Linear Free Energy Substituent Effect on Flavin Redox Chemistry

A Redox-Reversible Switch of DNA Hydrogen Bonding and Structure

Modulating molecular structure and function at the nanoscale drives innovation across wide-ranging technologies. Electrical control of the bonding of individual DNA base pairs endows DNA with precise nanoscale structural reconfigurability, benefiting efforts in DNA origami and actuation. Here, alloxazine DNA base surrogates were synthesized and incorporated into DNA duplexes to function as a redox-active switch of hydrogen bonding. Circular dichroism (CD) revealed that 24-mer DNA duplexes containing one or two alloxazines exhibited CD spectra and melting transitions similar to DNA with only canonical bases, indicating that the constructs adopt a B-form conformation. However, duplexes were not formed when four or more alloxazines were incorporated into a 24-mer strand. Thiolated duplexes incorporating alloxazines were self-assembled onto multiplexed gold electrodes and probed electrochemically. Square-wave voltammetry (SWV) revealed a substantial reduction peak centered at -0.272 V vs Ag/AgCl reference. Alternating between alloxazine oxidizing and reducing conditions modulated the SWV peak in a manner consistent with the formation and loss of hydrogen bonding, which disrupts the base pair stacking and redox efficiency of the DNA construct. These alternating signals support the assertion that alloxazine can function as a redox-active switch of hydrogen bonding, useful in controlling DNA and bioinspired assemblies.

Substrate Electronics Dominate the Rate of Reductive Dehalogenation Promoted by the Flavin-Dependent Iodotyrosine Deiodinase

Iodotyrosine deiodinase (IYD) is unusual in its reliance on flavin to promote reductive dehalogenation of halotyrosines under aerobic conditions. Applications of this activity can be envisioned for bioremediation, but expansion of its specificity requires an understanding of the mechanistic steps that limit the rate of turnover. Key processes capable of controlling steady-state turnover have now been evaluated and described in this study. While proton transfer is necessary for converting the electron-rich substrate into an electrophilic intermediate suitable for reduction, kinetic solvent deuterium isotope effects suggest that this process does not contribute to the overall efficiency of catalysis under neutral conditions. Similarly, reconstituting IYD with flavin analogues demonstrates that a change in reduction potential by as much as 132 mV affects kcat by less than 3-fold. Furthermore, kcat/Km does not correlate with reduction potential and indicates that electron transfer is also not rate determining. Catalytic efficiency is most sensitive to the electronic nature of its substrates. Electron-donating substituents on the ortho position of iodotyrosine stimulate catalysis and conversely electron-withdrawing substituents suppress catalysis. Effects on kcat and kcat/Km range from 22- to 100-fold and fit a linear free-energy correlation with a ρ ranging from -2.1 to -2.8 for human and bacterial IYD. These values are consistent with a rate-determining process of stabilizing the electrophilic and nonaromatic intermediate poised for reduction. Future engineering can now focus on efforts to stabilize this electrophilic intermediate over a broad series of phenolic substrates that are targeted for removal from our environment.

Reduced Flavin in Aqueous Solution Is Nonfluorescent

Flavins are blue-light-absorbing chromophores with rich redox activity. Biologically, the most important are riboflavin (vitamin B₂), flavin mononucleotide, and flavin adenine dinucleotide, the latter two of which are catalytic cofactors in enzymes. Flavins pivot between oxidized, one electron-, and two electron-reduced forms in different protonation states, depending on enzymatic requirements. Some flavoenzymes use light as a reagent for chemical bond formation, photoinduced electron transfer, or conformational changes required for light-sensitive signaling. Therefore, the photochemistry and photophysics of flavins have received wide attention. Fluorescence from oxidized flavin is often used to detect and track changes in flavin oxidation states. However, there have been conflicting reports over the past 45 years as to whether reduced flavin in solution has detectable fluorescence. Here, using single photon counting emission spectroscopy with rigorous sample preparation, we show definitively that reduced flavins are essentially nonfluorescent, having a quantum yield more than three orders of magnitude lower than oxidized flavin. This result will force a re-evaluation of experiments and models that assumed otherwise.

A Self‐Assembling Flavin for Visible Photooxidation

A new flavin‐based gelator is reported which forms micellar structures at high pH and gels at low pH. This flavin can be used for the photooxidation of thiols under visible light, with the catalytic efficiency being linked to the self‐assembled structures present.

