[1,+,t/m,]m, where , is the ratio of initial crack length to rupture velocity. For a local hardening mechanism (negative feedback), we find that transient creep dominates, with 0,<,m,<,1. Crack growth in this stage is stable and decelerating. For a local softening mechanism (positive feedback), m,<,0, and crack growth is unstable and accelerating. In this case a quasi-static instability criterion , , , can be defined at a finite failure time, resulting in the localization of damage and the formation of a throughgoing fracture. In the hybrid model, transient creep dominates in the early stages of damage and accelerating creep in the latter stages. At intermediate times the linear superposition of the two mechanisms spontaneously produces an apparent steady-state phase of relatively constant strain rate, with a power-law rheology, as observed in laboratory creep test data. The predicted acoustic emission event rates in the transient and accelerating phases are identical to the modified Omori laws for aftershocks and foreshocks, respectively, and provide a physical meaning for the empirical constants measured. At intermediate times, the event rate tends to a relatively constant background rate. The requirement for a finite event rate at the time of the main shock can be satisfied by modifying the instability criterion to having a finite crack velocity at the dynamic failure time, dx/dt , VR,, where VR is the dynamic rupture velocity. The same hybrid model can be modified to account for dynamic loading (constant stress rate) boundary conditions, and predicts the observed loading rate dependence of the breaking strength. The resulting scaling exponents imply systematically more non-linear behaviour for dynamic loading. [source] Kinetics and mechanism of esterification of epoxy resin in presence of triphenylphosphineINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2004Neelam Pal The kinetics of esterification of bisphenol-A based epoxy resin with acrylic acid in presence of triphenylphosphine has been studied. The reaction exhibits a first-order rate dependence with respect to [Epoxy] and [Catalyst]. A first-order dependence of rate with respect to [Acid] has been observed during a particular kinetic run. However, a retarding effect of [Acid] on the rate has been observed by increasing the initial concentration of acid. A suitable mechanism consistent with the kinetic data is proposed and discussed. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 280,285, 2004 [source] Relationship Between Heart Rate Turbulence and Heart Rate, Heart Rate Variability, and Number of Ventricular Premature Beats in Coronary PatientsJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 7 2004IWONA CYGANKIEWICZ M.D., Ph.D. Introduction: Heart rate variability (HRV) illustrates regulation of the heart by the autonomic nervous system whereas heart rate turbulence (HRT) is believed to reflect baroreflex sensitivity. The aim of this study was to determine the association between HRT and HRV parameters and the relationship between HRT parameters and heart rate and number of ventricular premature beats (VPBs) used to calculate HRT parameters. Methods and Results: In 146 patients (117 males and 29 females; mean age 62 years) with coronary artery disease, a 24-hour ECG Holter monitoring was performed to calculate mean heart rate (RR interval), number of VPBs, time- and frequency-domain HRV parameters and two HRT parameters: turbulence onset (TO) and turbulence slope (TS). Univariate and multivariate regression analyses were performed to evaluate the association between tested parameters. Significant correlation between TS and mean RR interval was observed (r = 0.42; p < 0.001), while no association for TO vs. RR interval was found. TS values were significantly higher in patients with less than 10 VPBs/24 hours than in patients with more frequent VPBs. Significant associations between HRT and HRV parameters were found with TS showing stronger correlation with HRV parameters than TO (r value ranging from 0.35 to 0.62 for TS vs. ,0.16 to ,0.38 for TO). Conclusion: HRT parameters correlate strongly with HRV parameters indicating that HRT should be considered as a reflection of both baroreceptors response and overall autonomic tone. Heart rate dependence of turbulence slope indicates the need to adjust this parameter for heart rate. (J Cardiovasc Electrophysiol, Vol. 15, pp. 731-737, July 2004) [source] Independent Autonomic Modulation of Sinus Node and Ventricular Myocardium in Healthy Young Men During SleepJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 10 2000PETER KOWALLIK M.D. Autonomic Modulation of Sinus Node and Ventricle. Introduction. The aim of this study was to investigate whether autonomic modulation of ventricular repolarization may spontaneousiy differ from that of the sinoatrial node. Methods and Results. Onset of P waves. QRS complexes, and the apex and end of T waves were detected