David Howard Goldberg - US grants

This Collaborative Research Activities in Environmental Molecular Science (CRAEMS) Award to Johns Hopkins University is supported by the Special Projects Office of the Chemistry Division. The award supports studies by Gerald Meyer, Howard Fairbrother, David Goldberg, Kenneth Karlin and A. Lynn Roberts of dehalogenation chemistry by combining their expertise in synthesis, homogeneous and heterogeneous catalysis, biomimetic and bioinorganic chemistry, electrochemistry, surface science, and environmental chemistry. Specific goals of this multidisciplinary program are: 1) the development of new reductive and oxidative dehalogenation chemistries and the elucidation of their fundamental mechanisms; 2) application of these new findings to the sensing, remediation, and determination of the environmental fate of organohalide pollutants; and 3) provide a pedagogical platform that informs and educates the next generation of environmental chemists. The fundamental studies will emphasize oxidative and reductive cleavage of organohalides by copper (I) and metalloporphyrin complexes. Both stoichiometric and electrocatalytic processes will be studied. Photo-triggered and electrochemical dehalogenation will also be examined. Dehalogenation by bimetallic reductants and metal sulfide minerals will be explored. Finally, electron-beam-induced chlorocarbon reductive cleavage will be examined in water and ice media. Collaborations include those with two National Laboratories (Oak Ridge National Laboratory and Pacific Northwest National Laboratory) and with two industries (KDF Fluid Treatment Inc., and Environmental Technologies Group Inc.). To develop an understanding of environmental principles and processes at the molecular level, a new graduate course will be developed and taught. An outreach program involving underrepresented undergraduate researchers from nearby universities (Howard University and Morgan State University) will be implemented.

Seventeen of the top twenty-five organic pollutants in the U.S. are organohalides. Volatile organohalides also deplete ozone and change global climate. This interdisciplinary work aims at a molecular-level understanding of redox-mediated dehalogenation, from which "greener" chemical processes can be developed and pollution problems due to organic halides can be obviated. The educational aspects of this multidisciplinary program, in collaboration with government laboratories and industries, is designed to increase the awareness of students concerning real-world environmental problems and to provide hands-on experience.

DESCRIPTION: (Adapted from applicant's abstract)A large class of metalloproteins utilize a mixed nitrogen/sulfur coordination environment to hold the metal ion (Fe, Ni, Co, Cu, Zn) in the active site, including blue copper proteins, nitrile hydratase, alcohol dehydrogenase, and peptide deformylase (PDF). The long term objective of the proposed research is the development of the coordination chemistry of new nitrogen/sulfur metal complexes as it relates to the structure and functioning of certain of these metalloproteins. In particular, the theme of this proposal concerns the synthesis and reactivity of a new family of mixed nitrogen/sulfur metal complexes that are designed to be structural and functional models of peptide deformylase. Peptide deformylase (PDF) is a bacterial metallohydrolase enzyme with a tetrahedral Fe(II) center. It is currently under scrutiny as an attractive target for new antibiotic drugs. Study of the new class of L(N,S)-M (M = Fe, Ni, Co, Zn) complexes designed here should shed light on important structure/function relationships concerning PDF and other related metallohydrolases. These compounds should also exhibit new properties and reactivity that will provide valuable information in the study of other N,S-metal sites in biology such as nitrile hydratase, alcohol dehydrogenase, or mutant forms of carbonic anhydrase. Objectives of the propose research include the synthesis of ligands designed to mimic the 2 His, 1 Cys ligation found in PDF. New organic syntheses will be developed to prepare three-coordinate, tripodal dipyridyl- and diimidazolyl-alkylthiolate ligands. Complexes of the type [L(M(II)X] (L = N2Sthiolate; M = Fe, Ni, Co, Zn; X = halogen, monoanion, solvent, etc.] should be accessible. The hydrolytic capabilities of these complexes toward amide, and ester substrates will be determined. Mechanistic investigations (e.g. isolation of intermediates, kinetic studies) will be undertaken. Spectroscopic analysis (NMR, EPR, UV-vis, electrochemistry) will be used to characterize ground state complexes as well intermediates along the reaction path. The isolation and characterization of [LM(II)OH] complexes will be emphasized, given their presumed role as the active nucleophilic agent during hydrolytic cleavage.