Spatial effects of photosensitization on morphology of giant unilamellar vesicles

Singlet oxygen (1O2) formed through photosensitization may initiate oxidative destruction of biomembranes, however, the influence from the spatial organization of photosensitizers (PS) relative to membranes remains unclear. To clarify this issue, we loaded riboflavin 5'-(dihydrogen phosphate) monosodium (FMN-Na) as a hydrophilic PS into the lumen of halloysite nanotubes (HNTs), and attached the nanoassemblies (FMN-Na@HNTs), via Pickering effects, to the outer surfaces of giant unilamellar vesicles (GUVs) of phospholipids. We also prepared GUVs dopped with lumiflavin (LF) as a lipophilic PS having a 1O2 quantum yield comparable to FMN-Na. FMN-Na capsulated in HNT was characterized by a longer triplet excited state lifetime (12.1 μs) compared to FMN-Na free in solution (7.5 μs), and FMN-Na in both forms efficiently generated 1O2 upon illumination. The spatio-effects of PS on the photosensitized morphological changes of membranes were studied using conventional optical microscopy by monitoring GUV morphological changes. Upon light exposure (400-440 nm), the GUVs attached with FMN-Na@HNT merely experienced membrane deformation starting from the original spherical shape, ascribed to Type II photosensitization with 1O2 as oxidant. In contrast, photooxidation of LF dopped GUVs mainly led to membrane coarsening and budding assigned to Type I photosensitization. The spatial effects of PS on photosensitized morphological changes were related to the different lipid oxidation products generated through Type I and Type II photosensitized lipid oxidation.

Flavin-Helicene Amphiphilic Hybrids: Synthesis, Characterization, and Preparation of Surface-Supported Films

This work reports on the preparation and structural characterization of flavo[7]helicene 1 (flavin-[7]helicene conjugate), which was subsequently characterized at the molecular level in either an aqueous environment or an organic phase, at the supramolecular level in the form of polymeric layers, and also embedded in a lipidic mesophase environment to study the resulting properties of such a hybrid relative to its parent molecules. The flavin benzo[g]pteridin-2,4-dione (isoalloxazine) was selected for conjugation because of its photoactivity and reversible redox behavior. Compound 1 was prepared from 2-nitroso[6]helicene and 6-methylamino-3-methyluracil, and characterized using common structural and spectroscopic tools: circular dichroism (CD), circularly polarized luminescence (CPL) spectroscopy, cyclic voltammetry (CV), and DFT quantum calculations. In addition, a methodology that allows the loading of 1 enantiomers into an internally nanostructured lipid (1-monoolein) matrix was developed.

The redox potential of flavin derivatives as a mediator in biosensors

The two-electron reduction potential for a set of 393 flavin derivatives is presented in this article. These derivatives are substituted flavin on carbon 6, 7, 8, and 9 by coinage transition metals (Cu, Ag, and Au) and conjugated double bond hydrocarbons; and both groups are examined with and without functional groups such as OH, Cl, CH₃, COOH, and NO₂. In order to show the validity of the results, the reduction potential of human life molecules, which have experimental values, such as flavin adenine dinucleotide (FAD) and riboflavin (vitamin B₂) is calculated. The experimental value for FAD is - 0.22 V, while the obtained theoretical value is - 0.21 V, and the corresponding values for riboflavin are - 0.18 and - 0.19 V, respectively. Theoretical calculations have been carried out by DFT procedure with a 6-31+G** basis set and BLYP xc-functional for coinage transition metals substitution, and MPW1PW9 xc-functionals for conjugated double bond hydrocarbon substitution. Both xc-functionals are chosen by the DFT calibration procedure.

A quantitative evaluation of computational methods to accelerate the study of alloxazine-derived electroactive compounds for energy storage

Alloxazines are a promising class of organic electroactive compounds for application in aqueous redox flow batteries (ARFBs), whose redox properties need to be tuned further for higher performance. High-throughput computational screening (HTCS) enables rational and time-efficient study of energy storage compounds. We compared the performance of computational chemistry methods, including the force field based molecular mechanics, semi-empirical quantum mechanics, density functional tight binding, and density functional theory, on the basis of their accuracy and computational cost in predicting the redox potentials of alloxazines. Various energy-based descriptors, including the redox reaction energies and the frontier orbital energies of the reactant and product molecules, were considered. We found that the lowest unoccupied molecular orbital (LUMO) energy of the reactant molecules is the best performing chemical descriptor for alloxazines, which is in contrast to other classes of energy storage compounds, such as quinones that we reported earlier. Notably, we present a flexible in silico approach to accelerate both the singly and the HTCS studies, therewithal considering the level of accuracy versus measured electrochemical data, which is readily applicable for the discovery of alloxazine-derived organic compounds for energy storage in ARFBs.

