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…”

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…”

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

“……”

“……”

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

“…Importantly, elongation of the Co−OH 2 bond beyond 2.5 Å or complete removal of the lower ligand results in changes in the predicted Abs spectrum that are too dramatic to be consistent with our experimental data. Therefore, it is reasonable to conclude that, like free MeCbi + and AdoCbi + , the Me−Co 3+ CFeSP maintains a water ligand in the lower axial position; however the Co 3+ −O bond length is slightly lengthened for the protein-bound cofactor 8 TD-DFT/B3LYP predicted Abs spectra for QM/MM optimized MeCbi + models with lower axial ligand bond lengths fixed at values ranging from 1.800 to 2.700 Å.…”

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…”

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

“……”

“……”

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

“…Importantly, elongation of the Co−OH 2 bond beyond 2.5 Å or complete removal of the lower ligand results in changes in the predicted Abs spectrum that are too dramatic to be consistent with our experimental data. Therefore, it is reasonable to conclude that, like free MeCbi + and AdoCbi + , the Me−Co 3+ CFeSP maintains a water ligand in the lower axial position; however the Co 3+ −O bond length is slightly lengthened for the protein-bound cofactor 8 TD-DFT/B3LYP predicted Abs spectra for QM/MM optimized MeCbi + models with lower axial ligand bond lengths fixed at values ranging from 1.800 to 2.700 Å.…”

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…”

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

“……”

“……”

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

“…Importantly, elongation of the Co−OH 2 bond beyond 2.5 Å or complete removal of the lower ligand results in changes in the predicted Abs spectrum that are too dramatic to be consistent with our experimental data. Therefore, it is reasonable to conclude that, like free MeCbi + and AdoCbi + , the Me−Co 3+ CFeSP maintains a water ligand in the lower axial position; however the Co 3+ −O bond length is slightly lengthened for the protein-bound cofactor 8 TD-DFT/B3LYP predicted Abs spectra for QM/MM optimized MeCbi + models with lower axial ligand bond lengths fixed at values ranging from 1.800 to 2.700 Å.…”