Flavin Derivatives Research Articles

Assessment of Franck-Condon Methods for Computing Vibrationally Broadened UV-vis Absorption Spectra of Flavin Derivatives: Riboflavin, Roseoflavin, and 5-Thioflavin.

We address the performance of the vertical and adiabatic Franck-Condon (VFC/AFC) approaches combined with time-independent or time-dependent (TI/TD) formalisms in simulating the one-photon absorption spectra of three flavin compounds with distinct structural features. Calculations were done in the gas phase and in two solvents (water, benzene) for which experimental reference measurements are available. We utilized the independent mode displaced harmonic oscillator model without or with frequency alteration (IMDHO/IMDHO-FA) and also accounted for Duschinsky mixing effects. In the initial validation on the first excited singlet state of riboflavin, the range-separated functionals, CAM-B3LYP and ωB97xD, showed the best performance, but B3LYP also gave a good compromise between peak positions and spectral topology. Large basis sets were not mandatory to obtain high-quality spectra for the selected systems. The presence of a symmetry plane facilitated the computation of vibrationally broadened spectra, since different FC variants yield similar results and the harmonic approximation holds rather well. Compared with the AFC approach, the VFC approach performed equally well or even better for all three flavins while offering several advantages, such as avoiding error-prone geometry optimization procedures on excited-state surfaces. We also explored the advantages of curvilinear displacements and of a Duschinsky treatment for the AFC spectra in cases when a rotatable group is present on the chromophore. Taken together, our findings indicate that the combination of the VFC approach with the TD formalism and the IMDHO-FA model offers the best overall performance.

Photoinduced intramolecular charge transfer in an electronically modified flavin derivative: roseoflavin.

The photophysical properties of a push-pull flavin derivative, roseoflavin (RoF), are investigated in different surroundings at the molecular level, with focus on intramolecular charge transfer (ICT). Time-dependent density functional theory (TD-DFT, CAM-B3LYP functional) and DFT-based multireference configuration interaction (DFT/MRCI) are used to compute excited-state energies and one-electron properties of a truncated RoF model, roseolumiflavin (RoLF). Solvent effects are taken into account implicitly by the conductor-like polarizable continuum model and explicitly through a microsolvation scheme. In the gas phase, the calculations predict no crossing between the lowest locally excited (LE) and charge-transfer (CT) states upon twisting the dimethylamine donor group relative to the plane of the isoalloxazine acceptor moiety, whereas this crossing is found to be facile in solution (i.e., in water or benzene). Crossing of the LE and CT states facilitates ICT, which is the main cause of the fluorescence quenching and dual fluorescence character experimentally observed for roseoflavin in solution. The barrier for the ICT process is computed to be lower in water than in benzene, consistent with the enhanced ICT rates observed in more polar solvents. We present a detailed study of the molecular mechanism of the photoinduced ICT process in RoLF. For a typical donor-acceptor chromophore, three such mechanisms are discussed in the literature, which differ in the alignment of the donor and acceptor planes, namely, planar ICT (PICT), perpendicular-twisted ICT (TICT), and wagging ICT (WICT). Our theoretical results suggest that the TICT mechanism is favored in RoLF.

Phase‐Transfer Catalysis in Oxidations Based on the Covalent Bonding of Hydrogen Peroxide to Amphiphilic Flavinium Salts

AbstractThe amphiphilic flavin derivatives 1,10‐ethylene‐bridged 3‐alkylalloxazinium salts (alkyl=butyl, octyl, dodecyl and hexadecyl) were shown to catalyse the chemoselective oxidation of sulfides to sulfoxides with hydrogen peroxide in chloroform/water or dichloromethane/water systems. The efficiency of the catalytic oxidation depended substantially on the pH of the aqueous phase to reach maximum values at pH 7. Under such conditions, the complete conversion of aliphatic and aromatic sulfides occurred within a maximum of 24 h, whereas the non‐catalysed reaction gave only traces of products. This new concept of two‐phase oxidation utilises the activation and the increase in the lipophilicity of hydrogen peroxide by its covalent bonding to the alloxazinium catalyst. A practical application in the synthesis of modafinil was demonstrated.

ChemInform Abstract: Biomimetic Aerobic Oxidative Hydroxylation of Arylboronic Acids to Phenols Catalyzed by a Flavin Derivative.

