Dual control mechanism

Normally, if saccharides are substituted for malonic acid in classical B-Z system, oscillations are observed since Br -control mechanism operates. This has been ascribed to accumulation of Br in the system such that [Br ] exceeds the critical Br concentration. Critical [Br ] is necessary for oscillations to occur [1]. Hence, in order to reduce the prevailing [Br ] to the level of [Brj] critical following steps can be undertaken  

Comprehensive studies of the oscillatory features of B-Z reaction involving double substrate such as (i) glucose -F acetone (ii) fructose -F acetone and (iii) sucrose -F acetone have been made. One of the interesting features is that the systems display the lower and upper critical limits of [acetone] between which oscillations occurred on increasing [F]. Another significant observation was that beyond a certain [F], oscillations occurred even when no acetone was present in the system. Obviously, at this stage Br -control mechanism is not in operation [46] and an alternative free-radical control 

In a particular concentration range, two types of oscillations in Fructose [F] -F acetone oscillator separated by time pause have also been observed in the batch reactor. Oscillations that occur before time pause (first type oscillations are insensitive to addition of 

Br up to 10 M and are F + OA similar to that observed in the corresponding fructose oscillator without acetone. On the other hand, oscillations that occur after time pause (second type oscillations) are stopped when [Br ]10 M. This gives an indication that the first type is non-bromide ion-controlled while the second is Br -controlled. Computer simulation studies confirm that both control mechanisms are operative in the system [46]. 

Experimental studies on B-Z oscillator having fructose -F tartaric acid as double substrate have been undertaken by keeping either of the two organic substrates at a fixed concentration and varying the concentration of the other [48]. A modified mechanism by including following additional steps (7) to (13) in the FKN mechanism has been used to explain the observations. 

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Thus, both the G-l-OA as well as F-i-OA oscillators provide examples of dual control mechanism. Multiple-control mechanisms are a distinct possibility. 

A simple explanation of dual control mechanism can be easily provided. In the case of type I oscillations, initially in the presence of oxalic acid, a sufficient amount of HBr02 is produced, and in turn, by a sequence of reactions Br is in greater excess than critically needed for balance of positive and negative feedback. However, when G... 

In most of the cases, one comes across single control mechanism involving one set of autocatalytic and inhibitory reaction. However, in some cases, dual control mechanism is operative as discussed in Chapter 9. 

The recent discovery of the dual nature of the enzyme, involving potent synthetic as well as hydrolytic activities, has served as a basis for a variety of new control mechanisms and physiological roles for the catalyst. For example, hydrolytic activity of the enzyme is limited through competitive inhibition by PPi, carbamyl-P, Pi HC03 , ATP, ADP, and various other phosphoanhydrides. Glucose itself, through participation as phosphoryl acceptor alternative with water (30, 31, Jfl), may regulate its own release in an autocatalytic manner (9, 10, 169). 

Increased clearance of steroid hormones due to induction of hepatic biotransformation enzymes following chemical exposure often has been cited as a possible mechanism by which toxicants could lower circulating testosterone or 17/3-estradiol levels. While enhanced clearance of sex steroids has been demonstrated following chemical exposure and induction of hepatic biotransformation enzymes, elegant feedback control mechanisms tend to ensure that more hormone is produced and homeostasis is maintained (Figure 17.2). Enhanced clearance of sex steroids can contribute to endocrine disruption if the toxicity also results in impaired hormone synthesis (i.e., gonadal toxicity or interference with the feedback control of hormone synthesis). 2,3,7,8-Tetrachlorodibenzodioxin appears to lower circulating sex steroid levels via this dual effect. 

This retardation of reaction rate with increase in reactant partial pressure is characteristic of catalytic reactions controlled by a surface reaction mechanism. Langmuir-Hinshelwood surface reaction rate mechanisms for single and dual site mechanisms are respectively ... 