Benzo-Dipteridine Derivatives as Organic Cathodes for Li- and Na-ion Batteries

Organic-based electrodes for Li- and Na-ion batteries present attractive alternatives to commonly applied inorganic counterparts which can often carry with them supply-chain risks, safety concerns with thermal runaway, and adverse environmental impact. The ability to chemically direct the structure of organic electrodes through control over functional groups is of particular importance, as this provides a route to fine-tune electrochemical performance parameters. Here, we report two benzo-dipteridine derivatives, BF-Me₂ and BF-H₂ , as high-capacity electrodes for use in Li- and Na-ion batteries. These moieties permit binding of multiple Li-ions per molecule while simultaneously ensuring low solubility in the supporting electrolyte, often a precluding issue with organic electrodes. Both display excellent electrochemical stability, with discharge capacities of 142 and 182 mAh g^-1 after 100 cycles at a C/10 rate and Coulombic efficiencies of 96% and ∼ 100% demonstrated for BF-Me₂ and BF-H₂ , respectively. The application of a Na-ion cell has also been demonstrated, showing discharge capacities of 88.8 and 137 mAh g^-1 after 100 cycles at a C/2 rate for BF-Me₂ and BF-H₂ , respectively. This work provides an encouraging precedent for these and related structures to provide versatile, high-energy density, and long cycle-life electrochemical energy storage materials.

Microscopic Picture of the Solvent Reorganization During Electron Transfer to Flavin in Water

The redox potential of molecular species is largely modulated by its molecular environment so that a change of the environment will lead to a different redox potential. However, a detailed molecular picture of reorganization of the environment upon reduction is still unclear. To unravel the details of the solvent reorganization during electron transfer, we have performed density functional theory-based molecular dynamics (DFT-MD) and hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of the reduction of lumiflavin. Previously, we have calculated the reduction free energy curves of the redox half reactions of lumiflavin in water as a function of the instantaneous gap energy (ΔE) ( J. Chem. Theory Comput. 2013 , 9 , 3889 - 3899 ). In this work, we focus on finding the changes in the solvent environment that correlate with this ΔE reaction coordinate. Comparing the QM/MM simulations, in which the solvent is modeled with an empirical force field, with the (full) DFT-MD simulations, we find that the response through electronic polarization plays a significant role in the latter case. Also a small charge transfer between flavin and solvent is observed in the full DFT treatment. As a result, we find only in the case of the QM/MM model a strong correlation between ΔE and the (pairwise computed) electrostatic potential (ESP) at the flavin due to the solvent. By analyzing the contribution of the ESP at the flavin per solvent molecule, we cannot only distinguish between the different modes of hydration by solvent molecules that coordinate at the hydrophilic and hydrophobic sides of the flavin molecule but also quantify their contribution to the reorganization free energy by measuring the ESP fluctuations per solvent molecule.

Flavin Derivatives with Tailored Redox Properties: Synthesis, Characterization, and Electrochemical Behavior

This study establishes structure-property relationships for four synthetic flavin molecules as bioinspired redox mediators in electro- and photocatalysis applications. The studied flavin compounds were disubstituted with polar substituents at the N1 and N3 positions (alloxazine) or at the N3 and N10 positions (isoalloxazines). The electrochemical behavior of one such synthetic flavin analogue was examined in detail in aqueous solutions of varying pH in the range from 1 to 10. Cyclic voltammetry, used in conjunction with hydrodynamic (rotating disk electrode) voltammetry, showed quasi-reversible behavior consistent with freely diffusing molecules and an overall global 2e(-) , 2H(+) proton-coupled electron transfer scheme. UV/Vis spectroelectrochemical data was also employed to study the pH-dependent electrochemical behavior of this derivative. Substituent effects on the redox behavior were compared and contrasted for all the four compounds, and visualized within a scatter plot framework to afford comparison with prior knowledge on mostly natural flavins in aqueous media. Finally, a preliminary assessment of one of the synthetic flavins was performed of its electrocatalytic activity toward dioxygen reduction as a prelude to further (quantitative) studies of both freely diffusing and tethered molecules on various electrode surfaces.

Switching an O2 sensitive glucose oxidase bioelectrode into an almost insensitive one by cofactor redesign

In the 5-8 mM glucose concentration range, of particular interest for diabetes management, glucose oxidase bioelectrodes are O2 dependent, which decrease their efficiencies. By replacing the natural cofactor of glucose oxidase, we succeeded in turning an O2 sensitive bioelectrode into an almost insensitive one.

Natural riboflavin analogs

Riboflavin analogs have a good potential to serve as basic structures for the development of novel anti-infectives. Riboflavin analogs have multiple cellular targets, since riboflavin (as a precursor to flavin cofactors) is active at more than one site in the cell. As a result, the frequency of developing resistance to antimicrobials based on riboflavin analogs is expected to be significantly lower. The only known natural riboflavin analog with antibiotic function is roseoflavin from the bacterium Streptomyces davawensis. This antibiotic negatively affects flavoenzymes and FMN riboswitches. Another roseoflavin producer, Streptomyces cinnabarinus, was recently identified. Possibly, flavin analogs with antibiotic activity are more widespread than anticipated. The same could be true for flavin analogs yet to be discovered, which could constitute tools for cellular chemistry, thus allowing a further extension of the catalytic spectrum of flavoenzymes.

