Using Ultrafast XANES and FDMNES Simulations to Examine the Photochemistry of Adenosylcobalamin

Adenosylcobalamin (AdoCbl) is a member of the cobalamin family, a class of molecules that feature a cobalt center with a corrin ring and upper and lower ligands.  Adenosylcobalamin has an adenosyl upper ligand and dimethylbenzimidazole lower ligand.  The photochemistry of AdoCbl is important due to its central role in the CarH photoreceptor, found in bacterial species such as M. xanthus.  Time-resolved X-ray absorption near-edge structure (XANES) is a method of examining the excited states of molecules, with the capability of studying structural change.  The use of polarization selection allows for the deconvolution of structural change in molecule-fixed directions.  Extensive time-resolved XANES data have been collected on AdoCbl in varying solvents.  In order to assign specific spectral changes to structural changes in the AdoCbl, the XANES spectra must be simulated for comparison with the experimental difference spectra.  We use a method called the finite difference method near-edge structure (FDMNES) to simulate the ground and excited state spectra.  FDMNES allows for systematic examination of the structural manipulations needed to reproduce the experimental XANES data.  These simulations permit the identification of sequential structural changes.