Jason J. Smee

The bio-organometallic chemistry of active site iron in hydrogenases ☆

Abstract The recent X-ray crystal structure determinations of several hydrogenase enzymes have engaged the organometallic community owing to the presence of CN − and CO ligands bound to iron in the active sites. This review focuses primarily on the structural features of these metalloproteins and discusses synthetic efforts to develop small molecule models of the active sites. Specific attention is given to the use of infrared spectroscopy as an additional tool to probe different enzyme states. In addition, structurally dissimilar complexes which show some ability to facilitate either dihydrogen uptake or production, are reviewed as possible functional models. Insights from earlier work with metal hydride chemistry and recent theoretical studies are discussed in terms of functional mechanistic proposals.

Chloro-substituted dipicolinate vanadium complexes: synthesis, solution, solid-state, and insulin-enhancing properties.

Three vanadium complexes of chlorodipicolinic acid (4-chloro-2,6-dipicolinic acid) in oxidation states III, IV, and V were prepared and their properties characterized across the oxidation states. In addition, the series of hydroxylamido, methylhydroxylamido, dimethylhydroxylamido, and diethylhydroxylamido complexes were prepared from the chlorodipicolinato dioxovanadium(V) complex. The vanadium(V) compounds were characterized in solution by (51)V and (1)H NMR and in the solid-state by X-ray diffraction and (51)V NMR. Density Functional Theory (DFT) calculations were performed to evaluate the experimental parameters and further describes the electronic structure of the complex. The small structural changes that do occur in bond lengths and angles and partial charges on different atoms are minor compared to the charge features that are responsible for the majority of the electric field gradient tensor. The EPR parameters of the vanadium(IV) complex were characterized and compared to the corresponding dipicolinate complex. The chemical properties of the chlorodipicolinate compounds are discussed and correlated with their insulin-enhancing activity in streptozoticin (STZ) induced diabetic Wistar rats. The effect of the chloro-substitution on lowering diabetic hyperglycemia was evaluated and differences were found depending on the compounds oxidation state similar as was observed for the vanadium III, IV and V dipicolinate complexes (P. Buglyo, D.C. Crans, E.M. Nagy, R.L. Lindo, L. Yang, J.J. Smee, W. Jin, L.-H. Chi, M.E. Godzala III, G.R. Willsky, Inorg. Chem. 44 (2005) 5416-5427). However, a linear correlation of oxidation states with efficacy was not observed, which suggests that the differences in mode of action are not simply an issue of redox equivalents. Importantly, our results contrast the previous observation with the vanadium-picolinate complexes, where the halogen substituents increased the insulin-enhancing properties of the complex (T. Takino, H. Yasui, A. Yoshitake, Y. Hamajima, R. Matsushita, J. Takada, H. Sakurai, J. Biol. Inorg. Chem. 6 (2001) 133-142).

Anti-diabetic effects of sodium 4-amino-2,6-dipicolinatodioxovanadium(V) dihydrate in streptozotocin-induced diabetic rats.

The evaluation of the anti-diabetic effects of an organic vanadium(V) complex in streptozotocin (STZ)-induced diabetic rats was investigated. The STZ-induced diabetic rats were orally administrated with sodium 4-amino-2,6-dipicolinatodioxovanadium(V) dihydrate (V5dipic-NH(2)), a vanadium(V) coordination compound. The compound was administered through drinking water at a concentration of 0.1mg/mL for 20 days, and then the concentration was increased to 0.3mg/mL for the following 20 days. At the end of the experiment, V5dipic-NH(2) statistically significantly reduced the levels of blood glucose (P<0.01), serum total cholesterol (P<0.01), triglycerides (P<0.01) and the activities of serum aspartate amino transferase (P<0.05) and alkaline phosphatase (P<0.01) compared to untreated diabetic animals. After treatment with 0.3mg/mL V5dipic-NH(2), the oral glucose tolerance was improved in diabetic animals (P<0.01). In addition, the daily intake of elemental vanadium was markedly decreased in V5dipic-NH(2)-treated diabetic rats compared to vanadyl sulfate (VOSO(4))-treated diabetic rats, which suggested that the anti-diabetic activity of the element vanadium was elevated after being modified with an organic ligand. These results suggested that V5dipic-NH(2), as an organic vanadium compound, is more effective than inorganic vanadium salt at alleviating the symptoms of diabetes.