Flavoproteins are potential targets for the antibiotic roseoflavin in Escherichia coli

The riboflavin analog roseoflavin is an antibiotic produced by Streptomyces davawensis. Riboflavin transporters are responsible for roseoflavin uptake by target cells. Roseoflavin is converted to the flavin mononucleotide (FMN) analog roseoflavin mononucleotide (RoFMN) by flavokinase and to the flavin adenine dinucleotide (FAD) analog roseoflavin adenine dinucleotide (RoFAD) by FAD synthetase. In order to study the effect of RoFMN and RoFAD in the cytoplasm of target cells, Escherichia coli was used as a model. E. coli is predicted to contain 38 different FMN- or FAD-dependent proteins (flavoproteins). These proteins were overproduced in recombinant E. coli strains grown in the presence of sublethal amounts of roseoflavin. The flavoproteins were purified and analyzed with regard to their cofactor contents. It was found that 37 out of 38 flavoproteins contained either RoFMN or RoFAD. These cofactors have different physicochemical properties than FMN and FAD and were reported to reduce or completely abolish flavoprotein function.

The flavoenzyme azobenzene reductase AzoR from Escherichia coli binds roseoflavin mononucleotide (RoFMN) with high affinity and is less active in its RoFMN form

The Gram-positive bacterium Streptomyces davawensis is the only organism known to produce the antibiotic roseoflavin. Roseoflavin is a structural riboflavin analogue and is converted to the flavin mononucleotide (FMN) analogue roseoflavin mononucleotide (RoFMN) by flavokinase. FMN-dependent homodimeric azobenzene reductase (AzoR) (EC 1.7.1.6) from Escherichia coli was analyzed as a model enzyme. In vivo and in vitro experiments revealed that RoFMN binds to the AzoR apoenzyme with an even higher affinity compared to that of the "natural" cofactor FMN. Structural analysis (at a resolution of 1.07 Å) revealed that RoFMN binding did not affect the overall topology of the enzyme and also did not interfere with dimerization of AzoR. The AzoR-RoFMN holoenzyme complex was found to be less active (30% of AzoR-FMN activity) in a standard assay. We provide evidence that the different physicochemical properties of RoFMN are responsible for its reduced cofactor activity.

The antibiotics roseoflavin and 8-demethyl-8-amino-riboflavin from Streptomyces davawensis are metabolized by human flavokinase and human FAD synthetase

The non-pathogenic Gram-positive soil bacterium Streptomyces davawensis synthesizes the riboflavin (vitamin B(2)) analogs roseoflavin (RoF) and 8-demethyl-8-amino-riboflavin (AF). Both compounds are antibiotics. Notably, a number of other riboflavin analogs are currently under investigation with regard to the development of novel antiinfectives. As a first step towards understanding the metabolism of riboflavin analogs in humans, the key enzymes flavokinase (EC 2.7.1.26) and FAD synthetase (EC 2.7.7.2) were studied. Human flavokinase efficiently converted RoF and AF to roseoflavin mononucleotide (RoFMN) and 8-demethyl-8-amino-riboflavin mononucleotide (AFMN), respectively. Human FAD synthetase accepted RoFMN but not AFMN as a substrate. Consequently, roseoflavin adenine dinucleotide (RoFAD) was synthesized by the latter enzyme but not 8-demethyl-8-amino-riboflavin adenine dinucleotide (AFAD). The cofactor analogs RoFMN, AFMN and RoFAD have different physicochemical properties as compared to FMN and FAD. Thus, the cofactor analogs have the potential to render flavoenzymes inactive, which may negatively affect human metabolism. RoF, but not AF, was found to inhibit human flavokinase. In summary, we suggest that AF has a lower toxic potential and may be better suited as a lead structure to develop antimicrobial compounds.