Dr. David Goldberg, Department of Chemistry, Johns Hopkins University, is supported by the Inorganic, Bioinorganic and Organometallic Chemistry program, Division of Chemistry, National Science Foundation, through a Career Award for his work on the synthesis and characterization of novel corrole macrocycles that will incorporate and stabilize high-valent oxidation states of transition metal ions such as iron(IV), nickel(III), and copper(III). Metal-oxo species such as (corrole)Fe=O will be isolated, characterized and exploited as possible catalysts for the dehalogenation of chlorocarbons. In the educational front, a series of Chemistry Workshops will be developed for the Ingenuity Project, an outreach program for advanced inner-city students focused on mathematics and science. Also, a new graduate level bioinorganic chemistry course will be implemented in which critical reading of primary literature and presentation skills will be emphasized.

The new corrole macrocycles will find applications in activating molecular oxygen and hydrogen peroxide, and in their mediating reductive dehalogenation of environmentally significant organohalide substrates such as the recalcitrant carbon tetrachloride found in many groundwaters. Students involved in the Ingenuity Project will have access to potential role models and will help them discover the excitement of doing chemistry, and science in general.

With support from the Major Research Instrumentation (MRI) Program, the Department of Chemistry at Johns Hopkins University will acquire a X-ray diffractometer with CCD detector and low temperature capabilities for small molecule diffractometry. This equipment will enhance research in a number of areas including the following: a) bioinorganic modeling of metalloenzyme active sites; b) environmental inorganic chemistry and the development of copper mediated dehalogenation; c) synthesis, characterization and reactivity of aza-substituted corrole macrocycles; d) new metal-carbonyl diimine compounds of photochemical interest; e) photoinduced electron, energy, and atom transfer processes; and f) X-ray studies of metal-organic complexes for asymmetric catalysis. Faculty members from Morgan State University, Loyola College, Towson University, the University of Maryland in Baltimore County, the US Naval Academy and Gettysburg College will also have access to this equipment.

The X-ray diffractometer allows accurate and precise measurements of the full three dimensional structure of a molecule, including bond distances and angles, and it provides accurate information about the spatial arrangement of the molecule relative to the neighboring molecules. These studies will have an impact in a number of areas, including environmental chemistry, preparation of more efficient catalysts, and biochemistry.

This research award in the Inorganic, Bioinorganic and Organometallic Chemistry Program supports work by Professor David Goldberg at Johns Hopkins University to carry out fundamental studies on the synthesis and reactivity of a new class of transition metal porphyrin-like compounds known as corrolazines. Methodology is being developed to synthesize new metallocorrolazines, focusing on manganese and iron complexes. The corrolazine ligand is designed to stabilize high oxidation states, and the synthesis and characterization of high-valent metal-oxo species are targeted. The metals iron and manganese have been selected for study because of their widespread use in synthetic catalysts as well as in biologically-relevant environments such as heme proteins. The reactivity of these species in oxygen-atom-transfer and hydrogen-atom-transfer reactions is under investigation. The influence of the corrolazine ligand on the stability and reactivity of these high-valent metal species is being examined, as well as the role of ancillary ligands. High-valent metal-oxo complexes and related high oxidation state species play key roles in a number of catalytic processes and biological systems, including heme enzymes, but they are difficult to study because of their inherent instability. The corrolazine platform is designed to increase their stability and allow for their direct examination, providing fundamental insights into their spectroscopic properties, reactivity, and mechanism of action.

Through participation in this fundamental research, students at the undergraduate, graduate, and postdoctoral levels are receiving intensive training in state-of-the-art inorganic and bioinorganic chemical methods. They are gaining the skills necessary to become independent scientists, such as formulating and testing hypotheses and analyzing and interpreting experimental data in a conceptual framework. The knowledge gained from this research should help in the design of novel synthetic catalysts for industrial processes, and the research should also provide fundamental insights into biologically-relevant metalloenzymes.

