12 - substrates potentiometric titration

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

Modified poly(methacrylic acid) microparticles complexed with gadolinium(lll) (Gd ) ions were prepared at 100 nm by Michinobu [81]. The emulsion tcrpolymerization of methacryhc acid, ethyl acrylate, and aUyl methacrylate and the following complexation with Gd ions yielded the polymer particles with different Gd ion contents. Potentiometric titration of the complexation of the particle with Gd + ions indicated the formation of a very stable tris-carboxylate coordinate complex with the Gd + ion. The microparticles dispersed on a mica substrate were subjected to AFM, followed by MFM. AFM showed 100-nm-sized and monodispersed spherical images. The following MFM... 

Potentiometric titration has been used to assess the acid/base properties of a variety of solid surfaces ranging from carbon blacks to metal (hydr)oxides. It is the experimental vehicle we use to determine proton affinity, but its application does present some limitations. Our determination of the charging behavior of the substrate is limited by a practical consideration the concentration of protons must fall in the range equivalent to 3 < pH < 11 or otherwise the so-called buffering effect of water will introduce significant error in the measurements [1]. 

Ansyin and coworkers have explored the stoichiometric hydrolysis reactivity of zinc complexes of terpyridyl-type ligand having guanidinium or ammonium appendages (Fig. 8.24) using the RNA dimer adenylyl (3 -> 5 )phosphoadenine (ApA) [119]. The hydrolysis of this substrate yields adenosine 3 -monophos-phate, adenosine 2 -monophosphate, adenosine 2, 3 -cAMP, and adenosine. The optimal rate of reaction of Zn(II)-4 with ApA is found at pH 7.5, which is near the pK value of a zinc-coordinated water molecule in this system (pJCa=7.3 determined independently by potentiometric titration). This suggests the involvement of a zinc hydroxide species as the active complex for ApA hydrolysis. 

The common problems with those metallomicelles may be summarized as follows (1) Most of these complexes were prepared in situ and often were not isolated. Hence, the intended structures of the metallomicelles in solution or in the solid state were not verified. (2) The metal complexes in solution were not identified or characterized in rigorous thermodynamic senses by potentiometric pH titration, etc. The complexation constants and possible species distribution at various pH s were totally unknown. (3) Possible catalytically active species L-Mn+—OH were not identified by means of the thermodynamic pvalues. Those described were all obtained merely in kinetics. (4) The product (phosphate anion) inhibition was not determined. Accordingly, it often was not clear whether it was catalytic or not. (5) Often, the substrates studied were limited. (6) The kinetics was complex, probably as a result of the existence of various species in solution. Thus, in most of the cases only pseudo-first-order rates (e.g., with excess metal complexes) were given. No solid kinetic studies combined with thermodynamic studies have been presented. It is thus impossible to compare the catalytic efficiency of these metallomicelles with that of the natural system. Besides, different... 

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