FT-IR spectroscopic studies on the molecular mechanism for substrate specificity/activation of medium-chain acyl-CoA dehydrogenase

The interactions of acyl-CoA with medium-chain acyl-CoA dehydrogenases (MCADs) reconstituted with artificial FADs-i.e. 8-CN-, 7,8-Cl(2)-, 8-Cl-, 8-OCH(3)- and 8-NH(2)-FAD-were investigated by UV-visible absorption and FT-IR measurements. Although 8-NH(2)-FAD-MCAD did not oxidize acyl-CoA the wavelength of the absorption maximum of the flavin was altered by acyl-CoAs binding. Thus, 8-NH(2)-FAD-MCAD is one of the attractive materials for investigation of enzyme-substrate (ES) interaction in ES complex (the complex of oxidized MCAD with acyl-CoA). FT-IR difference spectra between non-labelled and [1-(13)C]-labelled acyl-CoA free in solution and bound to oxidized 8-NH(2)-FAD-MCAD were obtained. The broad 1668-cm(-1) band of free octanoyl-CoA assigned to the C(1) = O stretching vibration appeared as a sharp signal at 1626 cm(-1) in the case of the complex. The downward shift indicates a large polarization of C(1) = O, and the sharpness suggests that the orientation of the C(1) = O in the active-site cavity is fairly limited. The hydrogen-bond enthalpy change responsible for the polarization on the transfer of the substrate from aqueous solution to the active site of MCAD was estimated to be approximately 15 kcal/mol. The 1626-cm(-1) band is noticeably weakened in the case of acyl-CoA with acyl chains longer than C12 which are poor substrates for MCAD, suggesting that C(1) = O is likely to exist in multiple orientations in the active-site cavity, whence the band becomes obscured. A band identical to that of bound C8-CoA was observed in the case of C4-CoA which is a poor substrate, indicating the strong hydrogen bond at C(1) = O.

Model systems for flavoenzyme activity: an investigation of the role functionality attached to the C(7) position of the flavin unit has on redox and molecular recognition properties

We describe the role functionality attached to the C(7) position of a family of flavin derivatives has in tuning their redox and recognition properties and the subsequent exploitation of two of these derivatives as a three-component electrochemically controllable molecular switch.

Recognition Properties of Flavin Analogues with Bile Acid-Based Receptors: Role of Steric Effects in Hydrogen Bond Based Molecular Recognition

The recognition properties of 7,8-dimethyl flavin analogues by bile acid-based receptors that contain 2,6-diaminopyridine and the dioctylamide of 2,6-diaminopyridine in CHCl3 were determined. The results show that the bile acid-based receptors bind 7,8-dimethyl flavin analogues less effectively as compared to 7,8-unsubstituted flavins reported earlier, which is contrary to the known fact that the association constants increase with increasing electron-donating capacity of the substituents at the 7 and 8 positions of the flavin analogues.

Combined quantum mechanical and molecular mechanical simulations of one- and two-electron reduction potentials of flavin cofactor in water, medium-chain acyl-CoA dehydrogenase, and cholesterol oxidase

Flavin adenine dinucleotide (FAD) is a common cofactor in redox proteins, and its reduction potentials are controlled by the protein environment. This regulation is mainly responsible for the versatile catalytic functions of flavoenzymes. In this article, we report computations of the reduction potentials of FAD in medium-chain acyl-CoA dehydrogenase (MCAD) and cholesterol oxidase (CHOX). In addition, the reduction potentials of lumiflavin in aqueous solution have also been computed. Using molecular dynamics and free-energy perturbation techniques, we obtained the free-energy changes for two-electron/two-proton as well as one-electron/one-proton addition steps. We employed a combined quantum mechanical and molecular mechanical (QM/MM) potential, in which the flavin ring was represented by the self-consistent-charge density functional tight-binding (SCC-DFTB) method, while the rest of the enzyme-solvent system was treated by classical force fields. The computed two-electron/two-proton reduction potentials for lumiflavin and the two enzyme-bound FADs are in reasonable agreement with experimental data. The calculations also yielded the pKa values for the one-electron reduced semiquinone (FH*) and the fully reduced hydroquinone (FH2) forms. The pKa of the FAD semiquinone in CHOX was found to be around 4, which is 4 units lower than that in the enzyme-free state and 2 units lower than that in MCAD; this supports the notion that oxidases have a greater ability than dehydrogenases to stabilize anionic semiquinones. In MCAD, the flavin ring interacts with four hydrophobic residues and has a significantly bent structure, even in the oxidized state. The present study shows that this bending of the flavin imparts a significant destabilization (approximately 5 kcal/mol) to the oxidized state. The reduction potential of lumiflavin was also computed using DFT (M06-L and B3LYP functionals with 6-31+G(d,p) basis set) with the SM6 continuum solvation model, and the results are in good agreement with results from explicit free-energy simulations, which supports the conclusion that the SCC-DFTB/MM computation is reasonably accurate for both 1e(-)/1H+ and 2e(-)/2H+ reduction processes. These results suggest that the first coupled electron-proton addition is stepwise for both the free and the two enzyme-bound flavins. In contrast, the second coupled electron-proton addition is also stepwise for the free flavin but is likely to be concerted when the flavin is bound to either the dehydrogenase or the oxidase enzyme.

