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Photoinduced bond homolysis of B12 coenzymes. An FT-EPR study

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Abstract

A Fourier Transform Electron Paramagnetic Resonance (FT-EPR) study was made of free radicals produced by photoinduced homolytic cleavage of the Co—C bond in methyl- and 5′-adenosylcobalamine (B12 coenzymes) and R(4-t-butyl-pyridyl)cobaloximes, R = methyl or ethyl. Spectra of methyl and adenosyl free radicals generated by the cobalamines show Chemically Induced Dynamic Electron Polarization (CIDEP) produced in precursor radical pairs. The polarization pattern can be accounted for in terms of bond cleavage via a singlet excited state of the cobalamines. In the case of methylcobalamine the polarization pattern is wavelength dependent confirming earlier findings that bond cleavage occurs via two reaction channels. Spectra of the methyl and ethyl radicals given by the cobaloximes show a remarkably strong dependence on solvent and the identity of the axial ligand trans to the leaving alkyl group. This illustrates that the character of the excited state involved in the bond cleavage reaction is strongly dependent on axial ligation of the cobalt ion.

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Authors
  1. Alejandro P. Bussandri
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  2. Cecilia W. Kiarie
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  3. Hans Van Willigen
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Bussandri, A.P., Kiarie, C.W. & Van Willigen, H. Photoinduced bond homolysis of B12 coenzymes. An FT-EPR study. Research on Chemical Intermediates 28, 697–710 (2002). https://doi.org/10.1163/15685670260469366

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