DESCRIPTION (provided by applicant): Abstract: This proposal focuses on the synthesis and reactivity of small-molecule metal complexes as models of NxSy-Mn+ metalloproteins. The metalloenzymes peptide deformylase (PDF) and superoxide reductase (SOR) share common structural elements. They both contain a mononuclear iron center coordinated by an uncommon cysteinate thiolate and two or four histidine ligands. The PDF class of enzymes are important targets for antibacterial, antimalarial, and anticancer therapies. Most bacterial PDFs employ an iron(ll) center bound by His2Cys ligands and catalyze the hydrolytic cleavage of the N-terminal formyl group of nascent polypeptides. Substitution of Zn(ll) for Fe(ll) renders PDF inactive in most cases, even though Zn(ll) is normally Nature's preferred choice for catalyzing hydrolysis reactions. However, the zinc(ll) ion is active in eukaryotic PDFs, as well as in one type of bacterial PDF. These observations lead to the following questions: Why does the enzyme utilize a redox-active iron(ll) ion to perform a non-redox role? What causes the variation in activity for Zn"? What is the influence of the uncommon thiolate donor? What is the mechanism? The enzyme SOR contains a unique iron(ll) active site with one Cys and four His ligands, and catalyzes the reduction of O2" to H2O2. The fundamental chemistry of superoxide has been implicated in a number of diseases. Questions regarding SOR include: What is the nature of the O2" binding site? What intermediates are formed during reduction of O2~ by Fe"? What factors (e.g. redox potential, spin state, donor set) control O2~ reduction? What factors are critical for the release of H2O2? Principles of ligand design and coordination chemistry will be used to synthesize model complexes of PDF and SOR. The PDF models will be examined for their hydrolytic reactivity. The SOR models will be studied in reactions with O2~ and related oxygenic species. Both types of complexes will be interrogated in order to determine broad patterns of reactivity, mechanism, and structure/function relationships in N/S-ligated metal centers. Knowledge regarding the synthesis and reactivity of new NxSy ligands and there metal complexes will be advanced. Relevance: The information obtained from this work will be potentially useful for the design of new antibiotics, as well as the treatment of a wide variety of diseases in which reactive oxygen species are implicated (e.g. Parkinson's, Alzheimer's, cancer).

This proposal focuses on the synthesis and reactivity of transition metal porphyrinoid complexes that are designed to model key aspects of structure and function in heme enzymes. A subset of heme enzymes react with O2 and/or the related small molecules H2O2 and O2-, including Cytochrome P450 (CYP), dioxygenases such as tryptophan dioxygenase or indoleamine dioxygenase, peroxidases such as chloroperoxidase and myeloperoxidase, aromatic peroxygenase, catalases, heme oxygenases, and cytochrome c oxidase. Although these enzymes carry out a diverse array of critical functions, their proposed mechanisms of action have multiple intermediates in common. These intermediates include high-valent metal-oxo and metal-hydroxo species, such as FeIV(O)(porphyrin-radical-cation) (Compound-I) and FeIV(OH)(porphyrin) (protonated Compound-II) identified in CYP, as well as metal-dioxygen species such as FeIII(superoxo)(porphyrin). The factors that control the generation, stability, and reactivity of these species remains poorly understood in many cases, as do the overall mechanisms of action in which these species are proposed to play key roles. The proposed efforts focus on developing synthetic porphyrinoid analogs of M=O, M-OH, and M-O2 intermediates, and studying their reactivity in H-atom transfer (HAT), proton-coupled electron-transfer (PCET), O-atom transfer (OAT), and rebound processes. Catalytic oxidations of substrates with C-H bonds and O2 as oxidant will also be investigated. These analogs will also provide needed spectroscopic benchmarks (rR, Mössbauer, EPR, XAS) for comparison with, and identification of, the relevant biological species. A new series of Fe and Mn porphyrinoid complexes will be synthesized with corrole (Crl) and corrolazine (Cz) ligands. These porphyrinoid compounds are modified by ring contraction of the porphyrin nucleus, and are designed to stabilize metal-oxygen species such as high-valent M=O and M-OH complexes. In the previous funding period our group has shown that the Cz scaffold yields new M=O and M-OH complexes of biological relevance, and the study of these systems led to discoveries regarding the influence of oxidation state, spin state, coordination environment, and proximal and distal effects on HAT, PCET, and OAT reactivity. New corrole and corrolazine complexes will be prepared in this proposal that are designed to address two main goals 1) answer fundamental questions related to heme enzyme structure, function and mechanism and 2) gain fundamental knowledge regarding transition metal porphyrinoid complexes for the design and synthesis of new catalysts. Heme enzymes are of central importance to a range of disease states. The new knowledge to be gained regarding the mechanisms of action of these systems should help in the development of novel therapeutic and/or diagnostic strategies that target these enzymes.

