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Selective isolation of gold facilitated by second-sphere coordination with ?-cyclodextrin.

ABSTRACT: Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH?)?][AuBr?](?-cyclodextrin)?}n chain superstructure formed during the rapid co-precipitation of ?-cyclodextrin and KAuBr? in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes. From single-crystal X-ray analyses of six inclusion complexes between ?-, ?- and ?-cyclodextrins with KAuBr? and KAuCl?, we hypothesize that a perfect match in molecular recognition between ?-cyclodextrin and [AuBr?](-) leads to a near-axial orientation of the ion with respect to the ?-cyclodextrin channel, which facilitates a highly specific second-sphere coordination involving [AuBr?](-) and [K(OH?)?](+) and drives the co-precipitation of the 1:2 adduct. This discovery heralds a green host-guest procedure for gold recovery from gold-bearing raw materials making use of ?-cyclodextrin-an inexpensive and environmentally benign carbohydrate.

SUBMITTER: Liu Z

PROVIDER: S-EPMC3674257 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature

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Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin.

Liu Zhichang Z Frasconi Marco M Lei Juying J Brown Zachary J ZJ Zhu Zhixue Z Cao Dennis D Iehl Julien J Liu Guoliang G Fahrenbach Albert C AC Botros Youssry Y YY Farha Omar K OK Hupp Joseph T JT Mirkin Chad A CA Fraser Stoddart J J

Nature communications 20130101

Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH₂)₆][AuBr₄](α-cyclodextrin)₂}n chain superstructure formed during the rapid co-precipitation of α-cyclodextrin and KAuBr₄ in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes. From single-crystal X-ray anal ...[more]

PMID: 23673640

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Project description:The factor inhibiting HIF (FIH) is a proximate oxygen sensor for human cells, hydroxylating Asn(803) within the ?-subunit of the hypoxia inducible factor (HIF). FIH is an ?-ketoglutatrate (?KG)-dependent, non-heme Fe(II) dioxygenase, in which Fe(II) is coordinated by a (His(2)Asp) facial triad, ?KG, and H(2)O. Hydrogen bonding among the facial triad, the HIF-Asn(803) side chain, and various second-sphere residues suggests a functional role for the second coordination sphere in tuning the chemistry of the Fe(II) center. Point mutants of FIH were prepared to test the functional role of the ?KG-centered (Asn(205) and Asn(294)) or HIF-Asn(803)-centered (Arg(238) and Gln(239)) second-sphere residues. The second sphere was tested for local effects on priming Fe(II) to react with O(2), oxidative decarboxylation, and substrate positioning. Steady-sate kinetics were used to test for overall catalytic effects; autohydroxylation rates were used to test for priming and positioning, and electronic spectroscopy was used to assess the primary coordination sphere and the electrophilicity of ?KG. Asn(205) ? Ala and Asn(294) ? Ala mutants exhibited diminished rates of steady-state turnover, while minimally affecting autohydroxylation, consistent with impaired oxidative decarboxylation. Blue-shifted metal to ligand charge transfer transitions for (Fe+?KG)FIH indicated that these point mutations destabilized the ?* orbitals of ?KG, further supporting a slowed rate of oxidative decarboxylation. The Arg(238) ? Met mutant exhibited steady-state rates too low to measure and diminished product yields, suggesting impaired substrate positioning or priming; the Arg(238) ? Met mutant was capable of O(2) activation for the autohydroxylation reaction. The Gln(239) ? Asn mutant exhibited significantly slowed steady-state kinetics and diminished product yields, suggesting impaired substrate positioning or priming. As HIF binding to the Gln(239) ? Asn mutant stimulated autohydroxylation, it is more likely that this point mutant simply mispositions the HIF-Asn(803) side chain. This work combines kinetics and spectroscopy to show that these second-sphere hydrogen bonds play roles in promoting oxidative decarboxylation, priming Fe(II) to bind O(2), and positioning HIF-Asn(803).

Project description:In recent years, considerable progress has been made toward elucidating the geometric and electronic structures of thiol dioxygenases (TDOs). TDOs catalyze the conversion of substrates with a sulfhydryl group to their sulfinic acid derivatives via the addition of both oxygen atoms from molecular oxygen. All TDOs discovered to date belong to the family of cupin-type mononuclear nonheme Fe(II)-dependent metalloenzymes. While most members of this enzyme family bind the Fe cofactor by two histidines and one carboxylate side chain (2-His-1-carboxylate) to provide a monoanionic binding motif, TDOs feature a neutral three histidine (3-His) facial triad. In this Account, we present a bioinformatics analysis and multiple sequence alignment that highlight the significance of the secondary coordination sphere in tailoring the substrate specificity and reactivity among the different TDOs. These insights provide the framework within which important structural and functional features of the distinct TDOs are discussed.The best studied TDO is cysteine dioxygenase (CDO), which catalyzes the conversion of cysteine to cysteine sulfinic acid in both eukaryotes and prokaryotes. Crystal structures of resting and substrate-bound mammalian CDOs revealed two surprising structural motifs in the first- and second coordination spheres of the Fe center. The first is the presence of the abovementioned neutral 3-His facial triad that coordinates the Fe ion. The second is the existence of a covalent cross-link between the sulfur of Cys93 and an ortho carbon of Tyr157 (mouse CDO numbering scheme). While the exact role of this cross-link remains incompletely understood, various studies established that it is needed for proper substrate Cys positioning and gating solvent access to the active site. Intriguingly, bacterial CDOs lack the Cys-Tyr cross-link; yet, they are as active as cross-linked eukaryotic CDOs.The other known mammalian TDO is cysteamine dioxygenase (ADO). Initially, it was believed that ADO solely catalyzes the oxidation of cysteamine to hypotaurine. However, it has recently been shown that ADO additionally oxidizes N-terminal cysteine (Nt-Cys) peptides, which indicates that ADO may play a much more significant role in mammalian physiology than was originally anticipated. Though predicted on the basis of sequence alignment, site-directed mutagenesis, and spectroscopic studies, it was not until last year that two crystal structures, one of wild-type mouse ADO (solved by us) and the other of a variant of nickel-substituted human ADO, finally provided direct evidence that this enzyme also features a 3-His facial triad. These structures additionally revealed several features that are unique to ADO, including a putative cosubstrate O2 access tunnel that is lined by two Cys residues. Disulfide formation under conditions of high O2 levels may serve as a gating mechanism to prevent ADO from depleting organisms of Nt-Cys-containing molecules.The combination of kinetic and spectroscopic studies in conjunction with structural characterizations of TDOs has furthered our understanding of enzymatic sulfhydryl substrate regulation. In this article, we take advantage of the fact that the ADO X-ray crystal structures provided the final piece needed to compare and contrast key features of TDOs, an essential family of metalloenzymes found across all kingdoms of life.

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Selective isolation of gold facilitated by second-sphere coordination with ?-cyclodextrin.

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Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin.

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