TD-DFT insight into photodissociation of the Co-C bond in coenzyme B12.

Hui Liu,Pawel M. Kozlowski,Piotr Lodowski,Maria Jaworska,Karina Kornobis

doi:10.3389/fchem.2013.00041

Hui Liu, Pawel M. Kozlowski + Show 3 more

Open Access

https://doi.org/10.3389/fchem.2013.00041

Copy DOI

Abstract

Coenzyme B12 (AdoCbl) is one of the most biologically active forms of vitamin B12, and continues to be a topic of active research interest. The mechanism of Co-C bond cleavage in AdoCbl, and the corresponding enzymatic reactions are however, not well understood at the molecular level. In this work, time-dependent density functional theory (TD-DFT) has been applied to investigate the photodissociation of coenzyme B12. To reduce computational cost, while retaining the major spectroscopic features of AdoCbl, a truncated model based on ribosylcobalamin (RibCbl) was used to simulate Co-C photodissociation. Equilibrium geometries of RibCbl were obtained by optimization at the DFT/BP86/TZVP level of theory, and low-lying excited states were calculated by TD-DFT using the same functional and basis set. The calculated singlet states, and absorption spectra were simulated in both the gas phase, and water, using the polarizable continuum model (PCM). Both spectra were in reasonable agreement with experimental data, and potential energy curves based on vertical excitations were plotted to explore the nature of Co-C bond dissociation. It was found that a repulsive 3(σCo−C → σ*Co−C) triplet state became dissociative at large Co-C bond distance, similar to a previous observation for methylcobalamin (MeCbl). Furthermore, potential energy surfaces (PESs) obtained as a function of both Co-CRib and Co-NIm distances, identify the S1 state as a key intermediate generated during photoexcitation of RibCbl, attributed to a mixture of a metal-to-ligand charge transfer (MLCT) and a σ bonding-ligand charge transfer (SBLCT) states.

Highlights

The aim of this study is to provide the further insight into the photodissociation mechanism of the Co-C bond in the coenzyme B12 (AdoCbl, Figure 1) by time-dependent density functional theory (TD-DFT) computations
Molecular orbital (MO) energies and fragment contributions of RibCbl are collected in Table S5 for the gas phase, and Table S6 for water (PCM)
SUMMARY AND CONCLUSIONS The purpose of this study was to explore the mechanisms of Co-C bond scission in AdoCbl upon light exposure using the TD-DFT method

Summary

Introduction

Vitamin B12 derivatives (Figure 1) are a group of organometallic complexes that act as biologically active cofactors in many enzymatic reactions (Dolphin et al, 1982; Banerjee, 1997, 1999, 2001, 2003; Ludwig and Matthews, 1997; Kräutler et al, 1998; Marzilli, 1999; Toraya, 2000; Matthews, 2001; Banerjee and Ragsdale, 2003; Toraya, 2003; Brown, 2005; Randaccio et al, 2006, 2007) In addition to their pivotal roles in a variety of enzymatic processes, the B12 derivatives possess complex photophysical and photochemical properties (Endicott and Ferraudi, 1977; Endicott and Netzel, 1979; Rao and Symons, 1982; Chen and Chance, 1990, 1993; Sakaguchi et al, 1990; Chagovetz and Grissom, 1993; Grissom and Chagovetz, 1993; Lott et al, 1995; Natarajan and Grissom, 1996; Kruppa et al, 1997; Walker et al, 1998a,b; Shiang et al, 1999, 2006; Yoder et al, 2001; Cole et al, 2002; Sension et al, 2004, 2005a,b; Harris et al, 2007). The Co-R bond in cyannocobalimin (CNCbl) and non-alkylcobalamin analogs, such as azidocobalamin (N3Cbl) and aquocobalamin (H2OCbl+), are rather photostable (Shiang et al, 2006)

Objectives

Results

Conclusion