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Photochemical Spin Dynamics of the Vitamin B Derivative, Methylcobalamin. | LitMetric

AI Article Synopsis

  • Vitamin B derivatives are cobalt compounds crucial for metabolic processes and protection against harmful light, with two main types being MeCbl and AdoCbl.
  • Research indicates that the upper axial ligand only slightly alters the photochemical behavior of these compounds, and they primarily generate singlet radical pairs during photolysis.
  • Differences in the recombination rates and spin dynamics between MeCbl and AdoCbl highlight the significant role of their surrounding proteins in determining their unique biochemical properties.

Article Abstract

Derivatives of vitamin B are six-coordinate cobalt corrinoids found in humans, other animals, and microorganisms. By acting as enzymatic cofactors and photoreceptor chromophores, they serve vital metabolic and photoprotective functions. Depending on the context, the chemical mechanisms of the biologically active derivatives of B-methylcobalamin (MeCbl) and 5'-deoxyadenosylcobalamin (AdoCbl)-can be very different from one another. The extent to which this chemistry is tuned by the upper axial ligand, however, is not yet clear. Here, we have used a combination of time-resolved Fourier transform-electron paramagnetic resonance (FT-EPR), magnetic field effect experiments, and spin dynamic simulations to reveal that the upper axial ligand alone only results in relatively minor changes to the photochemical spin dynamics of B. By studying the photolysis of MeCbl, we find that, similar to AdoCbl, the initial (or "geminate") radical pairs (RPs) are born predominantly in the singlet spin state and thus originate from singlet excited-state precursors. This is in contrast to the triplet RPs and precursors proposed previously. Unlike AdoCbl, the extent of geminate recombination is limited following MeCbl photolysis, resulting in significant distortions to the FT-EPR signal caused by polarization from spin-correlated methyl-methyl radical "f-pairs" formed following rapid diffusion. Despite the photophysical mechanism that precedes photolysis of MeCbl showing wavelength dependence, the subsequent spin dynamics appear to be largely independent of excitation wavelength, again similar to AdoCbl. Our data finally provide clarity to what in the literature to date has been a confused and contradictory picture. We conclude that, although the upper axial position of MeCbl and AdoCbl does impact their reactivity to some extent, the remarkable biochemical diversity of these fascinating molecules is most likely a result of tuning by their protein environment.

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Source
http://dx.doi.org/10.1021/acs.jpcb.9b01969DOI Listing

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