Thermodynamic and Kinetic Data for the Base‐On/Base‐Off Equilibration of Alkylcobalamins

Abstract

The base-on/base-off equilibration of a series of alkylcobalamins (XCbl) was studied as a function of the electronic properties of X. A square scheme was developed to incorporate all species that participate in this equilibration. The equilibrium between five-and six-coordinate species was studied for several alkylcobalamins (protonated base-off) and alkylcobinamides as a function of pressure by using UV/Vis spectroscopy. In addition, the kinetics of the acid-induced base-

Kinetic and thermodynamic studies on the cyanation reactions and base-on/base-off equilibria of alkyl-13-epicobalamins

“…In this work, the fascinating influence of a single cobalt-carbon bond in tuning the reactivity of the Co() center was clearly demonstrated. [25][26][27][28][29] We found that the reaction of XCbl with CN Ϫ , where X = NCCH 2 , CF 3 and CN Ϫ , proceeded via the formation of an intermediate X(CN)Cbl, which subsequently reacts, in the presence of CN Ϫ , to give (CN) 2 Cbl Ϫ . 26 We also found that the mechanism of substituting the axial DMBz trans to the X group in a series of XCbl by CN Ϫ occurred via a dissociative mechanism.…”

“…We 27 have recently shown that evidence for the suggested dissociative mechanism for the reaction of XCbl with CN Ϫ could be obtained from the direct measurement of the dechelation rate constant of DMBz (k 1 ) through acidification of XCbl to produce the protonated base-off species. However, in the present study (X-13-epiCbl), pK base-off values are less than those reported for XCbl, viz.…”

Trans and cis influences and effects in cobalamins and in their simple models

“…Depending on the nature of X, the species represented on the left side can be apparently six-coordinate (as shown in Scheme 2), but it can be also apparently five-coordinate or a mixture of six-and five-coordinate. In any case, K represents the apparent acid dissociation constant of the species on the left side in Scheme 2 [42], independently of the coordination. When the trans X ligand varies from H 2 O to NO, an increase of about 6 orders of magnitude in K is observed.…”

X-ray Structural Characterization of Imidazolylcobalamin and Histidinylcobalamin: Cobalamin Models for Aquacobalamin Bound to the B12 Transporter Protein Transcobalamin

“…10,[25][26][27] In protein-free Cbls, α-DMB is difficult to displace from the α-axial site but can be displaced by cyanide, 28 by imidazole-type ligands, 29,30 and by protonation of the basic N of the imidazole moiety. 31 In comparison, substitution of the β-axial ligand of aquacobalamin (H 2 OCbl + , X = OH 2…”