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Cobalamins ligand substitution reactions

Thusius has studied the kinetics of cobalamin-ligand substitution reactions trans to the pendant 5,6-dimethylbenzimidazole ligand (Eqn. 59) for a variety of inorganic ligands including thiocyanate, azide, iodide and bromide ions [103]. [Pg.452]

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced... [Pg.35]

Cobalamin and Cobaloxime Substitution Reactions.— The substitution kinetics of vitamin Biaa with 8-azaguanine and 6-mercaptopurine have been investigated in aqueous media at 25 °C and ionic strength 0.1 M. The complex pH dependences are interpreted in terms of reactions of the base on aquocobalamin with protonated and deprotonated forms of the ligands. Rate constants are listed in Table 7 and are consistent with a dissociative interchange mechanism. A similar mechanism is likely in the substitution reaction of cyanocobalamin with [Fe(CN)e] . Both base on and base off forms of the coenzyme are reactive. [Pg.337]

Catalytic hydrogenation with platinum liberates the hydrocarbon from methylcobalamin (57) and from alkyl-Co-DMG complexes (161), but not from pentacyanides with primary alkyl, vinyl, or benzyl ligands, though the cr-allyl complex yields propylene (109). Sodium sand gives mixtures of hydrocarbons with the alkyl-Co-salen complexes (64). Dithioerythritol will liberate methane from a variety of methyl complexes [cobalamin, DMG, DMG-BF2, G, DPG, CHD, salen, and (DO)(DOH)pn] (156), as will 1,4-butanedithiol from the DMG complex (157), and certain unspecified thiols will reduce DMG complexes with substituted alkyl ligands (e.g., C0-CH2COOH ->CH3C00H) (163, 164). Reaction with thiols can also lead to the formation of thioethers (see Section C,3). [Pg.432]

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... [Pg.415]

The contributions from these terms frequently cannot be separated with the result that only the composite term k is known. One example for which the separation of these terms was possible, concerns the complex-formation reaction of aqua-cobalamin (vitamin Bn). Here the usually inert Co(III) center is labilized by the corrin ring, which induces a dissociative substitution mode. From the non-linear dependence of the observed pseudo-first-order rate constant on the entering ligand concentration for the reaction shown in Eq. (1.5), the precursor formation constant and rate-determining interchange constant can be determined, as can their pressure dependences. [Pg.14]


See other pages where Cobalamins ligand substitution reactions is mentioned: [Pg.52]    [Pg.128]    [Pg.638]    [Pg.303]    [Pg.404]    [Pg.638]    [Pg.404]    [Pg.36]    [Pg.124]    [Pg.451]    [Pg.6783]    [Pg.522]    [Pg.451]    [Pg.384]    [Pg.423]    [Pg.111]    [Pg.126]    [Pg.687]    [Pg.168]    [Pg.393]    [Pg.396]   
See also in sourсe #XX -- [ Pg.406 ]

See also in sourсe #XX -- [ Pg.406 ]




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