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-12 (i.e., cob(I)alamin or B-12s), a ubiquitous B-12 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 cob(I)alamin 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 cob(I)alamin. 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 pi 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 pi -> pi* transition followed by vibrational progression. Finally, the use of the full cob(I)alamin 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.