Synthesis, X-ray Structures, Electronic Properties, and O\u3csub\u3e2\u3c/sub\u3e/NO Reactivities of Thiol Dioxygenase Active-Site Models

Mononuclear non-heme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (Ph2TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate l-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substrate-bound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(Ph2TIP)(LS,N)]BPh4 complexes, where LS,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O2 to yield the corresponding sulfinic acid (RSO2H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O2-bound intermediates. Treatment of the TDO models with nitric oxide (NO)—a well-established surrogate of O2—led to a mixture of high-spin and low-spin {FeNO}7 species at low temperature (−70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O2/NO reactivity studies were carried out using previously reported TDO models that feature an anionic hydrotris(3-phenyl-5-methyl-pyrazolyl)borate (Ph,MeTp–) ligand. Though the O2 reactivities of the Ph2TIP- and Ph,MeTp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe–S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO

Tracing uptake and assimilation of NO2 in spruce needles with 13N

For the first time, spruce shoots (Picea abies [L.] Karst.) were fumigated in vivo with 13N-labelled NO2 with the aim of elucidating the mechanism of NO2− trapping in the apoplast of the substomatal cavity. Uptake by the needles could be monitored on-line, and a quantitative analysis of the activity records delivered a deposition velocity in agreement with the common dry deposition estimates and ruled out rapid export processes. A fast extraction procedure was applied which revealed that NO2 did not produce any detectable traces of nitrite. In needles in which the enzymes of nitrate reduction were not induced by prior fumigation with NO2, incorporation of NO2 was partially inhibited as compared to the fully induced shoots which took up and assimilated NO2 as expected from a constant influx. The only labelled inorganic species found in the extracts was nitrate (60%), whereas the rest of the label (40%) was assimilated organic nitrogen.A quantitative analysis of the data shows that the reaction of NO2 in the apoplast yields at least three times more nitrate than nitrite, so that the existing models about the apoplastic trapping reaction, disproportionation or antioxidant scavenging, which both postulate substantial production of nitrite, have to be reconsidere

Spectroscopic and Computational Comparisons of Thiolate-Ligated Ferric Nonheme Complexes to Cysteine Dioxygenase: Second-Sphere Effects on Substrate (Analogue) Positioning

Parallel spectroscopic and computational studies of iron(III) cysteine dioxygenase (CDO) and synthetic models are presented. The synthetic complexes utilize the ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (Ph2TIP), which mimics the facial three-histidine triad of CDO and other thiol dioxygenases. In addition to the previously reported [FeII(CysOEt)(Ph2TIP)]BPh4 (1; CysOEt is the ethyl ester of anionic l-cysteine), the formation and crystallographic characterization of [FeII(2-MTS)(Ph2TIP)]BPh4 (2) is reported, where the methyl 2-thiosalicylate anion (2-MTS) resembles the substrate of 3-mercaptopropionate dioxygenase (MDO). One-electron chemical oxidation of 1 and 2 yields ferric species that bind cyanide and azide anions, which have been used as spectroscopic probes of O2 binding in prior studies of FeIII-CDO. The six-coordinate FeIII-CN and FeIII-N3 adducts are examined with UV–vis absorption, electron paramagnetic resonance (EPR), and resonance Raman (rRaman) spectroscopies. In addition, UV–vis and rRaman studies of cysteine- and cyanide-bound FeIII-CDO are reported for both the wild-type (WT) enzyme and C93G variant, which lacks the Cys-Tyr cross-link that is present in the second coordination sphere of the WT active site. Density functional theory (DFT) and ab initio calculations are employed to provide geometric and electronic structure descriptions of the synthetic and enzymatic FeIII adducts. In particular, it is shown that the complete active space self-consistent field (CASSCF) method, in tandem with n-electron valence state second-order perturbation theory (NEVPT2), is capable of elucidating the structural basis of subtle shifts in EPR g values for low-spin FeIII species. Synopsis The geometric and electronic structures of thiolate-ligated FeIII complexes of relevance to the active sites of thiol dioxygenases have been elucidated with spectroscopic and computational methods. Data collected for the synthetic models are compared to those previously obtained for the analogous enzymatic species, and newly collected resonance Raman spectra of Cys- and CN-bound FeIII-CDO are presented. The combined enzymatic/synthetic approach reveals that second-sphere residues perturb the positions of substrate (analogues) coordinated to the nonheme iron site of CDO