In view of the uncertainty in determining whether single or dual site mechanisms controlled the dehydrogenation reaction, simple single site La ngmuir-Hinshel wood expressions for parallel and series coking were used to fit the experimental cokinjg data. For a parallel mechanism. 

The transport of a sub-critical Lennard-Jones fluid in a cylindrical mesopore is investigated here, using a combination of equilibrium and non-equilibrium as well as dual control volume grand canonical molecular dynamics methods. It is shown that all three techniques yield the same value of the transport coefficient for diffusely reflecting pore walls, even in the presence of viscous transport. It is also demonstrated that the classical Knudsen mechanism is not manifested, and that a combination of viscous flow and momentum exchange at the pore wall governs the transport over a wide range of densities. 

A discernible trend in modem pump design is towards the use of very small pistons (stroke volume around 100 pi). These tiny pistons have to be operated at very high driving speeds, to provide the flow rates required in analytical HPLC. However, when they are used in combination with electronic feedback pulse control mechanisms, as described above, they can provide extremely stable solvent delivery characteristics. Such is the efficacy of this approach that single piston pumps designed in this way are able to easily out-perform older dual piston pumps, and are consequently beginning to account for a major part of the LC pump market. 

The first step is to develop equations for all possible controlling steps. They will include, among many possibilities, dissociation of one or more of the reactants, the common single-site mechanism for reactant decomposition, the dual-site mechanism where two sites are involved in the decomposition, and half-site mechanism where two molecules are adsorbed on a single site. Having formulated sufficient models (usually 15 to 20 are even excess of 100), various experimental and sophisticated statistical methods are available for selecting the most probable model, as described in many texts (e.g., Froment and Bischoff, 1990). A comprehensive stepwise procedure (not discussed here) is suggested by Doraiswamy (2001). 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

Modern positive-pressure mechanical ventilators have been quite successful in treating patients with pulmonary disorders. Two major categories of breath dehvery modes for these ventilators are mandatory and spontaneous. The volume- and pressure-controlled mandatory breath delivery and the governing control equations for these modes are presented in this chapter. Similarly, CPAP and support pressure modes of spontaneous breath dehvery are described. Recent development of dual control modes that allow simultaneous monitoring and control of airway pressure and minute volume are also presented. 

Acetyl-CoA carboxylase is also inhibited by long-chain fatty acyl-CoA, and such inhibition is accompanied by enzyme depolymerization (91, 92, 95). Binding of 1 mole of palmityl-CoA per mole of rat liver acetyl-CoA carboxylase inhibits the enzyme (95). The Ti for palmityl-CoA, about 5 nM, is far lower than the critical micellar concentration of the thioester this indicates that the inhibition may be physiologically significant. If the allosteric control mechanisms of citrate promoted "substrate activation or fatty acyl-CoA mediated "feed back inhibition of fatty acid synthesis function at all under in vivo conditions, they must function as a dual mechanism (90). 

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

On-power refuelling provides the principal means for controlling reactivity in the CANDU 6. Additional reactivity control, independent of the safety shutdown systems, is achieved through use of reactivity control mechanisms. These include light-water zone compartments, absorber rods, and adjuster rods all are located between fiiel channels within the low pressure heavy water moderator and do not penetrate the heat transport system pressure boundary. The reactor is controlled by the dual redundant computer control system. The overall station control system is described in Section 5.7.2.3. 

This section identifies rechargeable or secondary battery requirements best suited for communications and surveillance and reconnaissance satellites. The battery power requirements are strictly dependent on several factors, including launch orbits such as LEO, elliptical, or GSO orbital height the type of stabilization technique used (i.e., mono-spin, dual-spin, or three-axis configuration) satellite operational life attitude control system and the overall DC power requirements needed to power the electronic and electrical subsystems, the electro-optical and microwave sensors, and the attitude and stabilization control mechanisms. 

Branson RD, MacIntyre NR. Dual control modes of mechanical ventilation. Respir Care 1996 41 294-305. 

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