ChemInform Abstract: Biomimetic Aerobic Oxidative Hydroxylation of Arylboronic Acids to Phenols Catalyzed by a Flavin Derivative.

Absolute Stereochemistry of a 4 a‐Hydroxyriboflavin Analogue of the Key Intermediate of the FAD‐Monooxygenase Cycle

The biological action of flavoenzymes, such as flavin adenine dinucleotide (FAD)-containing monooxygenase, involves the formation of oxygenated flavin derivatives, such as 4 a-hydroperoxyflavin and 4 a-hydroxyflavin, in which a new center of chirality is created at the 4 a position during the enzymatic reactions. So far, the absolute configuration of this center of chirality in natural 4 a-oxygenated flavins has remained unknown in spite of its key importance for the diverse functions of flavoenzymes. Herein, we report the 4 a-hydroxy adduct 3 of 3-benzyl-5-ethyl-10-(tetraacetyl-D-ribityl)flavinium (1), one of the key intermediates involved in the enantioselective organocatalytic oxidation of sulfides to sulfoxides. The 4 a-hydroxyflavin diastereomers (+)-3 and (-)-3, separated by HPLC, were characterized by electronic circular dichroism (CD) spectroscopy. Their absolute configurations at the 4 a position were, for the first time, determined by comparing experimental CD spectra with those calculated by means of time-dependent density functional theory (TDDFT) on DFT-optimized structures obtained after an extensive conformation analysis.

Photophysics of Flavin Derivatives Absorbing in the Blue-Green Region: Thioflavins As Potential Cofactors of Photoswitches

The purpose of this study was to find flavin derivatives with absorption maxima in the blue-green region of the visible spectrum that might be used as alternative cofactors in blue-light photoreceptors. To this end, the vertical absorption spectra of eight lumiflavin-related compounds were calculated by means of quantum chemical methods. The compounds differ from lumiflavin by the subsitution of an S atom for an O atom at the 2- and/or 4-positions of the isoalloxazine core, the substitution of an N atom for a CH group in the 6- and/or 9-positions, or an extension of the π system at the 7- and 8-positions. For the three most promising compounds, 2-thio-lumiflavin, 4-thio-lumiflavin, and 2,4-dithio-lumiflavin, the quantum chemical investigations were extended to include geometry relaxations in the excited states, rates for spin-forbidden transitions and an estimate of spectral shifts brought about by polar protic environments. We find these thiocarbonyl compounds to have very promising excited-state properties. They absorb in the blue-green wavelength regime around 500 nm, i.e., substantially red-shifted with respect to lumiflavin that is the cofactor of natural blue-light photoreceptors. Their triplet quantum yields are predicted to be close to unity while their triplet lifetimes are long enough to enable bimolecular photochemical reactions. The combination of these properties makes the thioflavins potentially suitable candidates as cofactors in biomimetic photoswitches.

Oxidation of phenolic compounds catalyzed by immobilized multi-enzyme systems with integrated hydrogen peroxide production

Suicide inactivation of peroxidases by hydrogen peroxide is the major deterrent to using such biocatalysts in oxidative processes. This has been successfully addressed by the in situ generation of H2O2. In this study, we have developed a novel multi-enzyme biocatalyst that has been immobilized on agarose-type carriers to oxidize phenols using oxygen and formic acid as indirect oxidants. This original system couples the in situ production of H2O2 to the phenol oxidation via an enzymatic cascade that involved three different enzymes (formate dehydrogenase, NADH-oxidase and peroxidase) and two different redox cofactors: nicotinamide and flavin derivatives. The cascade reaction only works when enzymes are immobilized on the solid support since soluble enzymes are dramatically inactivated under the reaction conditions. The highest oxidation efficiency was achieved by combining two different solid biocatalysts: (1) formate dehydrogenase and NADH-oxidase co-immobilized onto agarose beads activated with glyoxyl groups and (2) peroxidase immobilized onto agarose beads as well but activated with boronate groups. Unlike conventional peroxidase-mediated oxidations with exogenous H2O2, this novel system enables the quantitative oxidation of phenol without the addition of H2O2. Furthermore, this novel system can use a broad range of redox cofactors with similar oxidative effectiveness. Therefore, this novel immobilized tri-enzyme system removes important pollutants such as hydroxylated aromatic derivatives (phenol, 4-aminophenol, 2,4-dichloro-phenol or α-naphthol) using formic acid and molecular oxygen as substrates. In addition, this system generates CO2 as waste beyond the oxidized phenols that can be easily separated from the aqueous solution. The sustainability of this system is supported by an E-factor of 1.3 and an atom economy of 43%.

