A study of the cage mechanism for the homolytic cleavage of the cobalt-carbon bond in coenzyme B12 by varying the solvent viscosity

Gerards, Leonard E.H.; Bulthuis, H.; Bolster, M. W. G. De; Balt, S.

doi:10.1016/s0020-1693(00)80230-3

Cited by 19 publications

(16 citation statements)

References 32 publications

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“…Previous studies have established that homolytic Co−C bond dissociation rates for base-on cobalamins are much higher than for the corresponding base-off species, hence the relationship between k 0 and the base-on dissociation limit is given by eq 9 Measurements of k homol and its temperature dependence have previously been reported for Ado-B 12 under limiting trapping conditions in ethylene glycol, 3a,c in mixed water/glycol, and in aqueous pH 4.3 solution, as well as in aqueous pH 7.0 solution without added radical trap 3b…”

Section: Resultsmentioning

confidence: 99%

“…The Co−C bond dissociation energy of free coenzyme B 12 has been estimated to be in the range of 26 to 31 kcal/mol. − Under enzymatic reaction conditions the rate of Co−C bond dissociation is enhanced by a factor up to 10 10 compared with that of the free coenzyme. This Co−C bond-weakening has been attributed to steric effects associated with enzyme-induced conformational distortion of the corrin ring, although other interpretations are not precluded.…”

Section: Introductionmentioning

confidence: 99%

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Dealkylation of Coenzyme B₁₂ and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Jensen

Halpern

1999

J. Am. Chem. Soc.

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Rates and mechanisms of dealkylations of coenzyme B12, Ado-B12, and of five related organocobalamin compounds, including 2‘,5‘-dideoxyadenosyl, 3‘,5‘-dideoxyadenosyl, 2‘,3‘,5‘-trideoxyadenosyl, 1,5-dideoxyribofuranosyl, and tetrahydrofurfuryl complexes (2‘dAdo-B12, 3‘dAdo-B12, 2‘,3‘ddAdo-B12, 1dRF-B12, and THFF-B12, respectively), were determined in acidic solutions. In each case, competitive homolytic and acid-induced hydrolytic cobalt−carbon bond decomposition pathways were identified. Two mechanisms were observed for Co−C bond hydrolysis: the first, involving initial depurination followed by elimination from an organometallic intermediate, predominates for 2‘dAdo-B12 and 2‘,3‘-ddAdoB12; the second path, involving ring-opening protonation at the ribofuranosyl oxygen, analogous to hydrolyses of simple β-hydroxy and β-alkoxy complexes, predominates for the other four complexes. The rates of both hydrolysis pathways exhibited a marked dependence on the ligand functional groups. Ado-B12, the most substituted and most stable of the complexes, decomposes nearly 10 000-fold more slowly than the least stable, unsubstituted THFF-B12 complex. Systematic variation of the hydroxy and adenine substituents on the furanosyl ring afforded insights into the roles of these substituents in effecting this large stabilization toward hydrolysis. Because of the extreme hydrolytic stability of the coenzyme, biologically relevant homolytic dissociation of 5‘-deoxyadenosyl radical is competitive with hydrolysis over a wide pH range where the unprotonated, base-on form is kinetically dominant. Determination of the pH dependence of the dealkylation rates of Ado-B12, 2‘dAdo-B12, and 3‘dAdo-B12 afforded quantification of the competition between homolytic and hydrolytic paths. In contrast to hydrolysis, the limiting homolytic Co−C bond dissociation rate was found to be insensitive to hydroxy substitution. Finally, broader issues relevant to organocobalamin chemistry and also bearing on earlier observations, are considered, among them protonation equilibria and dealkylation kinetics of organocobalamins, and fitting procedures for axial base dissociation equilibria.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Dealkylation of Coenzyme B₁₂ and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Jensen

Halpern

1999

J. Am. Chem. Soc.

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“…This statement is shown by eq 4: the second term on the right-hand side contains a viscosity dependence (from the Stokes−Einstein equation: k dP ∝ D ∝ 1/η) such that k cP / k dP becomes much smaller than one as the viscosity approaches zero. Thus at zero viscosity 1/ Φ obsd will equal 1/φ pair . , A more explicit relationship of k dP to viscosity can also be used: k dP = (η° k dP °/η) = k dP /η rel , where η rel = η/η° and the superscript ° indicates the value of the parameter at a reference viscosity (taken as the solution without viscogen present in this study) 13a. Equations 5 and 6 are thus obtained…”

Section: Resultsmentioning

confidence: 99%

“…For example, cage effects are necessary to explain magnetic isotope , and chemically induced dynamic nuclear polarization (CIDNP) effects, rate−viscosity correlations, variations in products and yields as a function of medium, and variations in quantum yields as a function of medium . Examples of important reactions where cage effects are necessary to explain the kinetics include the initiation, propagation, and termination steps of radical polymerization reactions; the reactions of coenzyme B 12 12 and its model complexes; the reactions of hemes with O 2 ; and various electron-transfer reactions . New observations of cage effects and their impact on reactivity are reported regularly.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Radical Size and Mass on the Cage Recombination Efficiency of Photochemically Generated Radical Cage Pairs

