Many metalloenzymes have active sites composed of metals coordinated to S-donor ligands. Among the examples are Ni, Fe hydrogenases. These enzymes catalyze the two-electron redox chemistry of H2 and are believed to contain a heterodinuclear active site composed of a Ni center bridged to an Fe center by cysteinate ligands. The possible roles of the thiolate ligands in constructing the active site, in the redox chemistry of the active site, and in the binding of H+ are discussed in the context of an overview of the results of physical studies of the enzyme and dinuclear model compounds.
Electroabsorption studies of a diruthenium tetraiminoethylenedimacrocycle show that very little change in molecular dipole moment or polarizability accompanies excitation of the high-intensity band found in the near-infrared region. This supports earlier assignments of the band as a delocalized intervalence transition. These results, along with resonance Raman spectra of analogous Fe species, provide evidence for a three-center bonding scheme for valence delocalization.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSynthesis, Characterization, and Alkali Metal Stability Constants of a New Bis(phosphotriester) Macrobicyclic Polyether CryptandChristian B. Allan and Larry O. SpreerCite this: J. Org. Chem. 1994, 59, 25, 7695–7700Publication Date (Print):December 1, 1994Publication History Published online1 May 2002Published inissue 1 December 1994https://pubs.acs.org/doi/10.1021/jo00104a026https://doi.org/10.1021/jo00104a026research-articleACS PublicationsRequest reuse permissionsArticle Views243Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts
Neurotrophic corneal defects are difficult to heal and all too often lead to scarring and vision loss. Medical management is often of limited success. We describe the results of nine patients (ages 37–84) with chronic nonhealing neurotrophic corneal epithelial defects who were treated with thymosin beta 4 (Tβ4) sterile eye drops for 28 or 49 days with a follow‐up period of 30 days. Those with geographic defects (six patients) showed dramatic healing without clinically significant neovascularization. Stromal thinning was observed in one patient. Three patients with punctate epithelial defects did not have a demonstrable change in their clinical findings. Reduced ocular irritation was reported by all patients soon after treatment initiation. Results from these compassionate use cases indicate that Tβ4 may provide a novel, topical approach to wound healing in chronic nonhealing neurotrophic corneal ulcers.
Structural information obtained from the analysis of nickel K-edge X-ray absorption spectroscopic data of [NiFe]hydrogenases from Desulfovibrio gigas, Thiocapsa roseopersicina, Desulfovibrio desulfuricans (ATCC 27774), Escherichia coli (hydrogenase-1), Chromatium vinosum, and Alcaligenes eutrophus H16 (NAD+-reducing, soluble hydrogenase), poised in different redox states, is reported. The data allow the active-site structures of enzymes from several species to be compared, and allow the effects of redox poise on the structure of the nickel sites to be examined. In addition, the structure of the nickel site obtained from recent crystallographic studies of the D. gigas enzyme (Volbeda, A.; Charon, M.-H.; Piras, C.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580−587) is compared with the structural features obtained from the analysis of XAS data from the same enzyme. The nickel sites of all but the oxidized (as isolated) sample of A. eutrophus hydrogenase are quite similar. The nickel K-edge energies shift 0.9−1.5 eV to lower energy upon reduction from oxidized (forms A and B) to fully reduced forms. This value is comparable with no more than a one-electron metal-centered oxidation state change. With the exception of T. roseopersicina hydrogenase, most of the edge energy shift (∼0.8 eV) occurs upon reduction of the oxidized enzymes to the EPR-silent intermediate redox level (SI). Analysis of the XANES features assigned to 1s → 3d electronic transitions indicates that the shift in energy that occurs for reduction of the enzymes to the SI level may be attributed at least in part to an increase in the coordination number from five to six. The smallest edge energy shift is observed for the T. roseopersicina enzyme, where the XANES data indicate that the nickel center is always six-coordinate. With the exception of the oxidized sample of A. eutrophus hydrogenase, the EXAFS data are dominated by scattering from S-donor ligands at ∼2.2 Å. The enzyme obtained from T. roseopersicina also shows evidence for the presence of O,N-donor ligands. The data from A. eutrophus hydrogenase are unique in that they indicate that a significant structural change occurs upon reduction of the enzyme. EXAFS data obtained from the oxidized (as isolated) A. eutrophus enzyme indicate that the EXAFS is dominated by scattering from 3−4 N,O-donor atoms at 2.06(2) Å, with contributions from 2−3 S-donor ligands at 2.35(2) Å. This changes upon reduction to a more typical nickel site composed of ∼4 S-donor ligands at a Ni−S distance of 2.19(2) Å. Evidence for the presence of atoms in the 2.4−2.9 Å distance range is found in most samples, particularly the reduced enzymes (SI, form C, and R). The analysis of these data is complicated by the fact that it is difficult to distinguish between S and Fe scattering atoms at this distance, and by the potential presence of both S and another metal atom at similar distances. The results of EXAFS analysis are shown to be in general agreement with the published crystal structure of the D. gigas enzyme.
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