Axial Solvent Coordination in "Base-Off" Cob(II)alamin and Related Co(II)-Corrinates Revealed by 2D-EPR
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- 作者：Sabine Van Doorslaer ; Gunnar Jeschke ; Boris Epel ; Daniella Goldfarb ; Rü ; diger-Albert Eichel ; Bernhard Krä ; utler ; and Arthur Schweiger
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：May 14, 2003
- 年：2003
- 卷：125
- 期：19
- 页码：5915 - 5927
- 全文大小：274K
- 年卷期：v.125,no.19(May 14, 2003)
- ISSN：1520-5126
文摘
Detailed information on the structure of cobalt(II) corrinates is of interest in the context of studieson the coenzyme B12 catalyzed enzymatic reactions, where cob(II)alamin has been identified as a reactionintermediate. Cob(II)ester (heptamethyl cobyrinate perchlorate) is found to be soluble in both polar andnonpolar solvents and is therefore very suitable to study solvent effects on Co(II) corrinates. In the literature,Co(II) corrinates in solution are often addressed as four-coordinated Co(II) corrins. However, using acombination of continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) and pulse ENDOR(electron nuclear double resonance) at different microwave frequencies we clearly prove axial ligation forCob(II)ester and the base-off form of cob(II)alamin (B12r) in different solvents. This goal is achieved by theanalysis of the g values, and the hyperfine couplings of cobalt, some corrin nitrogens and hydrogens, andsolvent protons. These parameters are shown to be very sensitive to changes in the solvent ligation. Densityfunctional computations (DFT) facilitate largely the interpretation of the EPR data. In the CW-EPR spectrumof Cob(II)ester in methanol, a second component appears below 100 K. Different cooling experimentssuggest that this observation is related to the phase transition of methanol from the 
-phase to the glassystate. A detailed analysis of the EPR parameters indicates that this transition induces a change from afive-coordinated (above 100 K) to a six-coordinated (below 100 K) Co(II) corrin. In a CH3OH:H2O mixturethe phase-transition properties alter and only the five-coordinated form is detected for Cob(II)ester and forbase-off B12r at all temperatures. Our study thus shows that the characteristics of the solvent can have alarge influence on the structure of Co(II) corrinates and that comparison with the protein-embedded cofactorrequires some caution. Finally, the spectral similarities between Cob(II)ester and base-off B12r prove theanalogies in their electronic structure.
-phase to the glassystate. A detailed analysis of the EPR parameters indicates that this transition induces a change from afive-coordinated (above 100 K) to a six-coordinated (below 100 K) Co(II) corrin. In a CH3OH:H2O mixturethe phase-transition properties alter and only the five-coordinated form is detected for Cob(II)ester and forbase-off B12r at all temperatures. Our study thus shows that the characteristics of the solvent can have alarge influence on the structure of Co(II) corrinates and that comparison with the protein-embedded cofactorrequires some caution. Finally, the spectral similarities between Cob(II)ester and base-off B12r prove theanalogies in their electronic structure.