Innerspheres

A further interesting point comes from the comparison of the in vivo data with the other ligand substitution results in general. The in vivo half-lives listed are comparable with the rate data of the equatorial substitution and might suggest that the mechanisms responsible for the uptake/clearance of the radiopharmaceuticals, be they protein, peptide, or DNA interactions or innersphere redox reactions, might indeed be associated therewith. This is, however, just an observation and surely requires much more research to be well understood. 

Innersphere mechanisms, in redox kinetics of octacyanomolybdate and -tung-state ions, 40 280-282 Inorganic... 

Oxidation of alcohols at the expense of photoreduction of Cr (VI) was initially interpreted in terms of the photochemical reactivity of chromate(VI) esters, but time-resolved spectroscopic studies 253) proved that the photoreaction mode depends on the bond strength between chromate and secondary alcohol. In the case of the strong bond, innersphere transfer of two electrons was recorded (Fig. 3) ... 

The former case is illustrated in Fig. 4a, showing two alternative pathways for the redox cycles photocatalyzed by Fe(III) or Cu(II) chelate complexes (i) innersphere PET with ligand (L) as electron donor and (ii) outersphere PET with an external electron donor (D). The latter way of degradation by Fe(III) photocatalysts is of special meaning in the case of anthropogenic pollutants, such as herbicides and pesticides 164,186,189,258,259). 

These kinetics data are consistent with a preequilibrium dissociation of dmf from the molybdenum center to form a reactive five-coordinate species that rapidly reduces the Fe(III) center via an inner sphere (halogen transfer) reaction. Other one-electron atom transfer reactions are known in oxo-molybdenum chemistry (262). An innersphere (atom transfer) mechanism is not a viable model for intramolecular transfer in sulfite oxidase because in the enzyme the Mo and Fe centers are almost certainly held too far apart by the protein framework. Moreover, the 65-type heme center of sulfite oxidase is six-coordinate with axial histidine ligands from the protein and hence cannot participate in atom transfer reactions. 

Recent studies of Co (III) carboxypeptidases are also consistent with these proposals (60). The Co (III) enzyme is inactive toward both peptides and esters, but examination of ES complexes by stopped-fiow fluorescence demonstrates that the peptide still binds to the modified enzyme while the ester does not. Since Co (III) complexes are "exchange inert, these results suggest that an innersphere complex is formed between the ester and the metal atom during binding but not for the peptide. 

It should be mentioned that although the problem of the change in the innersphere coordination of aquoions of the lanthanide series was solved by means of the doubledouble effect in 1973s5 57), the work in this field is still continued by our group95, % as well as by others92,97-103. ... 

If this interpretation is correct, then the pKa of the phenolic hydroxyl of the intermediate must be perturbed to a higher value by at least 2 pka units (i.e., a value >8.75), while the pKaofthe -CH2OH group is perturbed to a value <9 in this ternary complex. The blue-shifted spectrum of the intermediate is consistent with structure 2. Studies with other substrates (5,26,27,39) strongly support a catalytic mechanism in which the hydride transfer step involves the interconversion of innersphere-coordinated aldehyde and innersphere-coordinated al-koxide ion. The data of Kvassman et al. (39) and Sartorius et al. (26) indicate a pKa for the coordinated alcohol of 6 or 7 is not unreasonable (21,40). 

A 10-nt RNA duplex containing sheared G A basepairs was shown to have an Mn ion bound to the N7 of a guanine residue and the phosphate oxygen of the preceding adenosine residue." A similar site is found in the hammerhead ribozyme between the phosphate of A9 and the N7 position of GlO.l (Figure 11)." This site in the hammerhead ribozyme has been investigated by EPR methods and has an apparent affinity of 10 M in 0.1 M NaCl, pH 7." In the complex folded structure of the group I P4-P6 RNA, it was found that Mn replaced Mg at all sites that included innersphere coordination to the RNA, but not at all sites bound by hexahydrated Mg . ... 

The effect of ionic strength and dielectric constant of the medium on the rate explains qualitatively the reaction between two negatively charged ions, as seen in Scheme 1. The moderate AH and AS values are favorable for electron transfer reaction. The negative value of AS suggests that the intermediate complex is more ordered than the reactants [11]. The observed modest enthalpy of activation and a higher-rate constant for the slow step indicate that the oxidation presumably occurs via an innersphere mechanism. This conclusion is supported by earlier observations [12]. 

The fourth OA mechanism involves an innersphere electron transfer reaction between the organometallic compound and R-X, leading to the one-electron oxidized complex R radical cage. The radical cage then undergoes... 

Electrocatalytic oxidation through a charge transfer-complex with innersphere electron transfer ... 

The role played by the Mg " ions in the catalytic reaction offers a noteworthy example of a bimetallic chemical catalysis of an electrochemical reduction where an electron-rich center, here the inxiC O") porphyrin, commences the reduction process in an innersphere feshion and an... 

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