Biomimetic aerobic oxidative hydroxylation of arylboronic acids to phenols catalysed by a flavin derivative

Flavin-catalysed oxidative hydroxylation of substituted arylboronic acids by molecular oxygen with the assistance of hydrazine or ascorbic acid resulted in phenols in high yields. This mild organocatalytic protocol is compatible with a variety of functional groups and it is alternatively usable for transformation of alkylboronic acids to alcohols. Reaction takes place also in water and fulfils criteria for a green procedure.

A novel flavin derivative reveals the impact of glucose on oxidative stress in adipocytes.

We present the synthesis and characterisation of naphthalimide-flavin redox sensor 1 (NpFR1), a novel flavin molecule that is almost non-fluorescent in reduced form, but oxidation gives rise to a 125-fold increase in fluorescence. Confocal microscopy experiments confirm that is sensitive to redox changes within living cells.

Excited State Charge Redistribution and Dynamics in the Donor-π-Acceptor Flavin Derivative ABFL

Chromophores containing a donor-π-acceptor (D-π-A) motif have been shown to exhibit many interesting photophysical properties. The lowest electronic transition of a flavin derivative containing this motif, azobenzylflavin (ABFL), has previously been shown to be highly sensitive to solvent environment and hydrogen bonding ligands. To better understand this sensitivity, we have investigated the excited state charge redistribution and dynamics of ABFL in a low-dielectric, non-hydrogen bonding solvent by steady-state Stark and femtosecond optical transient absorption spectroscopies. The Stark measurements reveal the difference dipole moment, Δμ01, between the ground and first excited states to be 22.3 ± 0.9 D. The direction of Δμ01 in the molecular frame was assigned with the aid of TD-DFT and finite field calculations, verifying the hypothesis that electron density moves from the diethylaniline donor to the flavin acceptor in the excited state. The magnitude of the difference dipole moment was used to estimate the hyperpolarizability of ABFL, β0 = 720 × 10(-30) esu. Subsequent excited state decay via charge recombination was shown to take place in a few picoseconds. The data was best fit to a kinetic model composed of a sub-picosecond internal conversion step from S2→S1, followed by a 5 ps decay to the ground state. A competing process involving formation of an additional long-lived state from S1 was also observed. Cyclic voltammetry shows one oxidation and two reduction waves and is completely reversible. This analysis lays the groundwork for developing new flavin dyads with the desired excited electronic state properties for applications such as nonlinear optical devices, molecular electronics applications, or dye-sensitized solar cells.

Photophysics of Structurally Modified Flavin Derivatives in the Blue‐Light Photoreceptor YtvA: A Combined Experimental and Theoretical Study

The light-induced processes of two flavin mononucleotide derivatives (1- and 5-deaza flavin mononucleotide, 1DFMN and 5DFMN), incorporated into the LOV domain of YtvA protein from Bacillus subtilis, were studied by a combination of experimental and computational methods. Quantum mechanics/molecular mechanics (QM/MM) calculations were carried out in which the QM part was treated by density functional theory (DFT) using the B3LYP functional for geometry optimizations and the DFT/MRCI method for spectroscopic properties, whereas the MM part was described by the CHARMM force field. 1DFMN is incorporated into the protein binding site, yielding a red-shifted absorption band (λ(max) =530 nm compared to YtvA wild-type λ(max) =445 nm), but does not undergo any LOV-typical photoreactions such as triplet and photoadduct formation. QM/MM computations confirmed the absence of a channel for triplet formation and located a radiation-free channel (through an S₁/S₀ conical intersection) along a hydrogen transfer path that might allow for fast deactivation. By contrast, 5DFMN-YtvA-LOV shows a blue-shifted absorption (λ(max) =410 nm) and undergoes similar photochemical processes to FMN in the wild-type protein, both with regard to the photophysics and the formation of a photoadduct with a flavin-cysteinyl covalent bond. The QM/MM calculations predict a mechanism that involves hydrogen transfer in the T₁ state, followed by intersystem crossing and adduct formation in the S₀ state for the forward reaction. Experimentally, in contrast to wild-type YtvA, dark-state recovery in 5DFMN-YtvA-LOV is not thermally driven but can only be accomplished after absorption of a second photon by the photoadduct, again via the triplet state. The QM/MM calculations suggest a photochemical mechanism for dark-state recovery that is accessible only for the adduct with a C4a--S bond but not for alternative adducts with a C5--S bond.

