Inner-spheric surface complex

The binding of a reductant or oxidant species to form an inner-spheric surface complex changes its electronic structure and thus influences its reductive and oxidative reactivity. As a consequence the following differences in thermodynamic and kinetic properties between dissolved and adsorbed species may be observed (Wehrli et al., 1989). 

The dissolution of quartz is accelerated by bi- or multidentate ligands such as oxalate or citrate at neutral pH-values. The effect is due to surface complex formation of these ligands to the Si02-surface (Bennett, 1991). In the higher pH-range the dissolution of quartz is increased by alkali cations (Bennett, 1991). Most likely these cations can form inner-spheric complexes with the =SiO groups. Such a complex formation is accompanied by a deprotonation of the oxygen atoms in the surface lattice (see Examples 2.4 and 5.1). This increase in C H leads to an increase in dissolution rate (see Fig. 5.9c). 

Similar to solution complexation, surface complexation can be distinguished between inner-spherical complexes (e.g. phosphate, fluoride, copper), where the ion is directly bound to the surface, and outer-spherical (e g. sodium, chloride) complexes where the ion is covered by a hydration sleeve with the attraction working only electrostatically. The inner-sphere complex is much stronger and not dependent on electrostatic attraction, i.e. a cation can also be sorbed on a positively charged surface (Drever 1997). 

Figure 8.26. Representative Fe(II)/Fe(III) redox couples at pH = 7. (phen = phen-anthroline sal = salicylate porph = porphyrin = valid for [HCO = 10 M.) Complex formation with Fe(II) and Fe(III) both on solid and solute phases has a dramatic effect on the redox potentials thus electron transfer by the Fe(II),Fe(III) system can occur at pH = 7 over the entire range of the stability of water (-0.5 to 1.1 V). (= Fe 0)2 Fe refers to Fe adsorbed inner-spherically to a surface of a hydrous ferric oxide. The range of redox potentials for heme derivatives given on the right illustrates the possibilities involved in bioinorganic systems.

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface oxide ions act as u-donor ligands, which increase the electron density of the coordinated metal ion. Cu(II) bound inner-spherically is a different chemical entity than if it were bound outer-spherically or present in the diffuse part of the double layer the inner-spheric Cu(II) has chemically different properties for example, a different redox potential [with regard to Cu(I)], and its equatorial waters are expected to exchange faster than in Cu(II). 

The mitochondrial complex that carries out ATP synthesis is called ATP synthase or sometimes FjFo-ATPase (for the reverse reaction it catalyzes). ATP synthase was observed in early electron micrographs of submitochondrial particles (prepared by sonication of inner membrane preparations) as round, 8.5-nm-diameter projections or particles on the inner membrane (Figure 21.23). In micrographs of native mitochondria, the projections appear on the matrixfacing surface of the inner membrane. Mild agitation removes the particles from isolated membrane preparations, and the isolated spherical particles catalyze ATP hydrolysis, the reverse reaction of the ATP synthase. Stripped of these particles, the membranes can still carry out electron transfer but cannot synthesize ATP. In one of the first reconstitution experiments with membrane proteins, Efraim Racker showed that adding the particles back to stripped membranes restored electron transfer-dependent ATP synthesis. 

Anotiier characteristic of die inner mitochondrial membrane is the presence of projections on the inside surface, which faces the mitochondrial matrix. See Fig. 18-14. These spherical 85-kDa particles, discovered by Fernandez Moran in 1962 and attached to die membrane tiirough a "stalk", display ATP-hydrolyzing (ATPase) activity. The latter was a clue that the knobs might participate in the synthesis of ATP during oxidative phosphorylation. In fact, tiiey are now recognized as a complex of proteins called coupling factor 1 (F ) or ATP synthase. 

The final type of manipulator has three rotational degrees of freedom. This is the most complex type to control, but it has increased flexibility. Fig. 2d shows this type of manipulator-the anthropomorphic arm. The work volume of a practical manipulator of this form is shown in Fig. 3. You will notice that it is basically spherical but has missing portions due to the presence of the arm itself and because the rotations cannot achieve a full 360 degrees. The scallops on the inner surface are caused by constraints imposed by the joints. 

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