1 research outputs found
Cob(I)alamin: Insight Into the Nature of Electronically Excited States Elucidated via Quantum Chemical Computations and Analysis of Absorption, CD and MCD Data
The nature of electronically excited states of the super-reduced
form of vitamin B<sub>12</sub> (i.e., cobÂ(I)Âalamin or B<sub>12s</sub>), a ubiquitous B<sub>12</sub> intermediate, was investigated by performing
quantum-chemical calculations within the time-dependent density functional
theory (TD-DFT) framework and by establishing their correspondence
to experimental data. Using response theory, the electronic absorption
(Abs), circular dichroism (CD) and magnetic CD (MCD) spectra of cobÂ(I)Âalamin
were simulated and directly compared with experiment. Several issues
have been taken into considerations while performing the TD-DFT calculations,
such as strong dependence on the applied exchange-correlation (XC)
functional or structural simplification imposed on the cobÂ(I)Âalamin.
In addition, the low-lying transitions were also validated by performing
CASSCF/MC-XQDPT2 calculations. By comparing computational results
with existing experimental data a new level of understanding of electronic
excitations has been established at the molecular level. The present
study extends and confirms conclusions reached for other cobalamins.
In particular, the better performance of the BP86 functional, rather
than hybrid-type, was observed in terms of the excitations associated
with both Co d and corrin π localized transitions. In addition,
the lowest energy band was associated with multiple metal-to-ligand
charge transfer excitations as opposed to the commonly assumed view
of a single π → π* transition followed by vibrational
progression. Finally, the use of the full cobÂ(I)Âalamin structure,
instead of simplified molecular models, shed new light on the spectral
analyses of cobalamin systems and revealed new challenges of this
approach related to long-range charge transfer excitations involving
side chains