Fungal lignin peroxidase does not produce the veratryl alcohol cation radical as a diffusible ligninolytic oxidant

11p.-5 fig.-1 tab. + 13 p.- Supporting InformationPeroxidases are considered essential agents of lignin degradation by white-rot basidiomycetes. However, low-molecular-weight oxidants likely have a primary role in lignin breakdown because many of these fungi delignify wood before its porosity has sufficiently increased for enzymes to infiltrate. It has been proposed that lignin peroxidases (LPs, EC 1.11.1.14) fulfill this role by oxidizing the secreted fungal metabolite veratryl alcohol (VA) to its aryl cation radical (VA+•), releasing it to act as a one-electron lignin oxidant within woody plant cell walls. Here, we attached the fluorescent oxidant sensor BODIPY 581/591 throughout beads with a nominal porosity of 6 kDa and assessed whether peroxidase-generated aryl cation radical systems could oxidize the beads. As positive control, we used the 1,2,4,5-tetramethoxybenzene (TMB) cation radical, generated from TMB by horseradish peroxidase. This control oxidized the beads to depths that increased with the amount of oxidant supplied, ultimately resulting in completely oxidized beads. A reaction-diffusion computer model yielded oxidation profiles that were within the 95% confidence intervals for the data. By contrast, bead oxidation caused by VA and the LPA isozyme of Phanerochaete chrysosporium was confined to a shallow shell of LP-accessible volume at the bead surface, regardless of how much oxidant was supplied. This finding contrasted with the modeling results, which showed that if the LP/VA system were to release VA+•, it would oxidize the bead interiors. We conclude that LPA releases insignificant quantities of VA+• and that a different mechanism produces small ligninolytic oxidants during white rot.This work was supported by United States Department of Energy, Office of Biological and Environmental Research Grant DE-SC0006929 (to K. E. H.,C. J. H.,and C. G. H.)Peer reviewe

Initial Report on Molecular and Electronic Structure of Spherical Multiferrocenyl/tin(IV) (Hydr)oxide [(FcSn)12O14(OH)6]X2 Clusters

Two spherical organic-inorganic ferrocene-tin (hydr)oxide clusters of general formula [(FcSn)12O14(OH)6]X2 (Fc = ferrocenyl, X = nitroso-dicyanmethanide, DCO- and benzoylcyanoxime, PCO- anions) were prepared by the direct hydrolysis of Fc2SnCl2 or FcSnCl3 precursors in the presence of light- and thermally stable Ag(DCO) or Ag(PCO) salts. Molecular structures of FcSnCl3Py2 (1), Fc2SnCl2Py2 (2), [(FcSn)12O14(OH)6](DCO)2 (3), and [(FcSn)12O14(OH)6](PCO)2 (4) were investigated by X-ray crystallography. Density function theory (DFT) and time-dependent density functional theory (TDDFT) calculations were conducted on FcSnCl3Py2, Fc2SnCl2Py2, and [(FcSn)12O14(OH)6]2+ compounds in order to elaborate electronic structures and assign transitions in UV-vis spectra of these systems. The DFT and TDDFT calculations suggest that the organometallic substituents in the [(FcSn)12O14(OH)6]2+ core are rather isolated from each other

Synthesis, Redox Properties, and Electronic Coupling in the Diferrocene Aza-dipyrromethene and azaBODIPY Donor–Acceptor Dyad with Direct Ferrocene−α-Pyrrole Bond

3,3′-Diferrocenylazadipyrromethene (<b>3</b>) and corresponding difluoroboryl (azaBODIPY) complex (<b>4</b>) were synthesized in several steps from ferrocenecarbaldehyde, following the well-explored chalcone-type synthetic approach. The novel diiron complexes, in which ferrocene groups are directly connected to the α-pyrrolic positions were characterized by a variety of spectroscopic techniques, electrochemistry, spectroelectrochemistry, and X-ray crystallography, while their electronic structure, redox properties, and UV–vis spectra were correlated with the density functional theory (DFT) and time-dependent DFT calculations

Syntheses and Excitation Transfer Studies of Near-Orthogonal Free-Base Porphyrin–Ruthenium Phthalocyanine Dyads and Pentad

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor–acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (<i>k</i>_ENT) was monitored by performing transient absorption and emission measurements using pump–probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental <i>k</i>_ENT values on the order of 10¹⁰–10¹¹ s^–1, which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed

Synthesis and Charge-Transfer Dynamics in a Ferrocene-Containing Organoboryl aza-BODIPY Donor–Acceptor Triad with Boron as the Hub

A <i>N</i>,<i>N</i>′-bis­(ferroceneacetylene)­boryl complex of 3,3′-diphenylazadiisoindolylmethene was synthesized by the reaction of an <i>N</i>,<i>N</i>′-difluoroboryl complex of 3,3′-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex was characterized by a variety of spectroscopic techniques, electrochemistry, and ultrafast time-resolved methods. Spectroscopy and redox behavior was correlated with the density functional theory (DFT) and time-dependent DFT calculations. An unexpected degree of coupling between the two Fc ligands was observed. Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7 ± 0.1 ps) photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a lifetime of 4.8 ± 0.1 μs

