Mechanism of Methyl Transfer Reaction between CHCo(dmgBF)py and PPhNi(Triphos).

DFT calculations were performed for the methyl group transfer reaction between CHCo (dmgBF)py and PPhNi(Triphos). The reaction mechanism and its energetics were investigated. This reaction is relevant to the catalytic mechanism of the enzyme acetyl coenzyme A synthase.

Theoretical Investigation of Iridium Complex with Aggregation-Induced Emission Properties.

The mechanism of aggregation-induced emission (AIE) for the bis(1-(2,4-difluorophenyl)-1H-pyrazole)(2-(20-hydroxyphenyl)-2-oxazoline)iridium(III) complex, denoted as Ir(dfppz)(oz), was investigated with use DFT and the TD-DFT level of theory. The mechanism of radiationless deactivation of the triplet state was elucidated. Such a mechanism requires an additional, photophysical triplet channel of the internal conversion (IC) type, which is activated as a result of intramolecular motion deforming the structure of the oz ligand and distorting the iridium coordination sphere.

Mechanical Modulation of the Solid-State Luminescence of Tricarbonyl Rhenium(I) Complexes through the Interplay between Two Triplet Excited States.

Mechanoresponsive luminescence (MRL) materials promise smart devices for sensing, optoelectronics and security. We present here the first report on the MRL activity of two Re complexes, opening up new opportunities for applications in these fields. Both complexes exhibit marked solid-state luminescence enhancement (SLE).

Structure and electronic spectra of neutral and protonated forms of anticonvulsant drug lamotrigine.

In this work, the geometry, acid-base properties, pK, electronic spectra, and fluorescence spectrum of anticonvulsant drug lamotrigine (LTG) are investigated with the DFT/TD-DFT method and PCM solvent model. Calculated transition with the B3LYP functional at 295 nm corresponds to experimental absorption transition at 306 nm in water. In acidic conditions, the computed maximum transition occurs at 249 nm, comparing with experimental one at 270 nm.

Mechanism of the photo-induced activation of CoC bond in methylcobalamin-dependent methionine synthase.

Methylcobalamin (MeCbl)-dependent enzyme methionine synthase (MetH), plays a critical role in the catalysis of methyl group transfer from methyltetrahydrofolate (CH-Hfolate) to homocysteine. It often performs a side reaction to generate cob(II)alamin through photolysis of the organometallic CoC σ bond. A hybrid QM/MM method has been applied to explore the photochemistry of MeCbl-bound MetH.

Off to the Races: Comparison of Excited State Dynamics in Vitamin B Derivatives Hydroxocobalamin and Aquocobalamin.

Ultrafast time-resolved spectroscopy was used to study the photochemistry of hydroxocobalamin (HOCbl) and aquocobalamin (HOCbl) in solution. Spectroscopic measurements and TD-DFT simulations provide a consistent picture of the spectroscopy and photochemistry. Excitation of HOCbl results in formation of an excited state followed by rapid internal conversion to the ground state (0.

Photolytic Properties of Antivitamins B.

Antivitamins B represent an important class of vitamin B analogues that have gained recent interest in several research areas. In particular, 4-ethylphenylcobalamin (EtPhCbl) and phenylethynylcobalamin (PhEtyCbl) exemplify two such antivitamins B which have been characterized structurally and chemically. From a spectroscopic point of view, EtPhCbl is photolabile with a very low quantum yield of photoproducts, while PhEtyCbl is incredibly photostable.

Photodissociation of ethylphenylcobalamin antivitamin B.

Biologically active forms of cobalamins are crucial cofactors in biochemical reactions and these metabolites can be inhibited by their structurally similar analogues known as antivitamins B. Phenylethynylcobalamin (PhEtyCbl) or 4-ethylphenylcobalamin (EtPhCbl) exemplify recently synthesized and structurally characterized antivitamins B. Herein, DFT and TD-DFT studies of EtPhCbl are provided to explore its photochemical behavior, which may lead to design of arylcobalamins that can be used as therapeutic agents in light activated drug applications.

Mechanism of Co-C photodissociation in adenosylcobalamin.

A mechanism of Co-C bond photodissociation in the base-on form of adenosylcobalamin (AdoCbl) was investigated by time-dependent density functional theory (TD-DFT). The key mechanistic step involves singlet radical pair (RP) generation from the first electronically excited state (S1). To connect TD-DFT calculations with ultra-fast excited state dynamics, the potential energy surface (PES) of the S1 state was constructed using Co-C and Co-NIm axial coordinates.