et al. 1998

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This study explored the effect of radical size, chain length, and mass on the radical cage effect. Radical cage pairs of the type [(RCp)(CO)3M•, •M(CO)3(CpR)] (M = Mo or W; CpR = various substituted cyclopentadienyl ligands) were generated by photolysis (λ = 540 nm) of the metal−metal bonds in (RCp)2M2(CO)6. The cage recombination efficiencies (denoted as F cP) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl4 (a metal-radical trap) as a function of solvent system viscosity. For the series of molecules (R3SiOCH2CH2Cp)2Mo2(CO)6 (R = Me, i-Pr, n-Pr, n-Hx), the F cP values were linearly proportional to mass 1/ 2 /radius2, in agreement with the predictions of Noyes' cage effect theory. It is also demonstrated that the difference in the cage recombination efficiencies between the [(MeCp)(CO)3Mo•, •Mo(CO)3(CpMe)] and [(MeCp)(CO)3W•, •W(CO)3(CpMe)] cage pairs cannot be ascribed to the different masses of the radicals. Rather, the difference is shown to be attributable to differences in the metal−metal bond energies or to differences in the spin−orbit coupling. In another comparison, F cP at any viscosity for [(MeCp)(CO)3Mo•, •Mo(CO)3(CpMe)] was shown to be identical to that of [Cp*(CO)3Mo•, •Mo(CO)3Cp*] (Cp* = η5-C5Me5) in tetrahydrofuran (THF)/tetraglyme solution. Rotation of the MeCp ring is fast compared to the time scale of diffusive separation (k dP) and radical recombination (k cP), and hence the effective volumes of the radicals in the solvent cage are nearly identical, which leads to similar F cP values.

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“…On the other hand, it has been pointed out, quite appropriately, that in order to use photolysis as a model or substitute for thermolysis, the photolysis mechanism must be well understood. − It has long been known that photolysis of alkylcobalamins under anaerobic conditions results in the homolytic cleavage of the carbon−cobalt bond to form an alkyl radical and a cob(II)alamin radical . Photoinduced homolysis occurs with substantial quantum yields, ranging from 0.1 to 0.5 for 6-coordinate base-on cobalamins. − Continuous wave measurements with excitation at 442 nm determined photolysis quantum yields of φ = 0.20 ± 0.03 for adenosylcobalamin and φ = 0.35 ± 0.03 for methylcobalamin .…”

Section: Introductionmentioning

confidence: 99%

Time-Resolved Spectroscopic Studies of B₁₂ Coenzymes: The Photolysis of Methylcobalamin Is Wavelength Dependent

et al. 1999

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Femtosecond to nanosecond transient absorption spectroscopy has been used to investigate the primary photochemistry of the B 12 coenzymes, methylcobalamin and 5′-deoxyadenosylcobalamin. Photolysis at excitation wavelengths in the near UV (400 nm) and visible (520-530 nm) are compared. Measurements were performed with femtosecond time resolution covering time delays of up to 9 ns. The photochemistry of methylcobalamin is found to depend strongly on excitation wavelength, while the photochemistry of adenosylcobalamin is essentially wavelength independent over the range studied. Excitation of methylcobalamin at 400 nm results in a partitioning between prompt bond homolysis and formation of a metastable cob(III)-alamin photoproduct as reported earlier [Walker, L. A., II; Jarrett, J. T.; Anderson, N. A.; Pullen, S. H.; Matthews, R. G.; Sension, R. J. J. Am. Chem. Soc. 1998, 120, 3597-3603]. Excitation of methylcobalamin at 520 nm in the visible R -band results only in formation of the metastable cob(III)alamin photoproduct. No prompt bond homolysis is observed. The metastable photoproduct partitions between formation of cob(II)-alamin (14 ( 5%) and recovery of the methylcobalamin starting material (86 ( 5%) on a 1.0 ( 0.1 ns time scale. The quantum yield for bond homolysis in methylcobalamin is determined by the wavelength-dependent partitioning between prompt homolysis and formation of the metastable photoproduct and by the partitioning of the metastable photoproduct between bond homolysis and ground-state recovery. In contrast, excitation of adenosylcobalamin at both 400 and 520 nm results in the development of a difference spectrum characteristic of the formation of cob(II)alamin on a picosecond time scale. The quantum yield for bond homolysis in this case is determined primarily by the competition between geminate recombination and diffusion to form solventseparated radical pairs. The caging fraction for adenosylcobalamin in aqueous solution at room temperature is 0.71 ( 0.05.

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A study of the cage mechanism for the homolytic cleavage of the cobalt-carbon bond in coenzyme B12 by varying the solvent viscosity

Cited by 19 publications

References 32 publications

Dealkylation of Coenzyme B₁₂ and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Dealkylation of Coenzyme B₁₂ and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

The Effect of Radical Size and Mass on the Cage Recombination Efficiency of Photochemically Generated Radical Cage Pairs

Time-Resolved Spectroscopic Studies of B₁₂ Coenzymes: The Photolysis of Methylcobalamin Is Wavelength Dependent

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A study of the cage mechanism for the homolytic cleavage of the cobalt-carbon bond in coenzyme B12 by varying the solvent viscosity

Cited by 19 publications

References 32 publications

Dealkylation of Coenzyme B12 and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Dealkylation of Coenzyme B12 and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

The Effect of Radical Size and Mass on the Cage Recombination Efficiency of Photochemically Generated Radical Cage Pairs

Time-Resolved Spectroscopic Studies of B12 Coenzymes: The Photolysis of Methylcobalamin Is Wavelength Dependent

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Dealkylation of Coenzyme B₁₂ and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Dealkylation of Coenzyme B₁₂ and Related Organocobalamins: Ligand Structural Effects on Rates and Mechanisms of Hydrolysis

Time-Resolved Spectroscopic Studies of B₁₂ Coenzymes: The Photolysis of Methylcobalamin Is Wavelength Dependent