heat to heat in high-resolution ECGs from nine healthy young men during the night. There were time-dependent fluctuations in the QT/RR slopes of consecutive 5-minute segments that could not he explained by the mean RR cycle length of the respective segment. Because the variahility found in QT intervals could not be explained hy either possible effects of rate dependence or hysteresis, autonomic effects were obvious. Power speetral analysis was performed for consecutive 5-minute segments of PP and QT techograms. In a given subject. trends in the time course of low-frequency (LF) and high-frequency (HF) power in PP and QT often were similar, but they were quite different at other times. The mean LF/HF ratio for QTend (0.75 ± 0.1) was different from that of PP (1.8 ± 0.2; P = 0.002), indicating differences in sympathovagal balance at the different anatomic sites. Furthermore, at a given mean heart rate, averaged QT intervals were different on a time scale of several minutes to hours. The QT/RR slope of 5-minute segments correlated significantly with the HF power of QT variability but not with that of PP variability, indicating effects of the autonomic nervous system on ventricular action potential restitution. Conclusion. These differences demonstrate that changes in sinus node automaticity are not necessarily indicative of the autonomic control of ventricular myocardium. (J Cardiavasc Electrophysiol, Vol. II, pp. 1063-1070. October 2000) [source] Estimation of the fragility index of indomethacin by DSC using the heating and cooling rate dependency of the glass transitionJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 6 2004Joaquim J. Moura Ramos Abstract In this study we have investigated the features of the glass transition relaxation of indomethacin using Differential Scanning Calorimetry (DSC). The purpose of this work is to provide an estimation of the activation energy at the glass transition temperature, as well as of the fragility index, of amorphous indomethacin from DSC data. To do so, the glass transition temperature region of amorphous indomethacin was characterized in both cooling and heating regimes. The activation energy for structural relaxation (directly related to glass fragility) was estimated from the heating and cooling rate dependence of the location of the DSC profile of the glass transition. The obtained results were similar in the heating and in the cooling modes. The results on the fragility index of indomethacin obtained in the present study, m,=,60 in the cooling mode and m,=,56 in the heating mode, are compared with other values previously published in the literature. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:1503,1507, 2004 [source] Steady-State versus Non-Steady-State QT-RR Relationships in 24-hour Holter RecordingsPACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 3 2000GILLES LANDE The aim of the present study was to investigate the QT-RR interval relationship in ambulatory ECG recordings with special emphasis on the physiological circumstances under which the QT-RR intervals follow a linear relation. Continuous ECG recordings make it possible to automatically measure QT duration in individual subjects under various physiological circumstances. However, identification of QT prolongation in Holter recordings is hampered by the rate dependence of QT duration. Comparison of QT duration and QT interval rate dependence between different individuals implies that the nature of the QT-RR relationship is defined in ambulatory ECG. Holter recordings were performed in healthy volunteers at baseline and after administration of dofetilide, a Class III antiarrhythmic drug. After dofetilide, beat-to-beat automated QT measurements on Holter tapes were compared with manually measured QT intervals on standard ECGs matched by time. The QT-RR relationship was analyzed at baseline in individual and group data during three different periods: 24-hour, daytime, and nighttime. Data were collected under steady-state or non-steady-state conditions of cycle length and fitted with various correction formulae. Our study demonstrated an excellent agreement between manually and automated measurements. The classic Bazett correction formula did not fit the QT-RR data points in individual or group data. When heart beats were selected for a steady rhythm during the preceding minute, QT-RR intervals fit a linear relationship during the day and night periods, but not during the 24-hour period in both individual and group data. In contrast, in the absence of beat selection, data fit a more complex curvilinear relationship irrespective of the period. Our study provides the basis for comparison of QT interval durations and QT-RR relationships between individuals and between groups of subjects. [source] Enthalpy relaxation in bisphenol-A polycarbonate/poly(methyl methacrylate) blendsPOLYMER ENGINEERING & SCIENCE, Issue 3 2007Maurizio Penco Bisphenol-A polycarbonate (PC)/poly(methyl methacrylate) (PMMA) blends (PC/PMMA) were prepared by melt mixing with and without a trans-esterification catalyst (tetrabutylammoniun tetraphenylborate), which is able to promote inter-exchange reactions between PC and PMMA. Evidences of the ester,ester exchange reaction taking place were pointed out via Fourier Transform Infra-Red and size exclusion chromatography analyses. A series of enthalpy relaxation measurements were carried out for the pure polymers and for blends thereof. The data were analyzed on the basis of the Tool,Narayanaswamy,Moynihan/Kohlraush,Williams,Watts model. This model characterizes the structural relaxation in the glass transition region by means of four parameters: the apparent activation energy (,h*), the nonlinearity (x), the nonexponentiality (,), and the pre-exponential factor (,0). The apparent activation energy can be determined from the cooling rate dependence of the fictive temperature (Tf) measured using DSC. ,h* values of homogeneous blend and of the starting polymers were determined in this way, while an estimation of x, , and ,0 was proposed. Comparisons were made between a single glass transition temperature PC/PMMA blend and homopolymers data. The experimental values of ,h* suggest that the degree of cooperativity decreases on going from the starting polymers to the blend. POLYM. ENG. SCI., 47:218,224, 2007. © 2007 Society of Plastics Engineers. [source] Dynamics and Rate-Dependence of the Spatial Angle between Ventricular Depolarization and Repolarization Wave Fronts during Exercise ECGANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 3 2010Tuomas Kenttä M.Sc. Background: QRS/T angle and the cosine of the angle between QRS and T-wave vectors (TCRT), measured from standard 12-lead electrocardiogram (ECG), have been used in risk stratification of patients. This study assessed the possible rate dependence of these variables during exercise ECG in healthy subjects. Methods: Forty healthy volunteers, 20 men and 20 women, aged 34.6 ± 3.4, underwent an exercise ECG testing. Twelve-lead ECG was recorded from each test subject and the spatial QRS/T angle and TCRT were automatically analyzed in a beat-to-beat manner with custom-made software. The individual TCRT/RR and QRST/RR patterns were fitted with seven different regression models, including a linear model and six nonlinear models. Results: TCRT and QRS/T angle showed a significant rate dependence, with decreased values at higher heart rates (HR). In individual subjects, the second-degree polynomic model was the best regression model for TCRT/RR and QRST/RR slopes. It provided the best fit for both exercise and recovery. The overall TCRT/RR and QRST/RR slopes were similar between men and women during exercise and recovery. However, women had predominantly higher TCRT and QRS/T values. With respect to time, the dynamics of TCRT differed significantly between men and women; with a steeper exercise slope in women (women, ,0.04/min vs ,0.02/min in men, P < 0.0001). In addition, evident hysteresis was observed in the TCRT/RR slopes; with higher TCRT values during exercise. Conclusions: The individual patterns of TCRT and QRS/T angle are affected by HR and gender. Delayed rate adaptation creates hysteresis in the TCRT/RR slopes. Ann Noninvasive Electrocardiol 2010;15(3):264,275 [source] Abnormalities of the Repolarization Characteristics of Patients with Heart Failure Progress with Symptom SeverityANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 3 2004Ch.B., Christopher C. E. Lang B.Sc. (Hons) Background:,Congestive heart failure is a common condition with high mortality. Many of these deaths are sudden and unexpected. Ventricular action potential, surface repolarization (QT interval), and dispersion of repolarization are prolonged in the failing heart, contributing to arrhythmogenesis and sudden death. We studied the relationship between QT and heart rate (RR interval) from ambulatory recordings using a novel method in patients with ischemic heart disease and varying degrees of left-ventricular impairment (IHD) and compared them to healthy subjects (HS). We compare the degree of abnormality with the functional impairment and ejection fraction. Methods:,Using a previously described automated method for continuous estimation of the QT/RR characteristic that incorporates a correction formula for compensation of QT adaptation lag (VERDA, Del Mar Reynolds Medical Ltd., Hertford, UK), we compared recordings from 41 IHD patients with age-matched HS. Results:,IHD Patients have prolonged 24-hour mean QTo (461 ms vs 426 ms, P < 0.01), and abnormal rate dependence relative to controls (24-hour mean slope: 0.20 vs 0.14, P < 0.001; J: 0.38 vs 0.28, P < 0.001). There is increased temporal variation in J with respect to HS. These abnormalities of repolarization increase with worsening NYHA class, but do not correlate with ejection fraction. Conclusions:,The use of a universal correction