Redox and Photoinduced Electron-Transfer Properties in Short Distance Organoboryl Ferrocene-Subphthalocyanine Dyads

Reaction between ferrocene lithium or ethynylferrocene magnesium bromide and (chloro)­boronsubphthalocyanine leads to formation of ferrocene- (<b>2</b>) and ethynylferrocene- (<b>3</b>) containing subphthalocyanine dyads with a direct organometallic B–C bond. New donor–acceptor dyads were characterized using UV–vis and magnetic circular dichroism (MCD) spectroscopies, NMR method, and X-ray crystallography. Redox potentials of the rigid donor–acceptor dyads <b>2</b> and <b>3</b> were studied using the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches and compared to the parent subphthalocyanine <b>1</b> and conformationally flexible subphthalocyanine ferrocenenylmethoxide (<b>4</b>) and ferrocenyl carboxylate (<b>5</b>) dyads reported earlier. It was found that the first oxidation process in dyads <b>2</b> and <b>3</b> is ferrocene-centered, while the first reduction as well as the second oxidation are centered at the subphthalocyanine ligand. Density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to probe the electronic structures and explain the UV–vis and MCD spectra of complexes <b>1</b>–<b>5</b>. DFT-PCM calculations suggest that the LUMO, LUMO+1, and HOMO-3 in new dyads <b>2</b> and <b>3</b> are centered at the subphthalocyanine ligand, while the HOMO to HOMO-2 in both dyads are predominantly ferrocene-centered. TDDFT-PCM calculations on compounds <b>1</b>–<b>5</b> are indicative of the π → π* transitions dominance in their UV–vis spectra, which is consistent with the experimental data. The excited state dynamics of the parent subphthalocyanine <b>1</b> and dyads <b>2</b>–<b>5</b> were investigated using time-resolved transient spectroscopy. In the dyads <b>2</b>–<b>5</b>, the initially excited state is rapidly (<2 ps) quenched by electron transfer from the ferrocene ligand. The lifetime of the charge transfer state demonstrates a systematic dependence on the structure of the bridge between the subphthalocyanine and ferrocene

Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs

Stepwise modification of the methyl groups at the α positions of BODIPY <b>1</b> was used for preparation of a series of mono- (<b>2</b>, <b>4</b>, and <b>6</b>) and diferrocene (<b>3</b>) substituted donor–acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor–acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes <b>2</b>–<b>4</b> and <b>6</b> confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad <b>3</b> is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV–vis and redox properties of organometallic compounds <b>2</b>–<b>4</b> and <b>6</b>. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY <b>1</b> and dyads <b>2</b>–<b>4</b> and <b>6</b> were investigated using time-resolved transient spectroscopy. In all organometallic dyads <b>2</b>–<b>4</b> and <b>6</b> the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs <b>2</b>–<b>4</b> and <b>6</b>

Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs

Stepwise modification of the methyl groups at the α positions of BODIPY <b>1</b> was used for preparation of a series of mono- (<b>2</b>, <b>4</b>, and <b>6</b>) and diferrocene (<b>3</b>) substituted donor–acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor–acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes <b>2</b>–<b>4</b> and <b>6</b> confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad <b>3</b> is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV–vis and redox properties of organometallic compounds <b>2</b>–<b>4</b> and <b>6</b>. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY <b>1</b> and dyads <b>2</b>–<b>4</b> and <b>6</b> were investigated using time-resolved transient spectroscopy. In all organometallic dyads <b>2</b>–<b>4</b> and <b>6</b> the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs <b>2</b>–<b>4</b> and <b>6</b>

Initial Report on Molecular and Electronic Structure of Spherical Multiferrocenyl/tin(IV) (Hydr)oxide [(FcSn)₁₂O₁₄(OH)₆]X₂ Clusters

Two spherical organic–inorganic ferrocene-tin (hydr)­oxide clusters of general formula [(FcSn)₁₂O₁₄­(OH)₆]­X₂ (Fc = ferrocenyl, X = nitroso-dicyanmethanide, DCO^– and benzoylcyanoxime, PCO^– anions) were prepared by the direct hydrolysis of Fc₂SnCl₂ or FcSnCl₃ precursors in the presence of light- and thermally stable Ag­(DCO) or Ag­(PCO) salts. Molecular structures of FcSnCl₃Py₂ (<b>1</b>), Fc₂SnCl₂Py₂ (<b>2</b>), [(FcSn)₁₂O₁₄­(OH)₆]­(DCO)₂ (<b>3</b>), and [(FcSn)₁₂O₁₄­(OH)₆]­(PCO)₂ (<b>4</b>) were investigated by X-ray crystallography. Density function theory (DFT) and time-dependent density functional theory (TDDFT) calculations were conducted on FcSnCl₃Py₂, Fc₂SnCl₂Py₂, and [(FcSn)₁₂O₁₄­(OH)₆]²⁺ compounds in order to elaborate electronic structures and assign transitions in UV–vis spectra of these systems. The DFT and TDDFT calculations suggest that the organometallic substituents in the [(FcSn)₁₂O₁₄­(OH)₆]²⁺ core are rather isolated from each other