CO2 dissolution and design aspects of a multiorifice oscillatory baffled column

Dissolution of CO2 in water was studied for a batch vertical multiorifice baffled column (MOBC) with varying orifice diameters (d0) of 6.4-30 mm and baffle open area (α) of 15-42%. Bubble size distributions (BSDs) and the overall volumetric CO2 mass transfer coefficient (KLa) were experimentally evaluated for very low superficial gas velocities, UG of 0.12-0.81 mm s-1, using 5% v/v CO2 in the inlet gas stream at a range of fluid oscillations (f = 0-10 Hz and x0 = 0-10 mm). Remarkably, baffles presenting large do = 30 mm and α = 36%, therefore in the range typically found for single-orifice oscillatory baffled columns, were outperformed with respect to BSD control and CO2 dissolution by the other baffle designs or the same aerated column operating without baffles or fluid oscillations. Flow visualization and bubble tracking experiments also presented in this study established that a small do of 10.5 mm combined with a small value of α = 15% generates sufficient, strong eddy mixing capable of generating and trapping an extremely large fraction of microbubbles in the MOBC. This resulted in increased interfacial area yielding KLa values up to 65 ± 12 h-1 in the range of the UG tested, representing up to 3-fold increase in the rate of CO2 dissolution when compared to the unbaffled, steady column. In addition, a modi fied oscillatory Reynolds number, Re′o and Strouhal number, St' were presented to assist on the design and scale-up of gas-liquid systems based on multiorifice oscillatory ba ffled columns. This work is relevant to gas-liquid or multiphase chemical and biological systems relying on efficient dissolution of gaseous compounds into a liquid medium.BBSRC -European Commissio

Absorption and luminescence spectroscopy of Zn2SiO4 willemite crystals doped with Co2+

The polarized absorption spectra of Zn2SiO4 willemite crystals doped with Co2+ consist of three band systems in the near-infrared and visible spectral region centered at 3800, 7000, and 17000 cm-1, respectively. In the tetrahedral approximation they are assigned to the d → d transitions 4A2 → 4T2, 4T1(4F), and 4T1(4P), respectively. The crystal field parameter 10Dq is 4000 cm-1, and the Racah parameters B and C are 740 and 3330 cm-1, respectively. From the fine structure in the origin region of the 4A2 → 4T2 absorption band it follows that the 4A2 ground-state splittings of Co2+ occupying the two crystallographically inequivalent Zn2+ sites of C1 symmetry are 13 and 15 cm-1, respectively. Only a weak emission originating at 15335 cm-1 is observed upon 4A2 → 4T1(4P) photoexcitation. It is assigned to the 2E → 4A2 sharp-line luminescence of Co2+ located in a minority site. No luminescence is observed from regularly incorporated Co2+ ions

Optical spectroscopy of Ni2+ doped crystals of Zn2SiO4

The polarized 15 K absorption spectra of Zn2SiO4 doped with Ni2+ consist of three-band systems in the near-infrared (NIR) and visible (VIS) spectral regions centered at 4700, 8500, and 16500 cm-1, respectively. In the tetrahedral approximation they are assigned to the d → d transitions 3T1 → 3T2, 3A2, and 3T1(P), respectively. The spectra are interpreted in terms of a Td →C2v site distortion and analyzed using the angular overlap model. The ligand field parameter 10Dq is 4500 cm-1, and the Racah parameters B and C are 780 and 3630 cm-1, respectively. By the inclusion of spin-orbit coupling in the AOM calculation it is possible to assign the rich fine structure in the origin regions of the 3T1 → 3T2, 3A2 absorptions to Ni2+ ions occupying the two crystallographically inequivalent Zn2+ sites of Zn2SiO4. © 1997 Elsevier Science B.V

Spectroscopic and Computational Studies of a Small-Molecule Functional Mimic of Iron Superoxide Dismutase, Iron 2,6-Diacetylpyridinebis(semioxamazide)

Iron 2,6-diacetylpyridinebis­(semioxamazide) (Fe­(dapsox)) is a heptacoordinate pentagonal bipyramidal, functional mimic of iron-dependent superoxide dismutase that has been well-characterized on the basis of kinetics and mechanistic studies; however, prior to our studies, its electronic structure had yet to be examined. This paper details our initial characterization of Fe­(dapsox) in both its reduced and oxidized states, by electronic absorption (Abs) and low-temperature magnetic circular dichroism spectroscopies. Density functional theory (DFT) geometry optimizations have yielded models in good agreement with the published crystal structures. Time-dependent DFT and INDO/S-CI calculations performed on these models successfully reproduce the experimental Abs spectra and identify intense, low-energy transitions in the reduced complex (Fe^II(H₂dapsox)) as metal-to-ligand charge transfer transitions, suggesting the presence of π-backbonding in this complex. This backbonding, along, with the proton uptake accompanying metal ion reduction, provides a compelling mechanism by which the metal-centered redox potential is correctly tuned for catalytic superoxide disproportionation