Synthesis, properties, and NAD+-NADH-type redox ability of 14-substituted 1,3-dimethyl-5,10-methanocycloundeca[4,5]pyrrolo[2,3-d]pyrimidine-2,4(1,3h)-dionylium tetrafluoroborates and their hydride adducts

A synthesis of 14-substituted 1,3-dimethyl-5,10-methanocycloundeca[4,5]pyrrolo[2,3-d]pyrimidine-2,4(1,3H)-dionylium tetrafluoroborates 11a,b(+).BF4- was accomplished by the methylation of 5,10-methanocycloundeca[4,5]pyrrolo[2,3-d]pyrimidine-2,4(1,3H)-dione derivatives with MeI and following anion-exchange reaction by treatment with 42% aq HBF(4). Compound 11b(+).BF4- was synthesized alternatively by the reaction of 1,6-methano[11]annulenylium tetrafluoroborate with 6-phenylamino-1,3-dimethyluracil and following oxidative cyclization reaction. Remarkable structural characteristics of 11a,b(+)were clarified on inspection of the UV-vis and NMR spectral data as well as X-ray crystal analyses. The stability of cations 11a,b(+)() is expressed by the pK(R+) values which were determined spectrophotometrically as 9.8 and 9.7, which are smaller by 1.4 and 1.2 pH units than those of the corresponding seven-membered ring cations, respectively; however, the values are larger by 3.6 and 3.5 pH units than that of the parent 1,6-methano[11]annulenylium ion (pK(R+) = 6.2). The feature is rationalized on the basis of the perturbation derived from the bond fixation of the parent cation and the electron-donating ability of pyrrolopyrimidine. The electrochemical reduction of 11a,b(+).BF4- exhibited reduction potential at -0.58 and -0.52 (V vs Ag/AgNO3) upon cyclic voltammetry (CV). Reaction of 11a(+).BF4- with hydride afforded mixures of the C13- and C11-adducts in a ratio with hydride afforded, on the other hand, the C13-adduct as a single product. In both cations, the methano-bridge seemed to control the nucleophilic attack to the C13 favorably with exo-selectivity. The photoinduced autorecycling oxidation reactions of 11a,b(+).BF4- toward some amines under aerobic conditions were carried out to give the corresponding imines (isolated by converting to the corresponding 2,4-dinitrophenylhydrazones) with the recycling number of 1.1 to 32.2. Furthermore, as an example of the NAD+-NADH models, the reduction of a pyruvate analogue and some carbonyl compounds with the hydride adducts of 11a,b+.BF4- was accomplished for the first time to give the corresponding alcohol derivatives.

15N solid-state NMR provides a sensitive probe of oxidized flavin reactive sites

Flavins are central to the reactivity of a wide variety of enzymes and electron transport proteins. There is great interest in understanding the basis for the different reactivities displayed by flavins in different protein contexts. We propose solid-state nuclear magnetic resonance (SS-NMR) as a tool for directly observing reactive positions of the flavin ring and thereby obtaining information on their frontier orbitals. We now report the SS-NMR signals of the redox-active nitrogens N1 and N5, as well as that of N3. The chemical shift tensor of N5 is over 720 ppm wide, in accordance with the predictions of theory and our calculations. The signal of N3 can be distinguished on the basis of coupling to 1H absent for N1 and N5, as well as the shift tensor span of only 170 ppm, consistent with N3's lower aromaticity and lack of a nonbonding lone pair. The isotropic shifts and spans of N5 and N1 reflect two opposite extremes of the chemical shift range for "pyridine-type" N's, consistent with their electrophilic and nucleophilic chemical reactivities, respectively. Upon flavin reduction, N5's chemical shift tensor contracts dramatically to a span of less than 110 ppm, and the isotropic chemical shift changes by approximately 300 ppm. Both are consistent with loss of N5's nonbonding lone pair and decreased aromaticity, and illustrate the responsiveness of the 15N chemical shift principal values to electronic structure. Thus. 15N chemical shift principal values promise to be valuable tools for understanding electronic differences that underlie variations in flavin reactivity, as well as the reactivities of other heterocyclic cofactors.

Model systems for flavoenzyme activity: recognition and redox modulation of flavin mononucleotide in water using nanoparticles

We have used mixed monolayer protected gold clusters (MMPCs) to provide flavoenzyme model systems with a high affinity and ability to modulate cofactor reduction potential.

The electrochemically-tuneable interactions between flavin-functionalised C60 derivatives and 2,6-diethylamidopyridine

We report the electrochemically-tuneable interactions between flavin-functionalised C60 derivatives and a diamidopyridine derivative.

