Inhibition, borate

A reaction looked at earlier simulates borate inhibition of serine proteinases.33 Resorufin acetate (234) is proposed as an attractive substrate to use with chymotrypsin since the absorbance of the product is several times more intense than that formed when the more usual p-nitrophcnyl acetate is used as a substrate. The steady-state values are the same for the two substrates, which is expected if the slow deacylation step involves a common intermediate. Experiments show that the acetate can bind to chymotrypsin other than at the active site.210 Brownian dynamics simulations of the encounter kinetics between the active site of an acetylcholinesterase and a charged substrate together with ah initio quantum chemical calculations using the 3-21G set to probe the transformation of the Michaelis complex into a covalently bound tetrahedral intermediate have been carried out.211 The Glu 199 residue located near the enzyme active triad boosts acetylcholinesterase activity by increasing the encounter rate due to the favourable modification of the electric field inside the enzyme and by stabilization of the TS for the first chemical step of catalysis.211... 

As shown in Figure 13.19a, phosphate and borate inhibit the dissolution of goethite by H2S. Similarly, the dissolution of lepidocrocite (7-FeOOH) by EDTA (Y" ) is inhibited by phosphate and arsenate (Figure 13.19b). Both in the reductive dissolution (by H2S) and the ligand-promoted dissolution (by... 

Although borate was also found to inhibit the antilectin activity of AFGP, the results were more difficult to interpret because borate inhibited the activity of the lectin itself in the hemagglutination assay. 

Boric complexes with p5T ocatechol and gaUic acid have been reported (43, 48). The stability constant of boric-catechol-3,5-disulphonic acid has been determined potentiometrically (56). Borate inhibits tyrosinase activity by forming 1 1 or 1 2 complexes with the substrates catechol or dopa. The stability constants of these complexes have been determined spectrophotometrically and are in the order of 5.3 X 10 and 6.7 X 10- , respectively (73). 

Yasunobu, K. T., and E. R. Norris Mechanism of borate inhibition of dipheno-loxidation by tyrosinase. J. Biol. Chem. 227, 473 (1957). 

Zinc Borate. Zinc borate is also effective in enhancing the flame-inhibiting powers of chlorine. Although zinc borate increases flame resistance, it is not as effective as antimony oxide, as is illustrated in Figure 3. However, zinc borate can be used in combination with antimony oxide to obtain equivalent and in some instances enhanced effects over what can be obtained using either of the two synergists alone (Table 9). 

In the process of inhibition polypyrocatechin borate interacts with polyethylene macroradicals to form the B—O—C bonds. This is confirmed by the fact that the absorption spectrum of polyethylene inhibited with polypyrocatechin borate revealed the bands in the region of 1350 cm" characteristic for the B—O—C bond. There is no such a band in the spectrum of pure polypyrocatechin borate after heating under the same conditions. Chemical analysis of boron in polyethylene provides support for the IR-spectroscopy data concerning the presence of chemically bonded boron in polyethylene after destruction. 

Inhibition of polyethylene thermal destruction by polypyrocatechin borate could be represented as follows. The initial molecular-chain scission of branched... 

Polypyrocatechin borate has a higher inhibitive efficiency relative to the reactions of the radical R2 than to that of the radical Ri, since the latter exerts a more severe decrease in the yield of propane and propylene than in the yield of ethane and ethylene, and favors an insignificant growth of polyethylene nonsaturation in the pro-... 

Some salts, notably chromates, dichromates, silicates, borates and cinna-mates, have marked inhibitive power and are very effective in closed-circuit water systems. Care must be taken to ensure that a sufficient quantity of such anodic inhibitors as chromates is added, as otherwise attack, though occurring at fewer points, may be more severe at these points. Chromates and dichromates have little inhibitive power in strongly acid solutions. 

Borates are often the base for light-duty cleaners associated with the cleaning of light metals, due to their inhibiting action and mild pH. They can also be used, to a certain extent, as a substitute for phosphates when a phosphate-free product is required. 

A large number of electrolytic treatments of magnesium, anodic or a.c., have been developed, in which adherent white or grey films consisting of fluoride, oxide, hydroxide, aluminate or basic carbonate are deposited from alkaline solutions containing caustic alkali, alkali carbonates, phosphates, pyrophosphates, cyanides, aluminates, oxalates, silicates, borates, etc. Some films are thin, and some are relatively thick. All are more or less absorbent and act as good bases for paint, though none contributes appreciable inhibition. All can, however, absorb chromates with consequent improvement of protective efficiency. 

Locomotive diesels As larger volumes of coolant are required in railway locomotives than in road vehicles, the cost of inhibition is proportionally greater. An additional factor is the possibility of cavitation attack of cylinder liners. These considerations place a restriction on the choice of inhibitors. In the past, chromates have been used at concentrations of up to 0-4%, but their use presents handling and disposal problems. Chromates cannot be used with ethanediol antifreeze solutions. A IS I borate-metasilicate at a concentration of 1 % has been used in the UK. Nitrate is added to this to improve inhibition of aluminium alloy corrosion. Tannins and soluble oils are also used, but probably to a lesser extent than in the past. The benzoate-nitrite formulation (formerly BS 3151) is effective and has been used by continental railways . ... 

