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.

The Chitosan as Dietary Fiber: An in vitro Comparative Study of Interactions with Drug and Nutritional Substances

“…Vitamin B12 belong to the cobalamins, a class of octahedral Co(III) complexes which contain a planar framework called a corrin, with the metal center coordinated in the equatorial position by the four corrin nitrogens. Her energies of excited states are sensitive to the nature of the ligands of center coordinated and are influenced by water content of the surrounding environment (Solheim et al, 2011). These characteristics may be the reason for the more significant increase of the spectral properties of the vitamin B12, with increasing concentration of chitosan (lower water content), compared with the vitamin B2.…”

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].…”

Theoretical investigations of photophysics and spectroscopic properties in B12 cofactors.

“…The interpretation of the simulated CD and MCD spectra implied the existence of a manifold of electronic excitations in the α/β part of the CNCbl absorption spectrum. 140 The data shown with correlated ab initio data (presented further in this paper) allows for a reliable justification of the suitability of a particular exchange-correlation functional to study electronic properties of CNCbl.…”

“…Moreover, it has been determined that multiple electronic transitions contribute to the α/β band observed for both cobalamins. 140 The above discussion demonstrates that the nature of the S 1 state of MeCbl cofactor remains elusive, since the existing experimental and theoretical predictions do not yield a consistent picture regarding its electronic structure. Clearly, a…”

“…Whereas the ground-state structural properties were essentially insensitive with respect to the functional used, in contrast, the excited states were very sensitive to the choice of XC functional. Recent theoretical studies 140,161,163,271 on cobalamins support the use of gradient-corrected functionals, such as BP86, when studying excited states of these systems compared to, for instance, the use of hybrid functionals.…”