Deprotonation of water molecules

These ions then precipitate as a hydrated iron(III) oxide, Fe203-H20, the brown, insoluble substance that we call rust. The oxide ions can be regarded as coming from deprotonation of water molecules and as immediately forming the hydrated solid by precipitation with the Fe3+ ions produced in reaction F ... 

The coordination of these oxygens at the monomolecular step will, of course, change as the mineral reacts with the aqueous solution because the MnOe octahedra shown with the wireframe (Figure 2) must detach as a surface complex as the step migrates. This detachment causes newly uncovered metals to reestablish their inner-coordination spheres by movement and deprotonation of water molecules. If the reaction proceeds at steady state, these water molecules dissociate to maintain a fixed charge density and a fixed numbers of different metal-ligand coordination numbers (7, 8). 

The sensing mechanism of metal oxides is dependent on their surface chemistries. These surface morphologies control the potential of the sensing electrode. The active sites originate from the protonation-deprotonation of water molecules attached to the metal (M) ions ... 

When a metal oxide surface is exposed to water, adsorption of water molecules takes place as shown in Equation 2.1. Cation sites can be considered as Lewis acids and interact with donor molecules like water through a combination of ion-dipole attraction and orbital overlap. Subsequent protonation and deprotonation of the surface hydroxyls produce charged oxide surfaces as shown in Equation 2.2 and Equation 2.3, respectively ... 

The acido-basic properties of water molecules are greatly affected in restricted media such as the active sites of enzymes, reverse micelles, etc. The ability of water to accept or yield a proton is indeed related to its H-bonded structure which is, in a confined environment, different from that of bulk water. Water acidity is then best described by the concept of proton-transfer efficiency -characterized by the rate constants of deprotonation and reprotonation of solutes - instead of the classical concept of pH. Such rate constants can be determined by means of fluorescent acidic or basic probes. 

For instance, Lowe et al. showed that the relaxivity of a series of macro-cyclic Gd(III) complexes bearing (3-arylsulfonamide groups is markedly pH-dependent (Fig. 15) on passing from about 8 s mM at pH < 4 to ca. 2.2 s mM at pH > 8 in one selected case (Chart 12, ligand 2) (130). It has been demonstrated that the observed decrease (about 4-fold) of ri is the result of a switch in the number of water molecules coordinated to the Gd(III) ion from 2 (at low pH values) to 0 (at basic pHs). This corresponds to a change in the coordination ability of the p-arylsulfonamide arm that binds the metal ion only when it is in the deprotonated form (Fig. 15). 

In addition, many oxyanion holes have been discovered in which water is one of the hydrogen bonding donors. These waters are clearly defined in the respective electron density maps. In some enzymes these waters are part of a trail of water molecules [80, 81]. When water is used as the hydrogen bond donor, very often the intermediate that is stabilized is not the tetrahedral intermediate but an enolate, which is derived by base-catalyzed deprotonation of the Ca-atom of the... 

In solutions with pH < 5, aluminum occurs as the octahedral hexahydrate, A1(H20)6 ", usually abbreviated as AP+. As a solution becomes less acidic, A1(H20)6 " undergoes successive deprotonations to yield Al(OH) +, A1(0H)2+, and soluble Al(OH)3, with a decreasing and variable number of water molecules. Neutral solutions give an Al(OH)3 precipitate that redissolves, owing to formation of tetrahedral aluminate, Al(OH)4, the primary soluble AP+ species at pH > 6.2. The four successive deprotonations squeeze into a narrow range 5.5 < pH < 6.2 the deprotonations from octahedral hexahydrate to tetrahedral aluminate occur cooperatively. ... 

For the in vitro simulation the reagents of weak base B are triethylamine, iV-methy-morpholine, imidazole and 2,6-lutidine. In human carbonic anhydrase II a couple of water molecules connected to His 64 and Thr 199 through hydrogen bond may deprotonate a zinc-bound water molecule[17,... 

What is the shape of the molecule which results from the deprotonation of water, i.e. the hydroxide anion ... 

The difference between the simple and the complex salts lies in the capability of the ions to deprotonate the water molecules associated with them. For simple ions this is not probable. The hydrolyzed and deprotonated metal cations form multinuclear complexes through oxolation-olation reactions discussed below. These reactions are dependent, not only on the pH, but also on the total metal concentration. By using the conditional constant approach this is illustrated as the dependency of log a(Fe(OH)) plotted as a function of the pH for a range of concentrations of Fe + ions in Figure 8.12. ... 

Fig. 11. The structure of the metal ion site in adenosine deaminase. The drawing is adapted from the data in Ref. 105. The Zn serves to activate a water molecule for nucleophilic attack while also orienting a carboxylate ligand to serve as a general base to deprotonate the water molecule.

Figure 4 Different subsites within the same active site, (a) The main active site feature of the serum paraoxonase PON1 is the cataiytic caicium ion, which lies at the bottom of a deep and hydrophobic active site, and is thought to act as the oxyanion hole of PONs. The native function, hydrolysis of lactones, is mediated by a His115-His134 dyad, that deprotonates a water molecule to generate the attacking hydroxide. Although the same dyad appears to mediate the promiscuous arylesterase activity of PON1, the promiscuous phosphotriesterase activity (shown here for paraoxon) appears to be independent, and mediated by other residues that act as a base, or nucleophile. Indeed, mutations of both His residues may increase the...

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