formula to compare dynamic QT data in IHD patients is inappropriate. The observed progressive abnormalities may be responsible for the high incidence of sudden death through promotion of arrhythmias. [source] Beat-to-Beat QT Dynamics in Healthy SubjectsANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 1 2004Berit T. Jensen M.D. Background: Measures of QT dynamics express repolarization abnormalities that carry prognostic information, but the reproducibility of beat-to-beat QT dynamics has never been established. The QT interval is prolonged at night, but how the circadian rhythm and heart rate influence the dynamic QT measurements is still unsettled. The aims of the present study were: (1) to describe the reproducibility of beat-to-beat QT dynamics with respect to intrasubject, between-subject, and between-observer variability and (2) to describe the normal range, circadian variation, and heart rate dependence of QT dynamics. Methods: Ambulatory Holter recordings were performed three times on 20 healthy volunteers and were analyzed by two experienced cardiologists. Slope and intercept of the QT/RR regression, the variability of QT and R-R intervals expressed as the standard deviation, and the relation between QT and RR variability expressed as a variability ratio were measured among other QT dynamics. Results: The reproducibility of all QT dynamics was good. All QT dynamics showed circadian variation when calculated on an hourly basis. The day/night variation in slope could be explained by the differences in heart rate, whereas the day/night variation in intercept was heart rate independent. Conclusion: The present study shows that reliable automatic QT measurements could be performed, encouraging further evaluation of the clinical value of QT dynamics in risk stratification of cardiac patients. [source] Effects of ionic strength on lysozyme uptake rates in cation exchangers.BIOTECHNOLOGY & BIOENGINEERING, Issue 2 2005I: Uptake in SP Sepharose FF Fluorescence scanning confocal microscopy was used in parallel with batch uptake and breakthrough measurements of transport rates to study the effect of ionic strength on the uptake of lysozyme into SP Sepharose FF. In all cases the adsorption isotherms were near-rectangular. As described previously, the intraparticle profiles changed from slow-moving self-sharpening fronts at low salt concentration, to fast-moving diffuse profiles at high salt concentration, and batch uptake rates correspondingly increased with increasing salt concentration. Shrinking core and homogeneous diffusion frameworks were used successfully to obtain effective diffusivities for the low salt and high salt conditions, respectively. The prediction of column breakthrough was generally good using these frameworks, except for low-salt uptake results. In those cases, the compressibility of the stationary phase coupled with the shrinking core behavior appears to reduce the mass transfer rates at particle-particle contacts, leading to shallower breakthrough curves. In contrast, the fast uptake rates at high ionic strength appear to reduce the importance of mass transfer limitations at the particle contacts, but the confocal results do show a flow rate dependence on the uptake profiles, suggesting that external mass transfer becomes more limiting at high ionic strength. These results show that the complexity of behavior observable at the microscopic scale is directly manifested at the column scale and provides a phenomenological basis to interpret and predict column breakthrough. In addition, the results provide heuristics for the optimization of chromatographic conditions. © 2005 Wiley Periodicals, Inc. [source] Temperature and strain rate dependences of yield stress of polypropylene composites reinforced with carbon nanofibersPOLYMER COMPOSITES, Issue 12 2009S.P. Bao Polypropylene (PP) nanocomposites filled with 0.1, 0.3, 0.5, and 1.0 wt% carbon nanofiber (CNF) were prepared via melt compounding in a twin-screw extruder followed by injection molding. The effects of CNF additions on the structure, mechanical and tensile yielding behavior of PP were investigated. TEM and SEM observations showed that CNFs were dispersed uniformly within PP matrix. Tensile test showed that the yield strength and Young's modulus of PP were improved considerably by adding very low CNF loadings. The reinforcing effect of CNF was also verified from the dynamic mechanical analysis. Impact measurement revealed that the CNF additions were beneficial to enhance the impact toughness of PP. The yield stress of the PP/CNF nanocomposites was found to be strain rate and temperature dependent. The yielding responses of PP/CNF nanocomposites can be described successfully by the Erying's equation and a reinforcing index n. The structure and mechanical property relationship of the nanocomposites is discussed. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers [source] | |||||||||||||||||||