Source:http://linkedlifedata.com/resource/pubmed/id/11578192
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| Predicate | Object |
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| rdf:type | |
| lifeskim:mentions | |
| pubmed:issue |
21
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| pubmed:dateCreated |
2001-10-1
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| pubmed:abstractText |
Ligand substitution equilibria of different alkylcobalamins (RCbl, R = Me, CH(2)Br, CH(2)CF(3), CHF(2), CF(3)) with cyanide have been studied. It was found that CN(-) first substitutes the 5,6-dimethylbenzimidazole (Bzm) moiety in the alpha-position, followed by substitution of the alkyl group in the beta-position trans to Bzm. The formation constants K(CN) for the 1:1 cyanide adducts (R(CN)Cbl) were found to be 0.38 +/- 0.03, 0.43 +/- 0.03, and 123 +/- 9 M(-1) for R = Me, CH(2)Br, and CF(3), respectively. In the case of R = CH(2)CF(3), the 1:1 adduct decomposes in the dark with CN(-) to give (CN)(2)Cbl. The unfavorable formation constants for R = Me and CH(2)Br indicate the requirement of very high cyanide concentrations to produce the 1:1 complex, which cause the kinetics of the displacement of Bzm to be too fast to follow kinetically. The kinetics of the displacement of Bzm by CN(-) could be followed for R = CH(2)CF(3) and CF(3) to form CF(3)CH(2)(CN)Cbl and CF(3)(CN)Cbl, respectively, in the rate-determining step. Both reactions show saturation kinetics at high cyanide concentration, and the limiting rate constants are characterized by the activation parameters: R = CH(2)CF(3), DeltaH = 71 +/- 1 kJ mol(-1), DeltaS = -25 +/- 4 J K(-1) mol(-1), and DeltaV = +8.9 +/- 1.0 cm(3) mol(-1); R = CF(3), DeltaH = 77 +/- 3 kJ mol(-1), DeltaS = +44 +/- 11 J K(-1) mol(-1), and DeltaV = +14.8 +/- 0.8 cm(3) mol(-1), respectively. These parameters are interpreted in terms of an I(d) and D mechanism for R = CH(2)CF(3) and CF(3), respectively. The results of the study enable the formulation of a general mechanism that can account for the substitution behavior of all investigated alkylcobalamins including coenzyme B(12).
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| pubmed:grant | |
| pubmed:language |
eng
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| pubmed:journal | |
| pubmed:citationSubset |
IM
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| pubmed:chemical | |
| pubmed:status |
MEDLINE
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| pubmed:month |
Oct
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| pubmed:issn |
0020-1669
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| pubmed:author | |
| pubmed:issnType |
Print
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| pubmed:day |
8
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| pubmed:volume |
40
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| pubmed:owner |
NLM
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| pubmed:authorsComplete |
Y
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| pubmed:pagination |
5440-7
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| pubmed:dateRevised |
2007-11-14
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| pubmed:meshHeading | |
| pubmed:year |
2001
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| pubmed:articleTitle |
Equilibrium and kinetic studies on the reactions of alkylcobalamins with cyanide.
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| pubmed:affiliation |
Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
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| pubmed:publicationType |
Journal Article,
Research Support, U.S. Gov't, P.H.S.,
Research Support, Non-U.S. Gov't
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