| 1. |
- Eilers, Gerriet, et al.
(författare)
-
Ligand versus metal protonation of an iron hydrogenase active site mimic
- 2007
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 13:25, s. 7075-7084
-
Tidskriftsartikel (refereegranskat)abstract
- The protonation behavior of the iron hydrogenase active-site mimic [Fe2(u-adt)(CO)4(PMe3)2] (1; adt=N-benzyl-azadithiolate) has been investigated by spectroscopic, electrochemical, and computational methods. The combination of an adt bridge and electron-donating phosphine ligands allows protonation of either the adt nitrogen to give [Fe2(μ-Hadt)(CO)4(PMe3)2]+ ([1H]+), the Fe-Fe bond to give [Fe2-(μ-adt)(μ-H)(CO)4(PMe3)2]+ ([1Hy]+), or both sites simultaneously to give [Fe2(μ-Hadt)(μ-H)(CO)4(PMe3)2]2+ ([1HHy]2+). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, 1H, and 31PNMR spectroscopy. The solution structures of all protonation states feature a basal/basal orientation of the phosphine ligands, which contrasts with the basal/apical structure of 1 in the solid state. Density functional calculations have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1H]+ (pKa =12) is the initial, metastable protonation product while the hydride [1Hy]+ (pKa=15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1H]+ to [1Hy]+ does not occur spontaneously. However, it can be catalyzed by HCl (k=2.2M-1s-1), which results in the selective formation of cation [1Hy]+. The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pKa=8) and the ammonium proton (pKa=5) of the doubly protonated cationic complex [1HHy]2+ are considerably more acidic than in the singly protonated analogues. The formation of dication [1HHy]2+ from cation [1H]+ is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k= 0.15 M-1s-1), while the dication is formed substantially faster (k > 102 M-1 s-1) with hydrobromic acid. Electrochemically, 1 undergoes irreversible reduction at -2.2V versus ferrocene, and this potential shifts to -1.6, - 1.1, and -1.0 V for the cationic complexes [1H]+, [1Hy]+, and [1HHy]2+, respectively, upon protonation. The doubly protonated form [1HHy]2+ is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.
|
|
| 2. |
- Elfgren, Lennart, et al.
(författare)
-
Fracture mechanics approaches to modeling the pullout of anchor bolts
- 1992
-
Ingår i: Concrete Design Based on Fracture Mechanics. - Detroit : American Concrete Institute. ; , s. 63-77
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Summarizes and presents preliminary results of a round-robin analysis of anchor bolts organized by RILEM TC 90-FMA, Fracture Mechanics of Concrete-Applications. The analyses employed finite element models using fracture mechanics approaches for the most part. The assumptions used in establishing the material/cracking models varied with investigator and included linear elastic fracture mechanics (LEFM), the fictitious crack model (FCM) with linear softening or non-linear softening, a fixed crack line, a variable crack line with non-rotating cracks or rotating cracks. Crack propagation was determined using Mode I parameters, in some cases, with consideration of mixed mode behavior.
|
|
| 3. |
- Erdem, Oezlen F., et al.
(författare)
-
A Model of the [FeFe] Hydrogenase Active Site with a Biologically Relevant Azadithiolate Bridge : A Spectroscopic and Theoretical Investigation
- 2011
-
Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 50:6, s. 1439-1443
-
Tidskriftsartikel (refereegranskat)abstract
- Convincing evidence for the presence of a nitrogen atom in the dithiolate bridge of the active site of native [FeFe] hydrogenases (B) is provided by a spectroscopic, electrochemical, and theoretical study of a well-characterized structural mimic of the [FeFe] hydrogenase subcluster (picture: 14N matched-HYSCORE spectrum of the model compound A). This result should help to understand the mechanism of dihydrogen conversion and production.
|
|
| 4. |
- Hansen-Schwartz, Jacob, et al.
(författare)
-
Protein kinase mediated upregulation of endothelin A, endothelin B and 5-hydroxytryptamine 1B/1D receptors during organ culture in rat basilar artery.
