Non bridging oxygens

Figure 14-7. Snapshots of the active site structures near the transition state of (top) the nucleophilic attack and (bottom) the exocyclic cleavage for the in-line monoanionic O2p mechanism of cleavage transesterification in the hairpin ribozyme. The yellow and red colored cartoon is for the substrate and ribozyme strands, respectively, and water molecules interacting with non-bridging oxygens and O5/ are shown...

Table 5. The effect upon PLCB( activity of replacing bridging and non-bridging oxygens around phosphorus in soluble phosphatidylcholine derivatives... 

From the mechanistic viewpoint, a more informative structure is the complex of PLCBc with inorganic phosphate (Pi), which also inhibits the enzyme (50% inhibition at 50 mmol l1 Pi) [65]. In this complex, which was obtained at 2.1 A resolution, the three zinc ions were coordinated to the two non-bridging phosphate oxygens (Fig. 7 a). Comparison of this structure with the native one (Fig. 6) reveals that one of the non-bridging oxygens on the phosphate replaced the bridging water molecule between Znl and Zn3, while the other oxygen displaced one of the waters coordinated to Zn2. 

X = average number of non-bridging oxygen ions per polyhedron ... 

The non-bridging oxygen ion so produced is now capable of dissociating another water molecule ... 

These equations may be summarized as follows, but the intermediate stages involving the non-bridging oxygen sites are of crucial importance, as demonstrated by the observed resistance of vitreous silica to aqueous attack ... 

As the number of non-bridging oxygen or sulphur atoms increases, the average length of the macromolecular chains decreases. The chemical reaction symbolised in (4.2) is strongly exothermic, and the mixing enthalpies are of the order of some hundreds of kilojoules. The magnitude... 

Since the relative dielectric constant of inorganic glasses is low, typically between 5 and 15, ionic species are strongly associated. For instance most of the Ag cations will be associated with non-bridging oxygens in AgPOj glass. Nevertheless, thermal vibrations allow a partial... 

Fig. 4.3 Schematic representation of interstitial cationic pair formation (a) and migration from one non bridging oxygen to another in a cation conducting glass (b).

Direct coordination of a metal ion to the non-bridging oxygen might render the phosphorus center more susceptible to nucleophilic attack (electrophilic catalysis Fig. 3e) or, alternatively, hydrogen bonding between a metal-... 

In natural melts, the presence of high field strength ions such as Fe , TF, and P, which, like silicon, preferentially assume a tetrahedral coordination with oxygen, complicates the structure, and the constitution of the anion matrix may not be deduced on the basis of the equations in section 6.1.2. Structural parameters valid for compositionally complex melts were proposed by Mysen et al. (1980) and Virgo et al. (1980) on the basis of the results of Raman spectroscopy. These parameters are NBO/Si and NBO/T (NBO = Non-Bridging Oxygen T groups all tetrahedrally coordinated cations—i.e., Fe ", TF, P ", ... 

Fig. 3. Plot of logio normalized ion-exchange rate at amorphous silica saturation vs. the amount of excess alkalis (Na, K), denoted by the molar ratio XAlk/(Al + IVB + FeT). All boron is treated as four-fold coordinated (IVB) and total iron (FeT) is regarded as ferric. The ion-exchange rate subtracts out the contribution of alkalis to solution from matrix dissolution. As the amount of excess alkali increases, the ion-exchange rate increases. This increase in rate reflects the increasing amount of alkalis in non-bridging oxygen (NBO) configurations. Error bars represent 2-

NAA NAAQS NBO NCP Neutron activation analysis National ambient air quality standards Non-bridging oxygen Non-carbonate portion... 

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