There has been much activity in this field of corrosion inhibition in recent years which appears to have been prompted by health and safety requirements. As with engine coolants, the use of nitrites, particularly where amines may also be present, needs to be considered carefully. Nitrites have been widely used in cutting, grinding, penetrating, drawing and hydraulic oils. Suggested replacements for nitrites and/or amines make use, inter alia, of various borate compounds, e.g. monoalkanolamide borates. Molybdates have also been proposed in conjunction with other inhibitors, e.g. carbox-ylates, phosphates, etc . Water-based metalworking fluids usually contain other additives in addition to corrosion inhibitors, e.g. for hard-water stability, anti-foam, bactericidal proderties and so on. Thus, claims are made for oil-in-water emulsions with bactericidal and anti-corrosion properties. 

The ways in which inhibitive anions affect the corrosion of zinc are mainly similar to those described above for iron. In inhibition by chromate, localised uptake of chromium has been shown to occur at low chromate concentrations and in the presence of chloride ions Thus under conditions unfavourable for inhibition, pore plugging occurs on zinc. Inhibitive anions also promote the passivation of zinc, e.g. passivation is much easier in solutions of the inhibitive anion, borate , than in solutions of the non-inhibitive anions, carbonate and bicarbonate , A critical... 

Mixtures of sodium borate and nitrite have been used to store plant. The solution may either be left in. If this is impracticable, for instance if the plant needs to be drained for maintenance purposes, then the solution may be drained from the plant after initial dosing so as to leave locations which are difficult to drain containing a residue of inhibitive solution . 

In our previous work [63], we studied the hydrolysis kinetics of lipase from Mucor javanicus in a modified Lewis cell (Fig. 4). Initial hydrolysis reaction rates (uri) were measured in the presence of lipase in the aqueous phase (borate buffer). Initial substrate (trilinolein) concentration (TLj) in the organic phase (octane) was between 0.05 and 8 mM. The presence of the interface with octane enhances hydrolysis [37]. Lineweaver-Burk plots of the kinetics curve (1/Uj.] = f( /TL)) gave straight lines, demonstrating that the hydrolysis reaction shows the expected kinetic behavior (Michaelis-Menten). Excess substrate results in reaction inhibition. Apparent parameters of the Michaelis equation were determined from the curve l/urj = f /TL) and substrate inhibition was determined from the curve 1/Uj.] =f(TL) ... 

The advantage of a two-phase lipoxygenation system lies in three points. They include avoidance of inhibition by the substrate as well as high solubility of the substrate in the organic phase and product recovery in the aqueous phase. Drouet et al. [36] improved the production yield of hydroperoxylinoleic acid at high concentrations of linoleic acid in highly stirred borate buffer-octane biphasic medium. 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

FIG. 2. Mechanism of phenylephrine (PE)-mediated wave-like [Ca2+] oscillations in the rabbit inferior vena cava. (A) PE-mediated [Ca2+]j oscillations are completely inhibited by 10 fiM cyclopiazonic acid (CPA), but the average [Ca2+ ]j remains elevated. (B) PE-mediated [Ca2+]j oscillations are abolished by 75 /iM 2-aminoethoxydiphenyl borate (2-APB). (C) Application of 10 piM nifedipine (Nif) reduced the frequency of PE-mediated [Ca2+]j oscillations while additional application of SKF96365 (SKF) completely abolished the remaining [Ca2+] oscillations. (D) Application of 100 /iM 2,4-dichlorobenzamil (2,4-DCB) completely inhibited nifedipine-resistant PE-induced [Ca2+]j oscillations and lowered the [Ca2+]j to a level that is slightly higher than baseline. Additional application of SKF96365 returned the [Ca2+]j level to baseline. (Experimental traces reproduced with permission from Lee et al 2001.)... 

The anion plays a crucial role. BArF and other bulky fluorinated tetra-arylbo-rates or tetraalkoxyaluminates are the most suitable anions. Hexafluorophos-phate-containing catalysts display high reactivity in the initial phase of the reaction, but suffer deactivation before the reaction reaches completion. Tetrafluoro-borate, triflate or other more strongly coordinating anions inhibit the catalyst. 

Trispyrazolylborates are models for tris-histidine active sites in zinc enzymes, e.g., the matrix metalloproteinases involved in breakdown of extracellular matrices. Inhibition of these metalloproteinases may prove valuable in the treatment of, inter alios, cancer and arthritis, so efforts are being made to find appropriate ligands to block the zinc active site. The search has recently moved on from hydroxamates to hydroxypyridinones - l-hydroxy-2-pyridinone is a cyclic analogue of hydroxamic acid. As reported in Section II.B.2 earlier, hydroxypyridinones form stable five-coordinate complexes on reaction with hydrotris(3,5-phenylmethylpyrazolyl)borate zinc hydroxide. Modeling studies suggest that hydroxypyridinonate ligands should be able to access the active site in the enzyme with ease (110). 

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