Electronic and Structural Properties of Low-lying Excited States of Vitamin B₁₂

Abstract

Time-dependent density functional theory (TD-DFT) has been applied to explore electronically excited states of vitamin B(12) (cyanocobalamin or CNCbl). To explain why the Co-C bond in CNCbl does not undergo photodissociation under conditions of simple photon excitation, electronically excited states have been computed along the Co-C(CN) stretched coordinate. It was found that the repulsive (3)(σ(Co-C) → σ*(Co-C)) triplet state drops in energy as the Co-C(CN) bond lengthens, but it does not become dissociative. Low-lying excited states were also computed as function of two axial bond lengths. Two energy minima have been located on the S(1)/CNCbl, as well as T(1)/CNCbl, surfaces. The full geometry optimization was carried out for each minimum and electronic properties associated with each optimized structure were analyzed in details. One minimum was described as excitation having mixed ππ*/MLCT (metal-to-ligand charge transfer) character, while the second as ligand-to-metal charge transfer (LMCT) transition. Neither of them, however, can be viewed as pure MLCT or LMCT transitions since additional excitation to or from σ-bonds (SB) of N-Co-C unit have also noticeable contributions. Inclusion of solvent altered the character of one of the excitations from ππ*/MLCT/SBLCT to ππ*/LMCT/LSBCT-type, and therefore, both of them gained significant contribution from LMCT/LSBCT transition. Finally, the nature of S(1) electronic state has been comparatively analyzed in CNCbl and MeCbl cobalamins.

Excited electronic states and internal conversion in cyanocobalamin

“…4) using TD-DFT methods with an implicit COSMO solvent model, the BP86 functional and the TZVPP basis set [30,35,37,38]. Excitation of CNCbl results in the initial Franck-Condon population of any of a number of states dominated by p!p* excitation of the corrin ring.…”

“…A more recent calculation exploring the S 1 potential energy surface has called this into question, suggesting instead that a ligand field state plays the crucial role in controlling the photodissociation [29]. In the non-alkyl cobalamins, the dissociation channel is much higher in energy and shifted far from the Franck-Condon region and thus is a non-relevant excited state pathway [30,[35][36][37][38].…”

The Nature of the Co-C Bond Cleavage Processes in Methylcob(II)Alamin and Adenosylcob(III)Alamin

“…Consequently, various properties have been studied, including electron densities and reduction potentials determination [69,76,88,91,96], the comparison of DFT and CASSCF electronic structure data of truncated models of methylcobalamin species [70,71,74,81,85], the infl uence of various factors on the Co-C bond forming and cleavage in methyl-and 5-adenosylcobalamin models [68,[72][73][74][75][76]79,80,83,84,89,90,92,94,97,99,100], and excited states, spectroscopy and photo-dissociation analysis of vitamin B 12 species [78,82,86,87,93,95,98]. We used DFT calculations on Me-Cbl models to calculate total energy as a function of Co-C bond distance as the bond is stretched [39].…”

CHAPTER 12. The Chemistry of Cobalamins

“…Cobalamins are organometallic compounds belonging to the corrin family and have the characteristic feature of a tetrapyrrole ring (corrin) in their moiety (Banerjee 1997;Jensen and Ryde 2009;Lodowski et al 2011). However, what makes cobalamins different is the cobalt atom they have in their molecule.…”

“…However, what makes cobalamins different is the cobalt atom they have in their molecule. This rare in nature cobalt-carbon (Co-C) bond gives cobalamins unique chemical properties, rendering them suitable molecules to act as cofactors (see Key Facts) in various enzymatic systems in mammals (Lodowski et al 2011). Recent advances in existing knowledge regarding the absorption and transport of cobalamins in human body are reviewed in details elsewhere (Quadros 2009).…”