Model Systems for Flavoenzyme Activity: Site-Isolated Redox Behavior in Flavin-Functionalized Random Polystyrene Copolymers

[reaction: see text] A model system has been developed to study the redox behaviors of flavin derivatives appended onto random polystyrene copolymers through "click" chemistry strategies. The results demonstrate that flavin units attached onto polymers exhibit site-isolated redox behaviors, yielding new materials with electrochemically tunable associations (K(a)(ox) = 450 M(-)(1), K(a)(red) = 18,200 M(-)(1)) to complementary diamidopyridine (DAP) functionality.

Role of active site residues and solvent in proton transfer and the modulation of flavin reduction potential in bacterial morphinone reductase

The reactions of several active site mutant forms of bacterial morphinone reductase (MR) with NADH and 2-cyclohexen-1-one as substrates have been studied by stopped-flow and steady-state kinetic methods and redox potentiometry. The enzymes were designed to (i) probe a role for potential proton donors (Tyr-72 and Tyr-356) in the oxidative half-reaction of MR; (ii) assess the function of a highly conserved tryptophan residue (Trp-106) in catalysis; (iii) investigate the role of Thr-32 in modulating the FMN reduction potential and catalysis. The Y72F and Y356F enzymes retained activity in both steady-state and stopped-flow kinetic studies, indicating they do not serve as key proton donors in the oxidative reaction of MR. Taken together with our recently published data (Messiha, H. L., Munro, A. W., Bruce, N. C., Barsukov, I., and Scrutton, N. S. (2005) J. Biol. Chem. 280, 4627-4631) that rule out roles for Cys-191 (corresponding with the proton donor, Tyr-196, in the structurally related OYE1 enzyme) and His-186 as proton donors, we infer solvent is the source of the proton in the oxidative half-reaction of MR. We demonstrate a key role for Thr-32 in modulating the reduction potential of the FMN, which is decreased approximately 50 mV in the T32A mutant MR. This effects a change in rate-limiting step in the catalytic cycle of the T32A enzyme with the oxidizing substrate 2-cyclohexenone. Despite the conservation of Trp-106 throughout the OYE family, we show this residue does not play a major role in catalysis, although affects on substrate and coenzyme binding are observed in a W106F enzyme. Our studies show some similarities, but also major differences, in the catalytic mechanism of MR and OYE1, and emphasize the need for caution in inferring mechanism by structural comparison of highly related enzymes in the absence of solution studies.

Antioxidant Properties of Natural and Synthetic Chromanol Derivatives: Study by Fast Kinetics and Electron Spin Resonance Spectroscopy

[structure: see text] Chromanol-type compounds act as antioxidants in biological systems by reduction of oxygen-centered radicals. Their efficiency is determined by the reaction rate constants for the primary antioxidative reaction as well as for disproportionation and recycling reactions of the antioxidant-derived radicals. We studied the reaction kinetics of three novel chromanols: cis- and trans-oxachromanol and the dimeric twin-chromanol, as well as ubichromanol and ubichromenol, in comparison to alpha-tocopherol and pentamethylchromanol. The antioxidant-derived radicals were identified by optical and electron spin resonance spectroscopy (ESR). The kinetics of the primary antioxidative reaction and the disproportionation of the chromanoxyl radicals were assessed by stopped-flow photometry in different organic solvents to simulate the different polarities associated with biomembranes. Furthermore, the reduction of the chromanoxyl radicals by ubiquinol and ascorbate was measured after laser-induced one-electron chromanol oxidation in ethanol and in a micellar system, respectively. The rate constants showed that twin-chromanol had better radical scavenging properties than alpha-tocopherol and a significantly slower disproportionation rate of its corresponding chromanoxyl radical. In addition, the radical derived from twin-chromanol is reduced by ubiquinol and ascorbate at a faster rate than the tocopheroxyl radical. Finally, twin-chromanol can deliver twice as many reducing equivalents, which makes this compound a promising new candidate as artificial antioxidant in biological systems.

Oxygen isotope effects on electron transfer to O2 probed using chemically modified flavins bound to glucose oxidase

Apo-glucose oxidase has been reconstituted with flavins modified in the 7 and 8 positions and characterized with regard to the catalytic rate of O(2) reduction and oxygen-18 isotope effects on this process. Kinetic studies as a function of driving force indicate a reorganization energy for electron transfer to O(2) of lambda = 28 kcal mol(-)(1) at optimal pH, which is similar to the value obtained earlier from temperature dependencies of rates (Roth, J. P.; Klinman, J. P. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 62-67). For the various enzyme-bound flavins, competitive oxygen-18 kinetic isotope effects fall within the narrow range of 1.0266(5) to 1.0279(6), apparently because of the dominant contribution of outer-sphere reorganization to the activation barrier; within the context of semiclassical and quantum mechanical electron transfer theories, the magnitude of the isotope effects reveals the importance of nuclear tunneling.