Synthesis and Characterization of Naphthalenediimide-Functionalized Flavin Derivatives

Two acceptor–acceptor dyads have been synthesized featuring a flavin moiety and a naphthalenediimide (NDI) unit. The NDI unit is linked to the flavin through a short spacer group via either the N(3) or N(10) positions of the flavin. We have investigated the UV-Vis and redox properties of these multi-electron accepting systems which indicate that these materials display the collective properties of their component systems. Fluorescence spectroscopy measurements have revealed that their emission properties are dominated by the flavin unit.

(Invited) Metal-Assisted Formation of an Extremely Long–Lived Charge-Separated State in a Porphyrin–Flavin Dyad

not Available.

Spectroscopic and Theoretical Study on Electronically Modified Chromophores in LOV Domains: 8‐Bromo‐ and 8‐Trifluoromethyl‐Substituted Flavins

Two chemically synthesized flavin derivatives, 8-trifluoromethyl- and 8-bromoriboflavin (8-CF(3)RF and 8-BrRF), were photochemically characterized in H(2)O and studied spectroscopically after incorporation into the LOV domain of the blue light photoreceptor YtvA from Bacillus subtilis. The spectroscopic studies were paralleled by high-level quantum chemical calculations. In solution, 8-BrRF showed a remarkably high triplet quantum yield (0.97, parent compound riboflavin, RF: 0.6) and a small fluorescence quantum yield (0.07, RF: 0.27). For 8-CF(3)RF, the triplet yield was 0.12, and the fluorescence quantum yield was 0.7. The high triplet yield of 8-BrRF is due to the bromine heavy atom effect causing a stronger spin-orbit coupling. Theoretical calculations reveal that the decreased triplet yield of 8-CF(3)RF is due to a smaller charge transfer and a less favorable energetic position of T(2), required for intersystem crossing from S(1) to T(1), as an effect of the electron-withdrawing CF(3) group. The reconstitution of the LOV domain with the new flavins resulted in the typical LOV photochemistry, consisting of triplet state formation and covalent binding of the chromophore, followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties.

Recent Developments in Flavin‐based Catalysis

What to do with vitamin B2: Flavins are ubiquitous cofactors for enzymes and have been explored extensively as organocatalysts. Recent developments in the synthesis and use of flavin derivatives are discussed in this Minireview.

Electrode-assisted catalytic water oxidation by a flavin derivative

The success of solar fuel technology relies on the development of efficient catalysts that can oxidize or reduce water. All molecular water-oxidation catalysts reported thus far are transition-metal complexes, however, here we report catalytic water oxidation to give oxygen by a fully organic compound, the N(5)-ethylflavinium ion, Et-Fl(+). Evolution of oxygen was detected during bulk electrolysis of aqueous Et-Fl(+) solutions at several potentials above +1.9V versus normal hydrogen electrode. The catalysis was found to occur on glassy carbon and platinum working electrodes, but no catalysis was observed on fluoride-doped tin-oxide electrodes. Based on spectroelectrochemical results and preliminary calculations with density functional theory, one possible mechanistic route is proposed in which the oxygen evolution occurs from a peroxide intermediate formed between the oxidized flavin pseudobase and the oxidized carbon electrode. These findings offer an organic alternative to the traditional water-oxidation catalysts based on transition metals.

Reverse Structural Genomics: AN UNUSUAL FLAVIN-BINDING SITE IN A PUTATIVE PROTEASE FROM BACTEROIDES THETAIOTAOMICRON

The structure of a putative protease from Bacteroides thetaiotaomicron features an unprecedented binding site for flavin mononucleotide. The flavin isoalloxazine ring is sandwiched between two tryptophan residues in the interface of the dimeric protein. We characterized the recombinant protein with regard to its affinity for naturally occurring flavin derivatives and several chemically modified flavin analogs. Dissociation constants were determined by isothermal titration calorimetry. The protein has high affinity to naturally occurring flavin derivatives, such as riboflavin, FMN, and FAD, as well as lumichrome, a photodegradation product of flavins. Similarly, chemically modified flavin analogs showed high affinity to the protein in the nanomolar range. Replacement of the tryptophan by phenylalanine gave rise to much weaker binding, whereas in the tryptophan to alanine variant, flavin binding was abolished. We propose that the protein is an unspecific scavenger of flavin compounds and may serve as a storage protein in vivo.

