Coordination chemistry of mononuclear non-heme iron oxygenase enzymes: probing differential or carboxylate and phenolate ligation through functional synthetic model systems

Thesis (Ph.D.)--Boston University PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at [email protected]. Thank you.Mononuclear non-heme iron oxygenase (MNO) enzymes utilize ferrous iron and dioxygen to perform a variety of thermodynamically challenging reactions at standard temperatures and pressures. The potent oxidizing power of these enzymatic systems has led to increased interest from the bioinorganic and synthetic organic communities. Presented herein is the preparation and characterization of an α-keto acid dependent synthetic system that closely models the active site electronic and dioxygen reactivity properties of the FeII/α-ketoglutarate dependent class of MNH iron oxygenase enzymes. The ferrous complex utilized possesses a facially coordinating N,N,O-donor ligand reminiscent of a common active site motif observed for MNO iron enzymes. The labile coordination sites opposite the ligand framework allow for the ligation of exogenous α-keto acid cofactor as well as the binding and activation of dioxygen. The coordination of exogenous α-keto acid cofactor has been shown to greatly enhance the rate of dioxygen reactivity of the ferrous complex and lead to the catalytic decarboxylation of the cofactor. The enhancement in rate is attributed to the coupling of the dioxygen reduction step to the oxidative decarboxylation of the bound cofactor, which is a thermodynamically favorable process. The oxidative decarboxylation pathway suggests the formation of a high valent iron-oxo intermediate, which has been further supported by the concentration dependence of solvent oxidation during catalysis. The mechanism of dioxygen reactivity was further probed by Hammett analysis using substituted aromatic α-keto acid cofactors. The data presented suggest that the model system prepared proceeds via a biomimetic mechanism capable of catalytic dioxygen activation and substrate oxidation under ambient conditions. Investigation of differential carboxylate and phenolate ligation as it pertains to MNO iron enzymes is also reported. The synthesis and characterization of both ferrous and ferric compounds containing ligands with similar ethylene diamine backbones and either one or two phenolate moities: 2-(((2-(dimethylarnino)ethyl)(methyl)amino)-methyl)phenol (N2O1-Ph) and 2,2'-((ethane-1,2-diylbis(methylazanediyl))bis-(methylene))diphenol (N2O2-Ph). The replacement of carboxylate moiety with a phenolate led to a significant decrease in reduction potential and subsequent enhancement in dioxygen sensitivity. This observation may provide insight into the reactivity of other iron containing enzymes with coordinated tyrosine residues, such as intradiol catechol dioxygenases

Depolymerization of Lignin via Co-pyrolysis with 1,4-Butanediol in a Microwave Reactor

The production of valuable compounds from low cost but abundant residual lignin has proven to be challenging. The lack of effective biochemical lignin depolymerization processes has led many to focus on thermochemical conversion methods. Bench scale microwave pyrolysis of lignin has been performed at 1200 W over the course of 15 min in the presence of a microwave absorber (activated charcoal). The liquid products obtained are composed of smaller polymeric components and moderate yields of monomeric phenols. However, upon the addition of 1,4-butanediol, repolymerization reactions that limit the yield of monomeric and other reduced molecular weight products are inhibited. A great reduction in the average molecular weight (∼85–90% decrease) of the liquid products was observed as well as an overall increase in liquid yield. At the optimized ratio of 2:1 lignin to 1,4-butanediol (w/w), the yield of selected monomeric phenols increased 3-fold to ∼3.4 wt % (based on feedstock), while the yield of monoaromatic hydrocarbons decreased by approximately 90%. The addition of the diol coreactant also led to a significant shift in selectivity toward the production of methoxy-phenols (guaiacols, syringols) over nonmethoxylated alkyl-phenols (phenol, cresols, etc.). The results obtained may lead to the development of novel lignin coprocessing methods

Fluidized Bed Catalytic Pyrolysis of Eucalyptus over HZSM-5: Effect of Acid Density and Gallium Modification on Catalyst Deactivation

Catalytic fast pyrolysis of eucalyptus wood was performed on a continuous laboratory-scale fluidized bed fast pyrolysis system. Catalytic activity was monitored from use of fresh catalyst up to a cumulative biomass/catalyst ratio (B/C) of 4:1 over extruded pellets of three different ZSM-5 catalysts by tracking CO, CO₂, H₂, and C₂H₄ production and bio-oil quality. The catalysts employed were extruded HZSM-5 with two different silica/alumina ratios (30 and 80) as well as one modified with Ga (SiO₂/Al₂O₃ = 30) by ion exchange, which was reduced under H₂ prior to pyrolysis. The deactivation of the catalysts over the course of the experiment was reflected in the decline in deoxygenation activity, following the order HZSM-5 (30) > HZSM-5 (80) > GaZSM-5 (30). HZSM-5 (30) lost most of its activity before a cumulative B/C of 2:1 was reached, while HZSM-5 (80) still showed significant deoxygenated activity at this exposure level. GaZSM-5 (30) still showed deoxygenation activity at B/C of >4:1. The improvement exhibited by HZSM-5 with an increasing SiO₂/Al₂O₃ ratio was attributed to reduced acid site density that decreased the propensity for coke formation as a result of reactions occurring between substrates at adjacent active acid sites. For reduced GaZSM-5, initial dehydrogenation activity aided in the production of aromatics by the olefin oligomerization and aromatization route up to B/C of ∼1.5:1, after which Ga became completely oxidized; however, the oxidized GaZSM-5 catalyst continued to exhibit improved decarbonylation and decarboxylation activities

