Oxyanionic effect

The family of serine proteases has been subjected to intensive studies of site-directed mutagenesis. These experiments provide unique information about the contributions of individual amino acids to kcat and KM. Some of the clearest conclusions have emerged from studies in subtilisin (Ref. 9), where the oxyanion intermediate is stabilized by t>e main-chain hydrogen bond of Ser 221 and an hydrogen bond from Asn 155 (Ref. 2). Replacement of Asn 155 (e.g., the Asn 155— Ala 155 described in Fig. 7.9) allows for a quantitative assessment of the effect of the protein dipoles on Ag. ... 

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

Chemical effects include stable compound formation and ionization, both of which decrease the population of free atoms in the sample vapour and thereby lower the measured absorbance. Examples of compound formation include reactions between alkaline earth metals and oxyanions such as aluminates, silicates and phosphates, as well as the formation of stable oxides of aluminium, vanadium, boron etc. 

Organo-modified natural zeolites as new tailored natural materials for removal of cations, anions and even organic pollutants may present fairly large potential for water utility companies. The topic of this study was to examine the oxyanions removal from waters by octadecylammonium-enriched inland clinoptilolite. The 18-carbon chain consisting surfactant attached on the clinoptilolite surface, as to the organic acids of living bodies comparable substances, makes the treatment process economic on scale and cost-effective as well.7... 

An increase in the coordination number of molybdenum takes place in the second protonation step, which has a dramatic effect on the value of K2. Instead of the typical decrease of 3 to 5 log units from the first to the second protonation constant, K2 has in this case about the same value as Kx. In fact, these unusual values for the protonation constants compared to those of other oxyanions, along with the thermodynamic parameters AH° and AS0, were the basis on which the change in coordination number in the second protonation step was first proposed (54). Previously the small difference between the first and second pK value was interpreted in terms of an anomalously high first protonation constant, assumed to be caused by an increase in the coordination number in the first step (2, 3, 54-57). 

Carbothioic O-acid esters (3.1 to be distinguished from carbothioic S-acid esters, R-CO-SR, discussed in Chapt. 7) are not hydrolyzed by serine peptidases. It has been postulated that the thiocarbonyl S-atom, which is larger and less prone to H-bonding than the carbonyl O-atom, cannot interact effectively with the oxyanion hole. The resulting inability to stabilize the transition state considerably reduces the rate of hydrolysis. 

As both elements are important environmental contaminants whose mobility and environmental impact depend on redox reactions, we expect that Cr and Se isotope analyses will be used widely as indicators of oxyanion reduction. It may also be possible to trace the origin of the Se or Cr via isotopic signatures, but only if the possible confounding effects of reduction can be constrained. 

It has been shown that the tetrahedral oxyanion formed on addition of the conjugate base of fluoroform to solvent DMF acts as a masked form of FsC with which to effect trifluoromethylation of aldehydes. " ... 

High-level quantum mechanical calculations have been used to explore the Horner-Wandsworth-Emmons reaction in the gas phase and also with a solvation contribution evaluated using the PCM/DIR method. Ring closure of the P—O bond (TS2), to form oxaphosphetane, is rate determining in the absence of solvation however, the oxyanion becomes a true intermediate, at an energy minimum on the reaction path, only in response to the effects of solvation, whereupon its formation by carbonyl addition (TSl) becomes rate limiting. Formation of F-product is always... 

The deactivating effect of a phenoxide oxyanion is removed in the ether series, but in cases such as 531 where ortholithiation can compete with lateral lithiation, mixtures of products are frequently obtained . The MOM acetal 532 is fully ortto-selective in its reaction with t-BuLi (Scheme 209/ . ... 

Hayes, KF. Papelis, C. Leckie, J.O. (1988) Modeling ionic strength Effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sd. 78 717—726 Hayes, KF. Roe, A.L. Brown, G.E. Hodgson, KO. Leckie, J.O. Parks, G.A. (1987) In-situ X-ray absorption study of surface complexes Selenium oxyanions on a-FeOOH. Sdence 238 783-786... 

The contributions of hydrogen bond donors to catalysis can be estimated by site-directed mutagenesis studies in cases where the hydrogen bond donor is located in the amino acid side chain. Deletion of the main chain NH is only possible by substituting the amino acid with a proUne. In all cases, the effects of the substitution to key enzyme kinetic parameters, and K, should be checked. Typically, the oxyanion hole residues contribute only Uttle to the binding of substrate [19-21]. This is reflected in the values, which typically remain very similar... 

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