Five-Coordination and Adduct Formation in CoIII−Corrinoids Dissecting Ligand Substitution into Its Component Steps

Abstract

The equilibria between the five-and six-coordinate species of alkylcobinamide and cobalamin (base-off) have been studied using high-pressure spectroscopic techniques. The

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

“…Values of the equilibrium constant { K = [six‐coordinate]/[five‐coordinate] as defined in Equation (1 )} as a function of pressure were calculated from the increase in absorbance at 490 nm using the relative optical density for the fully formed six‐coordinate and five‐coordinate species derived from the temperature‐dependence studies 5. The reaction volume, Δ V , was found to be −12.4 ± 1.0 and −12.5 ± 1.2 cm 3 mol −1 for the conversion of the five‐ to the six‐coordinate species in vinylcobinamide and MeCbl (base‐off), respectively 12. Their identical values underline the close parallel between Cbi’s and protonated Cbl’s, and their agreement with the theoretical value of −13.1 cm 3 mol −1 expected for the coordination of a water molecule to a five‐coordinate species confirm that conversion of the five‐ to the six‐coordinate form involves the coordination of one water molecule to the five‐coordinate form 12…”

“…We recently studied this equilibrium for both MeCbl (protonated base‐off) and vinylcobinamide,12 and found that increasing pressure displaces the observed equilibrium [Equation (1 )] from the five‐coordinate towards the six‐coordinate form for both complexes. A significant increase in the absorbance at ca.…”

“…Recently, the equilibrium between the five‐ and six‐coordinate base‐off forms of both methylcobalamin and vinylcobinamide was studied as a function of pressure, and it was found that this equilibrium is shifted towards the formation of the six‐coordinate species upon increasing pressure 12. This has led us to investigate the equilibrium between the five‐ and six‐coordinate species [shown by way of example for the protonated base‐off species 1 in Equation (1 )] for many of the available alkylcobalamins (protonated base‐off) and cobinamides, in order to investigate the influence of the group X on this equilibrium and to understand the mechanistic implications of its occurrence. …”

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[COIII(en)2(Me)H2O]2+: A Model for Coenzyme B12

“…This is expected, since the presence of CH 3 in the axial position exerts a strong trans influence and the coordination of the other axial ligand (H 2 O) is very weak. A similar observation was made in the case of alkylcobaloximes and alkylcobinamide COENZYME B 12 MODELS 555 [7,29,30]. In the case of cobinamide, increasing pressure of decreasing temperature shifts the equilibrium towards the 6-coordinate species.…”

“…It has been shown in the case of methylcobinamide that the reaction volume for this equilibrium is À 12.4 cm 3 mol À1 . This indicates that a H 2 O molecule is coordinated trans to the methyl group to form the 6-coordinate species [29].…”