New Synthesis, Properties, and Oxidizing Ability of 1,3-Dimethyl-5,10-methanocycloundeca[4,5]furo[2,3-d]pyrimidin- 2,4(1,3H)-dionylium Tetrafluoroborate

A novel synthesis of 1,3-dimethyl-5,10-methanocycloundeca[4,5]furo[2,3-d]pyrimidin-2,4(1,3H)-dionylium tetrafluoroborate (10(+).BF(4)(-)) was accomplished by the reaction of 3,8-methano[11]annulenone with dimethylbarbituric acid and following acidic cyclization, albeit in low yield. Remarkable structural characteristics were suggested on inspection of the spectral data and MO calculation, and it was clarified that the positive charge is largely localized at the C11. The pK(R+) value of cation 10(+) was determined spectrophotometrically to be 4.6, which is much smaller by 4.1 pH unit than that of 1,3-dimethyl-7,12-methanocycloundeca[4,5]furo[2,3-d]pyrimidin-2,4(1,3H)-dionylium tetrafluoroborate (pK(R+) = 8.7). This value is also smaller by 1.6 pH unit than that of the parent 1,6-methano[11]annulenylium ion (pK(R+) = 6.2). The feature is rationalized on the basis of the perturbation derived from the bond fixation of the parent cation. The electrochemical reduction of 10(+) exhibited less negative reduction potential at -0.39 (V vs Ag/AgNO(3)) upon cyclic voltammetry (CV). In a search for reactivity, reactions of 10(+) with some nucleophiles, hydride and diethylamine, were carried out to give mixtures of C11- and C13-adducts. In both reactions, the methano-bridge controls the nucleophilic attacks to the C13 to favor exo selectivity. The photoinduced autorecycling oxidation reactions of 10(+).BF(4)(-) toward some amines under aerobic conditions were carried out to give the corresponding imines (isolated by converting to the corresponding 2,4-dinitrophenylhydrazones) in 719-3286% yield (recycling number of 10(+).BF(4)(-): 7.2-32.9).

Preparation and Redox Properties ofN,N,N-1,3,5-Trialkylated Flavin Derivatives and Their Activity as Redox Catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor.

Novel Synthesis and Oxidizing Ability of Tropylium Ions Annulated with Two 2,4-Dimethylfuro[2,3-d]pyrimidine-1(2H),3(4H)-diones

Convenient preparation of novel tropylium ions annulated with two 2,4-dimethylfuro[2,3-d]pyrimidine-1(2H),3(4H)-diones, 12a(+).BF(4)(-) and 12b(+)().BF(4)(-), consists of a reaction of 2-methoxytropone with dimethylbarbituric acid to give 7,9-dimethyl-3-[1',3'-dimethyl-2'(1'H),4'(3'H),6'(5'H)-trioxopyrimidin-5'-ylidene]cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dione 8 and the following oxidative cyclization by using DDQ or photoirradiation under aerobic conditions. On the basis of the MO calculations, the selectivity of two types of oxidative cyclization reactions of 8 was rationalized. X-ray crystal analyses and MO calculations were carried out to clarify the structural characteristics of 12a(+). BF(4)(-) and 12b(+).BF(4)(-). The stability of cations 12a(+) and 12b(+) is expressed by the pK(R) + values which were determined spectrophotometrically as 8.8 and 8.6. The electrochemical reduction of 12a(+) and 12b(+) exhibited reduction potential at -0.63 and -0.62 (V vs Ag/AgNO(3)), respectively. Reactions of 12a(+)().BF(4)(-) and 12b(+)().BF(4)(-) with some nucleophiles, hydride and diethylamine, were carried out to clarify that the reactivity of 12a(+)().BF(4)(-) and 12b(+).BF(4)(-) was substantially dependent on the annulating position. The oxidizing ability of 12a(+).BF(4)(-) and 12b(+).BF(4)(-) toward alcohols and amines in the autorecycling process was demonstrated as well.

Model systems for flavoenzyme activity: relationships between cofactor structure, binding and redox properties

A series of flavins were synthesized bearing electron-withdrawing and -donating substituents. The electrochemical properties of these flavins in a nonpolar solvent were determined. The recognition of these flavins by a diamidopyridine (DAP) receptor and the effect this receptor has on flavin redox potential was also quantified. It was found that the DAP-flavin binding affinity and the reduction potentials (E(1/2)) for both the DAP-bound and unbound flavins correlated well with functions derived from linear free energy relationships (LFERs). These results provide insight and predictive capability for the interplay of electronics and redox state-specific interactions for both abiotic and enzymatic systems.