Characterization of Flavins in Roots of Fe-Deficient Strategy I Plants, with a Focus on Medicago truncatula

The root accumulation and excretion of riboflavin (Rbfl) and Rbfl derivatives have been studied in the model legume species Medicago truncatula, grown in hydroponics in two different Fe deficiency conditions, with and without CaCO(3). Using high resolution mass spectrometry techniques coupled to liquid chromatography, three different flavin derivatives not previously reported in plants, putatively identified as 7-hydroxy-Rbfl, 7α-hydroxy-Rbfl and 7-carboxy-Rbfl, were found along with Rbfl in Fe-deficient M. truncatula roots. In the presence of CaCO(3) most of the flavins were accumulated in the roots, whereas in the absence of CaCO(3) there was partial export to the nutrient solution. The major flavins in roots and nutrient solution were Rbfl and 7-hydroxy-Rbfl, respectively. Flavins were located in the root cortex and epidermal cells, preferentially in a root region near the apex that also exhibited increased ferric chelate reductase (FCR) activity. Six out of 15 different species of horticultural interest showed root increases in both Rbfl (four of them also having Rbfl derivatives) and FCR. No significant correlation was found between Rbfl and either phosphoenolpyruvate carboxylase or FCR activities, whereas the latter two showed a good correlation between them. The possible roles of Rbfl and Rbfl derivatives in roots and nutrient solutions are discussed. Medicago truncatula is proposed as a model system for flavin studies.

ChemInform Abstract: Flavin-6-carboxylic Acids as Novel and Simple Flavoenzyme Models. Nonenzymatic Stabilization of the Flavin Semiquinone Radical and the 4a-Hydroperoxyflavin by Intramolecular Hydrogen Bonding.

Novel flavin derivatives, 10-ethyl-3-methylisoalloxazine-6-carboxylic acid (1) and 10-ethyl-3-methylisoalloxazine-6,8-dicarboxylic acid (2), which have a carboxyl group at C(6) position, were prepared. Even in the absence of metal cations and under aerobic condition, these flavin derivatives produced the corresponding stable semiquinone radicals by the dithionite reduction in sodium phosphate buffer (pH 6.89). Hyperfine electron spin resonance (ESR) spectra for the flavin semiquinone radicals have been obtained and were stable even after 1 day under ambient circumstances

Riboflavin mobilization from eleocyte stores in the earthworm Dendrodrilus rubidus inhabiting aerially-contaminated Ni smelter soil

A 6-week reciprocal transfer laboratory exposure experiment was conducted with two populations of the epigeic earthworm Dendrodrilus rubidus; one population inhabited a site approx. 200 m downwind of an active Ni smelter co-contaminated with Ni and Cu (3648 and 977 μg g −1 d.w., respectively), the other inhabited uncontaminated soil. Worms transferred from unpolluted to Ni/Cu-polluted soil lost body mass (62%); they also had reduced (70%) total coelomocyte number, including autofluorescent eleocytes, and had significantly decreased (92%) riboflavin-derived fluorescence emission measured at 525 nm. Coelomocyte counts were low, and 525 nm emission was negligible in worms maintained on their native Ni/Cu soil. Earthworms and their coelomocytes were unaffected when transferred from Ni/Cu-polluted soil to unpolluted soil. In conclusion, exposing worms to stress-inducing factors, including metal pollution, alters the riboflavin status within the immune-competent cells of D. rubidus, but it requires further in vivo studies to establish whether the reduction in the fluorescence signal is predominantly due to depletion of riboflavin-containing eleocytes, or to riboflavin quenching, or to enzymatic conversion (and thus depletion) of stored riboflavin into its functional immune-potentiating flavin derivatives, FMN and FAD. The flavin budget of D. rubidus coelomocytes recovered by a reproducible extrusion procedure is a potentially useful biomarker for assessing sublethal stress in this early colonizer of disturbed soils.