Cobalt Substitution

Delmas and his co-workers have done extensive work on pyroaurite-type materials which has recently been reviewed [73], In addition to precipitation methods, they have prepared the materials by mild oxidative hydrolysis of nickelates that were prepared by thermal methods similar to those used for the preparation of LiNiOz [74]. A cobalt-substituted material NaCoA ( Ni( A02) was prepared by the reaction of Na20, Co304 and NiO at 800 °C under a stream of oxygen. The material was then treated with a 10 molL-1 NaCIO +4 molL 1 KOH solution for 15h to form the oxidized y -oxyhydroxide. The pyroau-... 

Price, N.M. and F.M.M. Morel. 1990. Cadmium and cobalt substitution for zinc in a marine diatom. Nature 344 658-660. 

Pichat S, Douchet C, Albarede F (2003) Zinc isotope variations in deep-sea carbonates from the eastern equatorial Pacific over the last 175 ka. Earth Planet Sci Lett 210 167-178 Price NM, Morel EMM (1990) Cadmium and cobalt substitution for zinc in a zinc-deficient marine diatom. Nature 344 658-660... 

The crystal structure of the cobalt-substituted enzyme was obtained with bicarbonate bound to the metal (Iverson et al. 2000). The structure shows Asn 202 and Gln75 hydrogen bonded to the metal-bound bicarbonate, suggestzing potential roles for these residues in either transition-state stabilization or orientation and polarization of CO2 for attack from the zinc-hydroxyl (Fig. 11.5). The crystal structure also shows three discrete conformations for Glu 84, suggesting a role for this residue in the transfer of protons out of the active site indeed, kinetic analyses of Glu 84 variants combined with chemical rescue experiments establish this residue as critical for proton transfer (Tripp and Ferry 2000). The location of Glu 62 adjacent to Glu 84 suggests a potential role in proton transfer as well. Although kinetic analyses of site-specific variants establish an essential role for Glu 62 in the CO2 hydration steps (Eqs. 11.3 and 11.4), the results were inconclusive regarding an additional role in proton transfer (Eqs. 11.5 and 11.6). 

Figure 11.5. The active site of cobalt-substituted Cam complexed with bicarbonate. Reprinted with permission from Iverson et al. (2000), copyright 2000 American Chemical Society.

These cobalt-substituted materials can also be prepared hydrothermally, and their cycling behavior... 

Schneider, G., Eklund, H., Cedergren-Zeppezauer, E., and Zeppezauer, M. (1983). Crystal structures of the active site in specifically metal-depleted and cobalt-substituted horse liver alcohol dehydrogenase derivatives. Proc. Natl. Acad. Set. U.S.A. 80, 5289-5293. 

Arthur Adamson I think Dr. Wilmarth and co-workers have probably supplied the better available evidence for the pentacoordinated intermediate in a cobalt substitution reaction. 

The cobalt-substituted carbonic anhydrase has been extensively studied as it offers easily measurable pH-dependent electronic spectra... 

Cobalt substitution of mouse R2 was undertaken by Anderssen et al.,m which was not oxidised under aerobic conditions, unlike the natural diferrous enzyme. Addition of 1 equivalent of Co(II) resulted in a spectrum at X-band at 10 K consistent with an isolated S = spin system, with j/ at ca. 5.4,4.1 and 2.0, and... 

Fig. 24. Atom-resolved STM image of a CoMoS nanocluster. The size is 51 Ax 52A, and Ft = -95.2mV and /. = 0.81 nA. Note the intense brim associated with the cobalt-substituted sulfur edge (shorter edges).

The hyperfine shifts of groups bound to the donor atom are largely dominated by the contact interaction, even if pseudocontact shift contributions are sizable and any quantitative use of the shifts should rely on the separated contributions. Longitudinal nuclear relaxation times can be used, and have been used in the case of cobalt substitute stellacyanin, to determine metal-proton distances [101]. The contribution of Curie relaxation, estimated from the field dependence of the linewidths, can be used both for assignment and to determine structural constrains [101]. 

In the case of cobalt substituted Zn-fingers [102], the differences between the chemical shifts for corresponding resonances in the Co(II) and Zn(II) complexes allow the determination of the orientation and anisotropy of the magnetic susceptibility tensor [103]. Similar studies are available for pseudotetrahedral Co(II) in the zinc site of superoxide dismutase [104] and five coordinated carbonic anhydrase derivatives [105]. 

Fig. 8.11. 600 MHz NOESY spectra of the four-cobalt cluster of cobalt-substituted metalloth-ionein. The spectra have been recorded with tm = 7 ms using a specially built probe featuring a 90° pulse of 3.7 p,s. (A) Spectrum obtained with a 125 kHz 16 bit ADC by folding the spectrum in both dimensions cross peaks g-b and c-i are folded. (B) Spectrum recorded using a fast ADC over a 250 kHz spectral width. The nine observed connectivities between geminal protons of cobalt-coordinated cysteine P-CH2 are summarized in the inset. Cross peaks marked with numbers are EXSY cross peaks to a minor species labeled Y in the inset [12].

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