The role of spin-orbit coupling in the photolysis of methylcobalamin.

The photolysis of the methylcobalamin cofactor (MeCbl) in its base-off form was investigated by considering the extent of spin-orbit coupling (SOC). Triplet Co-C photodissociation pathways previously invoked at the density functional theory level using Landau-Zener theory were further validated with ab initio calculations that combine SOC based on multi-state second order perturbation theory. It was determined that SOC is feasible between singlet and triplet states at elongated Co-C distances, leading to photodissociation from the state having dominant σ(dz(2)) character, by either direct coupling with the lowest singlet states or by crossing with SOC mixed triplets.

Photolytic properties of cobalamins: a theoretical perspective.

This Perspective Article highlights recent theoretical developments, and summarizes the current understanding of the photolytic properties of cobalamins from a computational point of view. The primary focus is on two alkyl cobalamins, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), as well as two non-alkyl cobalamins, cyanocobalamin (CNCbl) and hydroxocobalamin (HOCbl). Photolysis of alkyl cobalamins involves low-lying singlet excited states where photodissociation of the Co-C bond leads to formation of singlet-born alkyl/cob(ii)alamin radical pairs (RPs).

Electronically excited states of cob(ii)alamin: insights from CASSCF/XMCQDPT2 and TD-DFT calculations.

The low-lying excited states of cob(ii)alamin were investigated using time-dependent density functional theory (TD-DFT). The performance of TD-DFT calculations was further evaluated using CASSCF/XMCQDPT2, where both four-coordinate and five-coordinate models of cob(ii)alamin were considered. Dependence of electronic structure on the axial base was then investigated using TD-DFT.

Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies.

Hydroxocobalamin is a potential biocompatible source of photogenerated hydroxyl radicals localized in time and space. The photogeneration of hydroxyl radicals is studied using time-resolved spectroscopy and theoretical simulations. Radicals are only generated for wavelengths <350 nm through a mechanism that involves competition between prompt dissociation and internal conversion.

Mechanism of Co-C bond photolysis in methylcobalamin: influence of axial base.

A mechanism of Co-C bond photolysis in the base-off form of the methylcobalamin cofactor (MeCbl) and the influence of its axial base on Co-C bond photodissociation has been investigated by time-dependent density functional theory (TD-DFT). At low pH, the MeCbl cofactor adopts the base-off form in which the axial nitrogenous ligand is replaced by a water molecule. Ultrafast excited-state dynamics and photolysis studies have revealed that a new channel for rapid nonradiative decay in base-off MeCbl is opened, which competes with bond dissociation.

Mechanism of Co-C bond photolysis in the base-on form of methylcobalamin.

A mechanism of Co-C bond photodissociation in the base-on form of the methylcobalamin cofactor (MeCbl) has been investigated employing time-dependent density functional theory (TD-DFT), in which the key step involves singlet radical pair generation from the first electronically excited state (S1). The corresponding potential energy surface of the S1 state was constructed as a function of Co-C and Co-Naxial bond distances, and two possible photodissociation pathways were identified on the basis of energetic grounds. These pathways are distinguished by whether the Co-C bond (path A) or Co-Naxial bond (path B) elongates first.

[The incidence of hypothyroidism symptoms and risk factors for cardiovascular events in subclinical hypothyroidism].

Unlabelled: The clinical significance of subclinical hypothyroidism (SH) has not been determined. There are different opinions with regard to symptoms and clinical consequences of SH as well as effectiveness of treatment. Aim of study was the analysis of incidence of hypothyroidism symptoms and selected cardiovascular risk factors in patients with SH in comparison to euthyroid individuals and the evaluation of the effect of treatment of SH on the above parameters.

Mechanism of the S1 excited state internal conversion in vitamin B12.

To explain the photostability of vitamin B12, internal conversion of the S1 state was investigated using TD-DFT. The active coordinates for radiationless deactivation were determined to be elongated axial bonds, overcoming a 5.0 kcal mol(-1) energy barrier between the relaxed ligand-to-metal charge transfer (S1), and the ground (S0) states.

Redox potentials and protonation of the A-cluster from acetyl-CoA synthase. A density functional theory study.

