Inner sphere interaction

Subsequent to CO2 association in the hydrophobic pocket, the chemistry of turnover requires the intimate participation of zinc. The role of zinc is to promote a water molecule as a potent nucleophile, and this is a role which the zinc of carbonic anhydrase II shares with the metal ion of the zinc proteases (discussed in the next section). In fact, the zinc of carbonic anhydrase II promotes the ionization of its bound water so that the active enzyme is in the zinc-hydroxide form (Coleman, 1967 Lindskog and Coleman, 1973 Silverman and Lindskog, 1988). Studies of small-molecule complexes yield effective models of the carbonic anhydrase active site which are catalytically active in zinc-hydroxide forms (Woolley, 1975). In addition to its role in promoting a nucleophilic water molecule, the zinc of carbonic anhydrase II is a classical electrophilic catalyst that is, it stabilizes the developing negative charge of the transition state and product bicarbonate anion. This role does not require the inner-sphere interaction of zinc with the substrate C=0 in a precatalytic complex. 

The site bound ions accounts for its hydration state and are grouped either as outer-sphere or as inner-sphere [9, 11, 14]. In the later case, it is assumed that the water molecules in the hydration shell do not participate. The ions directly interact with the phosphate charges and anionic ligands [9, 11, 14], Since both outer- and inner-sphere interactions lead to formation of ion pairs, site bound ions are describable in terms of an association constant satisfying the law of mass action [9, 11, 14—16]. 

Galactose oxidase hinds a single copper ion within Domain 11 on the axis of the wheel. The active site (Fig. 5) is unhke any other biological copper complex, an appropriate distinction for this remarkable enzyme. To explore the site in more detail, the protein environment of the mononuclear copper center may be separated into (A) direcdy coordinated metal hgands (hrst shell, inner sphere interactions) and (B) the extended active site environment (the second shell or outer coordination sphere). 

This equation actually describes the interaction between the ion and the first solvation shell. Ions frequently form a very stable "aquo complex with H2O molecules, for example an Fe(H2O)6 complex. Besides this inner sphere interaction there is also an outer sphere one, leading to a corresponding arrangement of the H2O dipoles around the ions. The outer sphere interaction is given by the energy required if an ion with the inner solvation shell (radius /j -1- z-soi) is transferred from a vacuum into the solution, as derived by Born using the continuum model ... 

However, the d-d transitions of Ni(II) are certainly more influenced by inner-than outer-sphere complexation. In the case of weak complexes, inner-sphere interaction can be expected to dominate only at markedly reduced water activity, i.e., in case of concentrated solutions. Consequently, a visible spectrophotometric study, using high concentrations of the complex-forming anion, mostly provides information on inner-sphere complexes, and it is therefore not strictly comparable with e.g., potentiometric results obtained at considerably lower ligand concentrations. 

This simplified discussion of electron transfer for outer sphere interactions where the electron is transferred through solvent molecules is given to provide a conceptual basis for understanding this kind of reaction. Many electron transfer reactions are more complicated due to quantum mechanics effects (electron tunneling) and inner sphere interactions (Astruc, 1995). Basolo and Pearson (1967) give more details about electron transfer reactions. 

The work term for inner sphere interaction of the ion-pair can be evaluated as follows ... 

The coordination of several ions to La(III) in methanol has been studied by La NMR (Bunzii et al. 1987). The chemical shift of the La-NMR resonance is indeed governed by the composition of the iimer coordination sphere. The chemical shifts measured for several LaX methanolic solutions are shown in fig. 9. The large variation of these shifts versus the concentration of the salts is indicative of the presence of a significant inner-sphere interaction between La(III) and all the anions studied to form complexes of the type [LaX (solv)x] "". The absolute stability constants (table 8) for the anion complexation can be obtained from the following equations, where x is the mole fraction of LaX ", and where the solvation has been neglected ... 

Inner-Sphere Interactions Between Metal Ions and Ionic Liquid Extractants Bis(0,O -ethyl) dithiophosphate... 

First, the pyridine ring is protonated (Equation 1.25) the base dissociation constant of pyridine is K, = 1.49 x 10 . Next, the pyridinium ion undergoes a one-electron transfer to form the pyridinium radical (Equation 1.26). The reduction is coupled to a catalytic reaction of hydrogen generation (Equation 1.27). Hence, steps 2 and 3 constitute an EC sequence (where EC stands for an electrochemical step followed by a chemical step). A competing process, the inner-sphere interaction between the catalyst and the CO2 substrate, yields a radical pyridinium-C02 complex intermediate (Equation 1.28), which cleaves into a hydroxyformyl radical (Equation 1.29). When Pt and Pd electrodes are used, this anion radical can undergo a one-electron... 

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