Carboxylate shift

Andersson, M. E., H"gbom, M., Rinaldo-Matthis, A., Andersson, K. K., Sj berg, B.-M., and Nordlund, P., 1999, The crystal structnre of an azide complex of the diferrous R2 subunit of ribonucleotide reductase displays a novel carboxylate shift with important mechanistic implications for diiron-catalyzed oxygen activation. J. Amer. Chem. Soc. 121 2346n2352. 

Rardin, R. L., Tolman, W. B., and Lippard, S. J., 1991, Monodentate carboxylate complexes and the carboxylate shift Implications for polymetalloprotein structure and function. New. J. Chem. 15 417fi430. 

A carboxylate shift has formally occurred during the synthesis of 50, testifying to the flexibility of carboxylate coordination. Similar to the precursor lb, the two nickel(ll) ions in 50 are weakly antiferromagnetically coupled (/= 0.9cm ) (110). 

Dinucleating carboxylates support diiron(II) complexes that bind O2 rapidly in a simple bimolecular process (first order in the complex and first order in O2).92 The activation parameters estimated for the oxygenation of the XDK-Im complex, approximately A/T = 16 kJ/mol and AS = —120 J/(molK) are in accord with the values obtained for other diiron(II) complexes and correspond to a low-barrier O2 coordination at a vacant iron(II) center. A mechanism proposed for the oxygenation reaction includes the attack of the O2 molecule on the unsaturated (five-coordinate) iron(II) center concomitant with the carboxylate shift at this center followed by the coordination to the second, six-coordinate iron(II) center and additional ligand rearrangement (Figure 4.26). 

Variable-temperature F NMR spectroscopic studies of [Fe2(02CAr4 FPt) (THF)2] (19) from +20 °C to —80 °C revealed dynamic interconversion between windmill and paddlewheel isomers via carboxylate shifts. Structural characterization of two geometric isomers of (19), [Fe2(A-02CAr4 FPb 2-... 

Although compounds 3a and 4a yield very similar products following oxidation, the kinetics of the two reactions highlight important differences. Compound 4a is coordinatively saturated, but undergoes rapid carboxylate exchange at room temperature. The intermediate in the exchange process is the carboxylate-shifted form, which is probably stabilized by methanol coordination. Carboxylate shifts of this kind have been previously delineated, and the present data require such a shift for meaningful interpretation. Monomer/dimer equilibria have also been observed in some diiron(II) compounds and the consequences of such an equilibrium are manifest in the kinetics. 

Figure 6 The carboxylate shift mechanism proposed for the two-electron-one-proton transition from MMOHox to MMOH d (adapted from Lovell et al., 2001). ...

How this vitamin enters the process of blood formation is still entirely unknown. More can be said, however, about its biochemical action It is a component of the coenzyme to carboxylation reactions. The Bi2-coenzyme in which the cyanide group is replaced by adenine is instrumental in carboxyl shifts within molecules. One of these reactions is the rearrangement of methylmalonate to succinate (formulas in Chapt. XII-5), important in the degradation of several branched-chain fatty acids. 

Bi2 Cobalamin VI-6 In carboxyl shifts (in bound form) Pernicious anemia 0.001 mg... 

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