Bifurcation analysis of a dynamic power system with SVC devices

Bifurcation analysis is a very useful tool for power system stability assessment. In this paper, detailed investigation of power system bifurcation behaviour is presented. One and two parameter bifurcation analysis are conducted on a 3-bus power system. We also examined the impact of FACTS devices on power system stability through Hopf bifurcation analysis by taking static Var compensator (SVC) as an example. A simplified first-order model of the SVC device is included in the 3-bus sample system. Real and reactive powers are used as bifurcation parameter in the analysis to compare the system oscillatory properties with and without SVC. The simulation results indicate that the linearized system model with SVC enlarge the voltage stability boundary by moving Hopf bifurcation point to higher level of loading conditions. The installation of SVC increases the dynamic stability range of the system, however complicates the Hopf bifurcation behavior of the system

Core stability exercise in chronic low back pain

Exercise is commonly used in the management of chronic musculoskeletal conditions, including chronic low back pain (CLBP). The focus of exercise is varied and may include parameters ranging from strength and endurance training, to specific training of muscle coordination and control. The assumption underpinning these approaches is that improved neuromuscular function will restore or augment the control and support of the spine and pelvis. In a biomechanical model of CLBP, which assumes that pain recurrence is caused by repeated mechanical irritation of pain sensitive structures [1], it is proposed that this improved control and stability would reduce mechanical irritation and lead to pain relief [1]. Although this model provides explanation for the chronicity of LBP, perpetuation of pain is more complex, and contemporary neuroscience holds the view that chronic pain is mediated by a range of changes including both peripheral (eg, peripheral sensitization) and central neuroplastic changes [2]. Although this does not exclude the role of improved control of the lumbar spine and pelvis in management of CLBP, particularly when there is peripheral sensitization, it highlights the need to look beyond outdated simplistic models. One factor that this information highlights is that the refinement of control and coordination may be more important than simple strength and endurance training for the trunk muscles. The objective of this article is to discuss the rationale for core stability exercise in the management of CLBP, to consider critical factors for its implementation, and to review evidence for efficacy of the approach.

Thermal, chemical, and enzymatic stability of the cyclotide kalata B1: The importance of the cyclic cystine knot

The cyclotides constitute a recently discovered family of plant-derived peptides that have the unusual features of a head-to-tail cyclized backbone and a cystine knot core. These features are thought to contribute to their exceptional stability, as qualitatively observed during experiments aimed at sequencing and characterizing early members of the family. However, to date there has been no quantitative study of the thermal, chemical, or enzymatic stability of the cyclotides. In this study, we demonstrate the stability of the prototypic cyclotide kalata B1 to the chaotropic agents 6 M guanidine hydrochloride (GdHCl) and 8 M urea, to temperatures approaching boiling, to acid, and following incubation with a range of proteases, conditions under which most proteins readily unfold. NMR spectroscopy was used to demonstrate the thermal stability, while fluorescence and circular dichroism were used to monitor the chemical stability. Several variants of kalata B1 were also examined, including kalata 132, which has five amino acid substitutions from B1, two acyclic permutants in which the backbone was broken but the cystine knot was retained, and a two-disulfide bond mutant. Together, these allowed determinations of the relative roles of the cystine knot and the circular backbone on the stability of the cyclotides. Addition of a denaturant to kalata B1 or an acyclic permutant did not cause unfolding, but the two-disulfide derivative was less stable, despite having a similar three-dimensional structure. It appears that the cystine knot is more important than the circular backbone in the chemical stability of the cyclotides. Furthermore, the cystine knot of the cyclotides is more stable than those in similar-sized molecules, judging by a comparison with the conotoxin PVIIA. There was no evidence for enzymatic digestion of native kalata B1 as monitored by LC-MS, but the reduced form was susceptible to proteolysis by trypsin, endoproteinase Glu-C, and thermolysin. Fluorescence spectra of kalata B1 in the presence of dithiothreitol, a reducing agent, showed a marked increase in intensity thought to be due to removal of the quenching effect on the Trp residue by the neighboring Cys5-Cys17 disulfide bond. In general, the reduced peptides were significantly more susceptible to chemical or enzymatic breakdown than the oxidized species.