Structural Analysis of Pyrolytic Lignins Isolated from Switchgrass Fast-Pyrolysis Oil

Structural characterization of lignin extracted from the bio-oil produced by fast pyrolysis of switchgrass (Panicum virgatum) is reported. This is important for understanding the utility of lignin as a chemical feedstock in a pyrolysis-based biorefinery scheme. Pyrolysis induces a variety of structural changes to lignin in addition to reduction in molecular weight. The guaiacol structural units remain largely intact, and some hemicellulose stays covalently linked to the lignin. However, two-dimensional ¹H–¹³C HSQC NMR analysis shows an absence of γ-methylene hydrogens from β-O-4 linkages, implying that rearrangements in the propyl linking chains have occurred. Ferulate and hydroxyl phenol esters are still present in the pyrolyzed lignin, but at lower concentrations than in unpyrolyzed switchgrass lignin

Characterization of Water Coordination to Ferrous Nitrosyl Complexes with <i>fac</i>-N₂O, <i>cis</i>-N₂O₂, and N₂O₃ Donor Ligands

Electron paramagnetic resonance (EPR) experiments were done on a series of <i>S</i> = ³/₂ ferrous nitrosyl model complexes prepared with chelating ligands that mimic the 2-His-1-carboxylate facial triad iron binding motif of the mononuclear nonheme iron oxidases. These complexes formed a comparative family, {FeNO}⁷(N₂O_<i>x</i>)­(H₂O)_3–<i>x</i> with <i>x</i> = 1–3, where the labile coordination sites for the binding of NO and solvent water were fac for <i>x</i> = 1 and cis for <i>x</i> = 2. The continuous-wave EPR spectra of these three complexes were typical of high-spin <i>S</i> = ³/₂ transition-metal ions with resonances near <i>g</i> = 4 and 2. Orientation-selective hyperfine sublevel correlation (HYSCORE) spectra revealed cross peaks arising from the protons of coordinated water in a clean spectral window from <i>g</i> = 3.0 to 2.3. These cross peaks were absent for the {FeNO}⁷(N₂O₃) complex. HYSCORE spectra were analyzed using a straightforward model for defining the spin Hamiltonian parameters of bound water and showed that, for the {FeNO}⁷(N₂O₂)­(H₂O) complex, a single water conformer with an isotropic hyperfine coupling, <i>A</i>_iso = 0.0 ± 0.3 MHz, and a dipolar coupling of <i>T</i> = 4.8 ± 0.2 MHz could account for the data. For the {FeNO}⁷(N₂O)­(H₂O)₂ complex, the HYSCORE cross peaks assigned to coordinated water showed more frequency dispersion and were analyzed with discrete orientations and hyperfine couplings for the two water molecules that accounted for the observed orientation-selective contour shapes. The use of three-pulse electron spin echo envelope modulation (ESEEM) data to quantify the number of water ligands coordinated to the {FeNO}⁷ centers was explored. For this aspect of the study, HYSCORE spectra were important for defining a spectral window where empirical integration of ESEEM spectra would be the most accurate

Studies of iron (II) and iron (III) complexes with fac-N2O, cis-N2O2 and N2O3 donor ligands: models for the 2-His 1-carboxylate motif of non-heme iron monooxygenases

Enzymes in the oxygen-activating class of mononuclear non-heme iron oxygenases (MNOs) contain a highly conserved iron center facially ligated by two histidine nitrogen atoms and one carboxylate oxygen atom that leave one face of the metal center (three binding sites) open for coordination to cofactor, substrate, and/or dioxygen. A comparative family of [FeII/III(N2On)(L)4−n)]±x, n = 1–3, L = solvent or Cl−, model complexes, based on a ligand series that supports a facially ligated N,N,O core that is then modified to contain either one or two additional carboxylate chelate arms, has been structurally and spectroscopically characterized. EPR studies demonstrate that the high-spin d5 FeIIIg = 4.3 signal becomes more symmetrical as the number of carboxylate ligands decreases across the series Fe(N2O3), Fe(N2O2), and Fe(N2O1), reflecting an increase in the E/D strain of these complexes as the number of exchangeable/solvent coordination sites increases, paralleling the enhanced distribution of electronic structures that contribute to the spectral line shape. The observed systematic variations in the FeII–FeIII oxidation–reduction potentials illustrate the fundamental influence of differential carboxylate ligation. The trend towards lower reduction potential for the iron center across the [FeIII(N2O1)Cl3]−, [FeIII(N2O2)Cl2]− and [FeIII(N2O3)Cl]− series is consistent with replacement of the chloride anions with the more strongly donating anionic O-donor carboxylate ligands that are expected to stabilize the oxidized ferric state. This electrochemical trend parallels the observed dioxygen sensitivity of the three ferrous complexes (FeII(N2O1) \u3c FeII(N2O2) \u3c FeII(N2O3)), which form μ-oxo bridged ferric species upon exposure to air or oxygen atom donor (OAD) molecules. The observed oxygen sensitivity is particularly interesting and discussed in the context of α-ketoglutarate-dependent MNO enzyme mechanisms