- 2002
-
Ingår i: British Journal of Pharmacology. - : Wiley. - 1476-5381 .- 0007-1188. ; 137:1, s. 118-126
-
Tidskriftsartikel (refereegranskat)abstract
- 1 Organ culture has been shown to upregulate both endothelin (ET) and 5-hydroxytryptamine 1B/1D (5-HT(1B/1D)) receptors in rat cerebral arteries. The purpose of the present study was to investigate the involvement of protein kinases, especially protein kinases C (PKC) and A (PKA) in this process. 2 The effect of inhibiting protein kinases during organ culture with staurosporine (unspecific protein kinase inhitor), RO 31-7549 (specific inhibitor of classical PKC's) and H 89 (specific inhibitor of PKA) was examined using in vitro pharmacological examination of cultured vessel segments with ET-1 (unspecific ET(A) and ET(B) agonist), S6c (specific ET(B) agonist) and 5-CT (5-HT(1) agonist). Levels of mRNA coding for the ET(A), ET(B), 5-HT(1B) and 5-HT(1D) receptors were analysed using real-time RT-PCR. 3 Classical PKC's are critically involved in the appearance of the ET(B) receptor; co-culture with RO 31-7549 abolished the contractile response (6.9+/-1.8%) and reduced the ET(B) receptor mRNA by 44+/-4% as compared to the cultured control. Correlation between decreased ET(B) receptor mRNA and abolished contractile function indicates upstream involvement of PKC. 4 Inhibition of PKA generally had an enhancing effect on the induced changes giving rise to a 7-25% increase in E(max) in response to ET-1, S6c and 5-CT as compared to the cultured control. 5 Staurosporine inhibited the culture induced upregulation of the response of both the ET(A) and the 5-HT(1B/1D) receptors, but had no significant effect on the mRNA levels of these receptors. This lack of correlation indicates an additional downstream involvement of protein kinases. British Journal of Pharmacology (2002) 137, 118-126. doi:10.1038/sj.bjp.0704838
|
|
| 5. |
- Johansson, Magnus J, et al.
(författare)
-
Desymmetrization of prochiral phosphanes using derivatives of (-)-cytisine
- 2004
-
Ingår i: European Journal of Organic Chemistry. - : Wiley. - 1434-193X .- 1099-0690. ; :9, s. 1894-1896
-
Tidskriftsartikel (refereegranskat)abstract
- yHistorically, (-)-sparteine-sec-BuLi has been used to desym-metrize prochiral phosphanes. In this report, derivatives of an alkaloid extracted from the seeds of Laburnum anagyro-ides have been utilized to mimic (+)-sparteine, which is not readily available. In several cases, the enantioselectivities achieved with the (+)-Sparteine surrogates outperformed (-)-sparteine itself in the deprotonation of alkyl-substituted (as well as aryl-substituted) prochiral phosphane derivatives. In addition, use of these surrogates allows a new methodology for a chiral switch in phosphorus chemistry. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).
|
|
| 6. |
- Johansson, Magnus J, et al.
(författare)
-
New chiral amine ligands in the desymmetrization of prochiral phosphine boranes
- 2004
-
Ingår i: Tetrahedron: asymmetry. - : Elsevier BV. - 0957-4166 .- 1362-511X. ; 15:22, s. 3531-3538
-
Tidskriftsartikel (refereegranskat)abstract
- P-chirogenic phosphine ligands can be prepared via desymmetrization of prochiral phosphine boranes using s-BuLi.(-)-sparteine complexes. One limitation of this method, however, has been that (+)-sparteine is not easily accessible. Herein, we show that derivatives of another alkaloid, (-)-cytisine, are useful surrogates for (+)-sparteine in the desymmetrization of prochiral phenyl-, cyclohexyl- and tert-butyl dimethyl phosphine boranes, yielding chiral phosphine boranes in up to 92% ee. Other chiral diamines were also tested but did not give as high enantioselectivity as the (-)-cytisine derivatives.
|
|
| 7. |
- Kaur-Ghumaan, Sandeep, et al.
(författare)
-
Catalytic Hydrogen Evolution from Mononuclear Iron(II) Carbonyl Complexes as Minimal Functional Models of the [FeFe] Hydrogenase Active Site
- 2010
-
Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 49:43, s. 8033-8036
-
Tidskriftsartikel (refereegranskat)abstract
- How much iron does it take? Mononuclear complexes [FeII(3,6-R2bdt)(CO)2(PMe3)2] (bdt=1,2-C6H4(S−)2; R=H, Cl) can be reversibly protonated at the sulfur ligands, can catalyze the electrochemical reduction of protons, and are thus minimal functional models of the [FeFe] hydrogenases (see scheme). DFT calculations show that cleavage of an FeS bond leads to the generation of a free coordination site, which is crucial for the formation of hydrides that are key intermediates in the generation of hydrogen.
|
|
| 8. |
- Kositzki, Ramona, et al.