Stability of benzylpenicillin during continuous home intravenous therapy

Objectives: The aim of this study was to investigate the temperature profile of home intravenous (iv) antibiotic reservoirs and the stability of 16 megaunits of benzylpenicillin sodium in 120 mL of sodium chloride 0.9% at constant and variable temperatures. Methods: A Tinytag computerized thermometer recorded temperatures every minute in the home iv antibiotic reservoir pouches of nine patients over a 24 h period. Similar bags containing benzylpenicillin sodium (16 megaunits) were maintained either at a constant 36degreesC, 26degreesC or 21-22degreesC or were worn in a pouch by five healthy volunteers for a 24 h period. Other bags were stored at 3-5degreesC for 10 days. The bags were sampled at timed intervals and benzylpenicillin concentrations assayed by HPLC. Results: Median temperatures recorded in the infusion bags worn by the nine patients were in the range 16.7-34.1degreesC. For infusion bags maintained at 36degreesC, 26degreesC and 21-22degreesC, the concentrations of benzylpenicillin dropped below 90% of the initial concentration at a mean time of 5 h 18 min, 12 h 54 min and 13 h 20 min, respectively, whereas for bags worn by the healthy volunteers the mean time for 10% loss of benzylpenicillin was 9 h 20 min. In contrast, at 3-5degreesC, concentrations of benzylpenicillin only dropped below 90% of the initial concentration at 8 days. Conclusions: Significant temperature-dependent degradation of benzylpenicillin occurs during continuous home iv antibiotic programme infusions, which could result in loss of efficacy.

Stability criteria for unsupervised temporal association networks

A biologically realizable, unsupervised learning rule is described for the online extraction of object features, suitable for solving a range of object recognition tasks. Alterations to the basic learning rule are proposed which allow the rule to better suit the parameters of a given input space. One negative consequence of such modifications is the potential for learning instability. The criteria for such instability are modeled using digital filtering techniques and predicted regions of stability and instability tested. The result is a family of learning rules which can be tailored to the specific environment, improving both convergence times and accuracy over the standard learning rule, while simultaneously insuring learning stability.

Factors influencing the stability of cyclotides: Proteins with a circular backbone and cystine knot motif

Cyclotides are a large family of mini-proteins that have the distinguishing features of a head-to-tail cyclised backbone and a cystine knot formed by six conserved cysteine residues. They are present in plants from the Rubiaceae, Violaceae and Cucurbitaceae families. The unique structural features of the cyclotides make them extremely resistant to chemical, thermal and proteolytic degradation. In this article we review recent Studies from our laboratory that dissect the role of the individual structural elements in defining the stability of cyclotides. The resistance of cyclotides to chemical and proteolytic degradation is in large part due to the cystine knot, whereas the thermal stability is I composite of several features including the cystine knot, the cyclic backbone and the hydrogen bonding network. A range of biological activities of cyclotides is critically dependent oil the presence of the cyclic backbone.

A novel suite of cyclotides from Viola odorata: sequence variation and the implications for structure, function and stability

Cyclotides are a fascinating family of plant-derived peptides characterized by their head-to-tail cyclized backbone and knotted arrangement of three disulfide bonds. This conserved structural architecture, termed the CCK (cyclic cystine knot), is responsible for their exceptional resistance to thermal, chemical and enzymatic degradation. Cyclotides have a variety of biological activities, but their insecticidal activities suggest that their primary function is in plant defence. In the present study, we determined the cyclotide content of the sweet violet Viola odorata, a member of the Violaceae family. We identified 30 cyclotides from the aerial parts and roots of this plant, 13 of which are novel sequences. The new sequences provide information about the natural diversity of cyclotides and the role of particular residues in defining structure and function. As many of the biological activities of cyclotides appear to be associated with membrane interactions, we used haemolytic activity as a marker of bioactivity for a selection of the new cyclotides. The new cyclotides were tested for their ability to resist proteolysis by a range of enzymes and, in common with other cyclotides, were completely resistant to trypsin, pepsin and thermolysin. The results show that while biological activity varies with the sequence, the proteolytic stability of the framework does not, and appears to be an inherent feature of the cyclotide framework. The structure of one of the new cyclotides, cycloviolacin O14, was determined and shown to contain the CCK motif. This study confirms that cyclotides may be regarded as a natural combinatorial template that displays a variety of peptide epitopes most likely targeted to a range of plant pests and pathogens.

Heritability and Stability of Resting Blood Pressure in Australian Twins

In Australian twins participating in three different studies (1979-1996), the contribution of genetic and environmental influences to variation in resting systolic (SBP) and diastolic blood pressure (DBP) was studied. The sample consisted of 368 monozygotic and 335 dizygotic twin pairs with measurements for both individuals. Blood pressure measurements in two studies were available for 115 complete twin pairs, and 49 twin pairs had measurements in three studies. This allowed assessment of blood pressure tracking over an average period of 12 years in the age range of 23 to 45 years. Multivariate analyses showed significant heritability (h(2)) of blood pressure in all studies (SBP h(2) = 19%-56%, DBP h(2) = 37%-52%). In addition, the analyses showed that the blood pressure tracking was explained by the same set of genetic factors. These results replicate an earlier finding in Dutch twins that also showed stability of the contribution of genetic factors to blood pressure tracking.