title: Electronically excited states of vitamin B12 and methylcobalamin: theoretical analysis of absorption, CD, and MCD data.
abstract: Linear and quadratic response time-dependent density functional theory (TD-DFT) has been applied to investigate absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectra of cyanocobalamin (CNCbl) and methylcobalamin (MeCbl). Although electronically excited states of both cobalamins have been probed by applying different experimental techniques, their exact nature remains poorly understood from an electronic structure point of view. Recent theoretical studies have revealed a lot of relevant information about their properties but also left some unresolved issues related to the nature of individual transitions. In this contribution, not only Abs but also CD and MCD spectra of both cobalamins were computed for direct comparison with experiment. The results were evaluated with respect to the choice of exchange-correlation functional, basis set, and the environment (gas phase or solvent) used in the calculation. Taking into account the complexity of the CNCbl and MeCbl systems, reliable agreement between theory and experiment was achieved based on calculations employing the BP86 functional, particularly for the low-energy α/β bands. This spectral range has been traditionally interpreted as a vibrational progression associated with a single electronic excitation, but according to the present analysis for both cobalamins, these bands are best interpreted as consisting of multiple electronic transitions.
authors.affiliation: Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø, 9037 Tromsø, Norway.
authors.forename: Harald
authors.initials: H
authors.lastname: Solheim
authors.forename: Karina
authors.initials: K
authors.lastname: Kornobis
authors.forename: Kenneth
authors.initials: K
authors.lastname: Ruud
authors.forename: Pawel M
authors.initials: PM
authors.lastname: Kozlowski
mesh_terms: D000042:Absorption; D002942:Circular Dichroism / Q000379:methods; D004583:Electrons; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory; D014805:Vitamin B 12 / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: C019476:mecobalamin; D014805:Vitamin B 12