Density functional theory (DFT/BP86) and the polarized continuum model (PCM/ε = 20) have been applied to perform calculations for the A-cluster of acetyl-CoA synthase enzyme. The geometry optimization was carried out for the oxidized, one- and two-electron reduced A-cluster as well as for A-cluster with ligands important in the catalytic cycle, i.e.

Electronic structure of the S1 state in methylcobalamin: insight from CASSCF/MC-XQDPT2, EOM-CCSD, and TD-DFT calculations.

The methylcobalamin cofactor (MeCbl), which is one of the biologically active forms of vitamin B12, has been the subject of many spectroscopic and theoretical investigations. Traditionally, the lowest-energy part of the photoabsorption spectrum of MeCbl (the so-called α/β band) has been interpreted as an S0→S1 electronic transition dominated by π→π* excitations associated with the C=C stretching of the corrin ring. However, a more quantitative band-shape analysis of the α/β spectral region, along with circular dichroism (CD), magnetic CD, and resonance Raman data, has revealed the presence of a second electronic transition that involves the Co-C(Me) bond weakening.

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

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.

The Cobalt-Methyl Bond Dissociation in Methylcobalamin: New Benchmark Analysis Based on Density Functional Theory and Completely Renormalized Coupled-Cluster Calculations.

The Co-CMe bond dissociation in methylcobalamin (MeCbl), modeled by the Im-[Co(III)corrin]-Me(+) system consisting of 58 atoms, is examined using the coupled-cluster (CC), density-functional theory (DFT), complete-active-space self-consistent-field (CASSCF), and CASSCF-based second-order perturbation theory (CASPT2) approaches. The multilevel variant of the local cluster-in-molecule framework, employing the completely renormalized (CR) CC method with singles, doubles, and noniterative triples, termed CR-CC(2,3), to describe higher-order electron correlation effects in the region where the Co-CMe bond breaking takes place, and the canonical CC approach with singles and doubles (CCSD) to capture the remaining correlation effects, abbreviated as CR-CC(2,3)/CCSD, is used to obtain the benchmark potential energy curve characterizing the Co-CMe dissociation in the MeCbl cofactor. The Co-CMe bond dissociation energy (BDE) resulting from the CR-CC(2,3)/CCSD calculations for the Im-[Co(III)corrin]-Me(+) system using the 6-31G* basis set, corrected for the zero-point energies (ZPEs) and the effect of replacing the 6-31G* basis by 6-311++G**, is about 38 kcal/mol, in excellent agreement with the experimental values characterizing MeCbl of 37 ± 3 and 36 ± 4 kcal/mol.

Electronic and structural properties of low-lying excited states of vitamin B12.

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.

Role of the Axial Base in the Modulation of the Cob(I)alamin Electronic Properties: Insight from QM/MM, DFT, and CASSCF Calculations.

Quantum chemical computations are used to study the electronic and structural properties of the cob(I)alamin intermediate of the cobalamin-dependent methionine synthase (MetH). QM(DFT)/MM calculations on the methylcobalamin (MeCbl) binding domain of MetH reveal that the transfer of the methyl group to the substrate is associated with the displacement of the histidine axial base (His759). The axial base oscillates between a His-on form in the Me-cob(III)lamin:MetH resting state, where the Co-N(His759) distance is 2.

Electronic structure of cofactor-substrate reactant complex involved in the methyl transfer reaction catalyzed by cobalamin-dependent methionine synthase.

Complete active space self-consistent field (CASSCF) computations followed by the second-order perturbation theory have been applied to investigate the electronic properties of a structural mimic of the reactant complex formed in the catalytic cycle of cobalamin-dependent methionine synthase (MetH). Two different structural models have been employed to analyze the reaction complex between methylcobalamin (MeCbl) and homocysteine (Hcy). The first model, referred to as the small model (SM), is based on a truncated corrin ring with inner conjugated macrocycle only and has symmetry constrain with respect to methylthiolate.

Electronically excited states of vitamin B12: benchmark calculations including time-dependent density functional theory and correlated ab initio methods.

Time-dependent density functional theory (TD-DFT) and correlated ab initio methods have been applied to explore the electronically excited states of vitamin B(12) (cyanocobalamin or CNCbl). Different experimental techniques have been used to probe the excited states of CNCbl, revealing many issues that remain poorly understood from an electronic structure point of view. Due to its efficient scaling with size, TD-DFT emerges as one of the most practical tools that can be used to study the electronic properties of these fairly complex molecules.