(författare)
-
Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry
- 2017
-
Ingår i: Dalton Transactions. - : ROYAL SOC CHEMISTRY. - 1477-9226 .- 1477-9234. ; 46:37, s. 12544-12557
-
Tidskriftsartikel (refereegranskat)abstract
- Synthetic diiron compounds of the general formula Fe-2(mu-S2R)(CO)(n)(L)(6-n) (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and K beta main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(I) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds.
|
|
| 9. |
- Leidel, Nils, et al.
(författare)
-
Electronic Structure of an [FeFe] Hydrogenase Model Complex in Solution Revealed by X-ray Absorption Spectroscopy Using Narrow-Band Emission Detection
- 2012
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:34, s. 14142-14157
-
Tidskriftsartikel (refereegranskat)abstract
- High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe-2(mu-adt)(CO)(4)(PMe3)(2)] (1, adt = S-CH2-(NCH2Ph)-CH2-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1(powder)), in MeCN solution (1'), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy protonation of the Fe-Fe bond to form a bridging metal hydride), relations between the molecular structures and the electronic configurations were determined. EXAFS analysis and DFT geometry optimization suggested prevailing rotational isomers in MeCN, which were similar to the crystal structure or exhibited rotation of the (CO) ligands at Fe1 (1(CO), 1Hy(CO)) and in addition of the phenyl ring (1H(CO,ph), 1HHy(CO,ph)), leading to an elongated solvent-exposed Fe-Fe bond. Isomer formation, adt-N protonation, and hydride binding caused spectral changes of core-to-valence (pre-edge of the Fe K-shell absorption) and of valence-to-core (K beta(2,5) emission) electronic transitions, and of K alpha RIXS data, which were quantitatively reproduced by DFT. The study reveals (1) the composition of molecular orbitals, for example, with dominant Fe-d character, showing variations in symmetry and apparent oxidation state at the two Fe ions and a drop in MO energies by similar to 1 eV upon each protonation step, (2) the HOMO-LUMO energy gaps, of similar to 2.3 eV for 1(powder) and similar to 2.0 eV for 1', and (3) the splitting between iron d(z(2)) and d(x(2-)y(2)) levels of similar to 0.5 eV for the nonhydride and similar to 0.9 eV for the hydride states. Good correlations of reduction potentials to LUMO energies and oxidation potentials to HOMO energies were obtained. Two routes of facilitated bridging hydride binding thereby are suggested, involving ligand rotation at Fe1 for 1Hy(CO) or adt-N protonation for 1HHy(CO,ph). XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.
|
|
| 10. |
- Löscher, Simone, et al.
(författare)
-
Facilitated hydride binding in an Fe-Fe hydrogenase active-site biomimic revealed by X-ray absorption spectroscopy and DFT calculations
- 2007
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 46:26, s. 11094-11105
-
Tidskriftsartikel (refereegranskat)abstract
- Iron-only hydrogenases are high-efficiency biocatalysts for the synthesis and cleavage of molecular hydrogen. Their active site is a diiron center, which carries CO and CN ligands. Remarkably, the two iron atoms likely are connected by a non-protein azadithiolate (adt = S-CH2-NH-CH2-S). To dwell on the role of the adt in H-2 catalysis, a specific biomimetic diiron compound, 1 = [Fe-2(mu-adt-CH2-Ph)(CO)(4)(PMe3)(2)], with unprecedented positive reduction potential, has been synthesized and crystallized previously. It comprises two protonation sites, the N-benzyl-adt nitrogen that can hold a proton (H) and the Fe-Fe bond that will formally carry a hydride (Hy). We investigated changes in the solution structure of 1 in its four different protonation states (1', [1H](+), [1HHy](2+), and [1Hy](+)) by X-ray absorption spectroscopy at the iron K-edge. EXAFS reveals that already protonation at the adt nitrogen atom causes a change of the ligand geometry involving a significant lengthening of the Fe-Fe distance and CO and PMe3 repositioning, respectively, thereby facilitating the subsequent binding of a bridging hydride. Hydride binding clearly is discernible in the XANES spectra of [1HHy](2+) and [1Hy](+). DIFT calculations are in excellent agreement with the experimentally derived structural parameters and provide complementary insights into the electronic structure of the four protonation states. In the iron-only hydrogenases, protonation of the putative adt ligand may cause the bridging CO to move to a terminal position, thereby preparing the active site for hydride binding en route to H2 formation.
|
|