Pulmonary Immunotoxic Potentials of Metals Are Governed by Select Physicochemical Properties: Vanadium Agents

The in situ reactions of metal ions/complexes are important in understanding the mechanisms by which environmental and occupational metal particles alter lung immune responses. A better understanding of these reactions in situ will also allow for the improved specificity and controlled toxicity of novel metallocompounds to be used as inhaled diagnostics or therapeutics. Our previous work showed that inhalation of metals (e.g., chromium, vanadium, nickel) caused altered lung immune cell function and host resistance. The data also suggested that the degree of immunomodulation induced depended not only on the amount of metal deposited, but also the compound used. If specificity governs pulmonary immunomodulatory potential, it follows that physicochemical properties inherent to the metal have a role in the elicited effects. We hypothe-size that major determinants of any metal compounds potential are its redox behavior, valency (generally referred to as oxidation state and considered speciation in chemical literature), and/or solubility. In accord with the extensive work carried out with vanadium (chemical symbol V) compounds showing the importance of form used, differences in potential for a range of V agents (pentavalent [VV] insoluble vanadium pentoxide and soluble sodium metavanadate, tetravalent [VIV] vanadyl dipicolinate, and trivalent [VIII] bis(dipicolinato)vanadium) were quantified based on induced changes in local bacterial resistance after host inhalation of each agent at 100 μ g V/m3 (5 hr/d for 5 d). Differences in effect between VV forms indicated that solubility was a critical property in in situ pulmonary immunotoxicity. Among the soluble forms, oxidizing vanadate had the greatest impact on resistance; reducing VIII altered resistance to a lesser extent. Both the VIV and insoluble VV had no effect. When data was analyzed in the context of pre-infection lung V burdens, soluble V agents with different oxidation states induced varying responses, supporting the hypothesis that differences in immunomodulatory potential might be attributed to redox behavior or valency. Our findings both provide a basis for understanding why some metals could be a greater health risk than others (when encountered in equal amounts) and will assist in the design of inhalable metallopharmaceuticals by allowing researchers to preempt selection of certain metal ions or complexes for use in such products.

Effects of metal compounds with distinct physicochemical properties on iron homeostasis and antibacterial activity in the lungs: chromium and vanadium

In situ reactions of metal ions or their compounds are important mechanisms by which particles alter lung immune responses. The authors hypothesized that major determinants of the immunomodulatory effect of any metal include its redox behavior/properties, oxidation state, and/or solubility, and that the toxicities arising from differences in physicochemical parameters are manifest, in part, via differential shifts in lung iron (Fe) homeostasis. To test the hypotheses, immunomodulatory potentials for both pentavalent vanadium (VV; as soluble metavanadate or insoluble vanadium pentoxide) and hexavalent chromium (CrVI; as soluble sodium chromate or insoluble calcium chromate) were quantified in rats after inhalation (5 h/day for 5 days) of each at 100 μg metal/m3. Differences in effects on local bacterial resistance between the two VV, and between each CrVI, agents suggested that solubility might be a determinant of in situ immunotoxicity. For the soluble forms, VV had a greater impact on resistance than CrVI, indicating that redox behavior/properties was likely also a determinant. The soluble VV agent was the strongest immunomodulant. Regarding Fe homeostasis, both VV agents had dramatic effects on airway Fe levels. Both also impacted local immune/airway epithelial cell Fe levels in that there were significant increases in production of select cytokines/chemokines whose genes are subject to regulation by HIF-1 (whose intracellular longevity is related to cell Fe status). Our findings contribute to a better understanding of the role that metal compound properties play in respiratory disease pathogenesis and provide a rationale for differing pulmonary immunotoxicities of commonly encountered ambient metal pollutants.

Models for Terminal Ni(S-Cysteine) Modification in [NiFe]Hydrogenases by Iodoacetamide and Iodoacetate

The cis-dithiolate complex [N,N′-bis(2-mercaptoethyl-2-methylpropyl)-1,5-diazacyclooctane]nickel(II) (Ni-1*) reacts with stoichiometric amounts of iodoacetamide to yield S-alkylated, mono- and diacetamide complexes, [(AA)Ni-1*][I] and [(AA)2Ni-1*][I]2. Their molecular structures are established by X-ray crystallography and find the former in pseudo-square planar geometry with no additional coordination of the amide functionality, while the latter is an octahedral N2S2O2NiII complex. The assignment of the axial coordination ligands as amide oxygen atoms is consistent with IR-spectroscopic ν(C=O) results both in the solid and solution states. The complexes are further characterized by UV/Vis spectroscopy, conductance measurements, and electrochemical studies. Comparisons are drawn between the alkylation of these simple dithiolate complexes and the loss of [NiFe]hydrogenase activity upon addition of alkylating agents.

TETRAVANADATE, DECAVANADATE, KEGGIN AND DAWSON OXOTUNGSTATES INHIBIT GROWTH OF S. cerevisiae

The Willsky group had previously shown that vanadate inhibits growth. By using the combination of yeast growth and 51V NMR spectroscopy we show that the inhibiting species is V4. Similar effects were observed when a structurally similar, chemically stable molybdate tetramer was tested. To extend this result further, detailed studies were conducted exploring the effect of V10 on cell growth and it was observed that V10 is a poorer inhibitor than V4. Furthermore, it appears that the cell concentrates the absorbed vanadium into an acidic compartment where the cell-associated V10 will be generated. Spectroscopic studies suggest that V10 does enter and/or is formed inside the cell, although the mechanism is not known. An initial screen of the inhibitory effects of a series of Keggin and Dawson oxometalate at the 5.0mM level was conducted. Both the Keggin and Dawson structures were found to be more potent inhibitors than V10, but the Dawson complexes were found to be slightly more potent than the Keggins. Further studies at lower concentrations are necessary to effectively compare the oxometalates’ potency.