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- B.S., University of Missouri-Columbia, MO 1978
- M.A., University of Missouri-Columbia, MO 1980
- Ph.D., New Mexico State University, NM 1983
- Postdoctoral, University of Illinois, IL 1984
- Postdoctoral, University of Guelph, Canada 1985
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My research at NMSU has evolved due to my changing interests and due to the pressures of funding. At present I have three programs underway. These are (1) reactivity of molecules in confined media (reverse micelles); (2) the exploration of the fundamental and environmental chemistry of the ferrate ion, FeO42-, and (3) the chemistry of amavadin and its derivatives (HIDCs).
Reactivity of Molecules in Confined Media
The uptake, biotransformation and eventual metabolism of metallodrugs involve hydrolysis, electron transfer or both. Many drugs undergo such transformations before they reach the site of action. Since these processes are important for drug efficacy, a better understanding of how lipid /membrane interfacial environments affect drug stability and processing is necessary. Rates for these drug activation reactions are generally measured in aqueous media and not much is known about how these rates are changed in vivo. Since studies with cell and animal systems are very complex, simple model systems are needed from which relevant information can be derived. Investigations into these effects are important because it is increasingly recognized that confined environments can alter reactivity. This makes aqueous studies inadequate to understand membrane-associated processes. Therefore, systematic investigations to understand the effects of location near or at interfaces on reaction rates are required. It is our all-encompassing hypothesis that the reaction rates of ligand exchange, ligand hydrolysis and electron transfer involving molecules at or near to membrane surfaces are substantially altered from aqueous rates. To probe this hypothesis we have initiated a series of studies in Reverse Micelles (RMs) as models for lipid or membrane interfaces. Thus far we have reported the dramatic effect (complete shut-down) for the iron(II) reduction of cobalt(III) complexes in reverse micelles. We have studied the ascorbic acid reductions of vanadium(V) and iron(III) complexes. The latter exhibits a 6 order increase in reactivity when the RM size is small. In addition, we have studied ligand substitutions in the vanadium(V)-catechol system.. It is interesting to note that the dissociation rates are substantially retarded in small RMs. This has direct impact on metallodrugs such as cis-platin, which is used to treat cancer and requires ligand dissociation to activate the drug.
Hypervalent Iron Chemistry (Ferrate Chemistry)
The study of iron in its high oxidation states has recently been shown to be of significance in the understanding of organic oxidations as well as biological transformation of molecules. For example, in the function of methane monooxygenase (MMO) or ribonucleotide reductase (RNR), the production of non-heme iron(IV) complexes has been proposed to the active to carry out oxidative processes. In addition, the ferrous ion catalysis of epoxidation reactions involving either hydrogen peroxide (Fenton’s Reagent) or molecular oxygen apparently involves the production of an iron(IV) or iron(V) species. In order to understand these important processes, a knowledge of iron in its higher oxidation states (>+3) is necessary. To date few complexes of hypervalent iron have been synthesized and characterized. Some examples of these include [Fe(dtc)3+ and Fe(diars)3]. The oldest, and perhaps most important hypervalent iron complex is potassium ferrate, K2FeO4, where iron is in the +6 oxidation state. Although this species has been known for almost 300 years, relatively little is known regarding its chemistry.
While iron(VI) is not a biologically relevant oxidation state, it is a useful starting point since it avoids the necessity of oxidation of iron(II) or iron(III), typically with H2O2 or O2, to achieve the important iron(V) or iron(IV) oxidation states.
This work impacts the basic understanding of biological O2 oxidations by non-heme iron oxidases, an ever developing and important research area in the last decade. Some of these enzymes are thought or known to proceed via iron(V) or iron(IV) intermediates. Other groups use peroxides or oxygen to achieve these high states which often “muddies the waters” by the possible formation of peroxo or superoxo iron intermediates. Our approach is unique for the study of “simple” iron(V) or iron(IV) since we start with the +6 state and “work our way down” by a series of controlled 1- or 2-electron reductions.
Thus far, our work has demonstrated that iron(VI) oxidations proceed via inner and outer-sphere pathways, depending on the reductant. These studies include the oxidation of sulfur, nitrogen, selenium and arsenic centers. We have demonstrated oxygen transfer during the reaction as well as proposed iron(VI)-reductant complexes. The intermediates have been proposed based on kinetic behavior of the reaction. Most recently, we have reported the first observable bridged intermediate using phosphorus reducing agents and submitted a manuscript for publication in January of this year. This mechanism, e.g., the oxidation of hypophosphite, occurs via oxygen transfer with simultaneous breaking a P-H bond. We have also reported the oxidation of aniline and its derivatives which occurs via one or two electron pathways-dependent on the substitution site on the reductant. It is interesting to note that for aniline the product is 100% cis-azobenzene. This is the only chemical process for selective production of the cis isomer and is an important clue as to the mechanism for this reaction which involves formation of an observable iron(VI) imido intermediate that uses iron(IV) to couple the two nitrogen centers.
Since ferric oxyhydroxides are typically the final iron product, a study of this chemistry will provide many examples of “green chemistry.” In the same vein, we have studied the ability of ferrate to act as an oxidative remediation agent. This work was originally funded by Los Alamos National Laboratory and has had generous support since then. We have studied the oxidation of organic chelators, hydrazines, and arsenic(III) compounds. In addition, we have also used the floculative abilities of the ferric oxyhydroxides to remove radionuclides, selenate, antimonate and arsenate from wastewaters. Our work on the removal of arsenic from water received an award by the National Academy of Environmental Engineering. We have also recently completed work on the ferrate(VI) degradation of high energy compounds such as HMX.
This work has been funded through ACS/PRF, DOE/LANL, DOE/WERC, Lockheed Engineering/NASA, and WRRI.
The literature methods for the production of potassium ferrate are difficult and probably represent the primary reason why ferrate chemistry has not been extensively examined. Current synthetic methods consist of the hypochlorite oxidation of ferric ion in strongly alkaline solutions. This must be done in a fume hood and typical preparation times are around 4-5 hours. In order to circumvent these problems, we have developed an entirely new method for ferrate synthesis that received a patent in 2000.
The Chelation Chemistry of N-Hydroxyiminodicarboxylic Acids (HIDCs)
Over the past few years we have reported the electron transfer chemistry of the unique vanadium complex amavadin. This compound naturally occurs in the mushroom Amanita muscaria and possesses the highly unusual absence of a vanadium(IV) center without an oxo ligand, e.g., V=O, in aqueous media. We have examined the electron self exchange between the IV/V oxidation states of amavadin and have shown it to be about 3 orders of magnitude faster than for the aqueous vanadium couple.
Because of the reported high affinity of HIDCs for vanadium(IV), -log(ß2)= 23 which is believed to be due to the ligand selectivity for the V=O unit, we are exploring the kinetics of this binding. The innovative aspect of this work is the use of an inexpensive and relatively unexplored set of novel chelators, HIDCs to selectively bind oxometal ions such as UO22+(aq) as models for PuO22+(aq) ions. This is significant as we develop nuclear energy and have to deal with waste remediation. Current chelators lack significant ability to discriminate between oxometal ions over non-oxometal ions, such as iron(III), that are also found in high concentrations in waste streams . This necessitates the use of large amounts of chelators and the generation of large quantities of secondary waste materials for disposal. This project takes advantage of our strengths in mechanistic chemistry, and introduces the organic synthesis of a novel set of ligands and their competitive binding studies.