PROJECT SUMMARY This proposal focuses on mononuclear nonheme iron complexes and enzymes that activate dioxygen and oxygenate substrates. An important class of nonheme iron enzymes is the thiol dioxygenases, which utilize a single iron center and O2 to oxidize thiol substrates to sulfinic acids. Mammalian cysteine dioxygenase (CDO) and bacterial 3-mercaptopropionate dioxygenase (p3MDO) are two enzymes in this class. Proper functioning of CDO is important for maintaining the appropriate levels of cysteine and producing cysteine sulfinic acid as part of cysteine metabolism in mammals. Loss of CDO function has been linked to a number of diseases including Parkinson's and Alzheimer's disease, as well as certain types of cancer. The mechanism of action of these enzymes is poorly understood. Efforts described in this proposal include the design and synthesis of a new series of iron complexes that activate O2 and carry out selective substrate oxidation reactions including S-oxygenation, similar to CDO and p3MDO. This work is also relevant to the larger class of nonheme iron oxygenases. The modular organic ligand scaffold surrounding the metal ion will be rationally adjusted to examine structure/function relationships. The study of these complexes will provide fundamental knowledge that will contribute to delineating enzyme mechanisms and to designing selective biomimetic iron oxidation catalysts. The O2 reactivity of new nonheme iron complexes bearing sulfur ligands will be examined by methods designed to trap and/or characterize unstable Fe/O2-derived species. These methods include the use of low temperatures to trap unstable species during O2 activation, and spectroscopic methods such as stopped-flow UV-vis coupled with rapid-freeze-quench trapping, resonance Raman, EPR, low-temperature ESIMS, and Mössbauer. Density functional theory (DFT) calculations will support and guide the spectroscopic and mechanistic studies. A key aspect of the proposed work also includes select, parallel studies on the enzymes CDO and p3MDO. New oxygen intermediates will be targeted, including the characterization of a promising O2-derived transient species already observed for CDO. The nitric oxide chemistry (NO) of both synthetic complexes and enzymes will be studied, as NO is a well-known and informative surrogate for O2. The spin states and electronic structures of the new FeNO species will be determined, and second and third sphere interactions in CDO will be assessed. The fundamental knowledge to be obtained should provide major advances in our understanding of the mechanisms of a large class of nonheme iron oxygenases/oxidases.

The support provided by the Chemical Synthesis Program of the Chemistry Division allows the research group of Professor David Goldberg in the Department of Chemistry at Johns Hopkins University to develop a new family of iron-containing compounds that activating nitrogen oxide or NOx species. The chemical changes of NOx species, including nitric oxide and its related molecules nitroxyl (HNO) and nitrite ion (NO2-), are critically important for a number of biochemical processes, including cell signaling, immune response, and therapeutic strategies. Nitric oxide is a key neurotransmitter in the brain and throughout the nervous system. There is also significant current interest in the role of HNO in the brain and how it is generated. In this project, Dr. Goldberg and his group construct new iron complexes that bind and react with NOx species. They carry out chemical changes on these molecules related to enzymatic processes. Graduate and undergraduate students are educated and trained in experimental chemical science. An outreach program for students underrepresented in science involving a Baltimore City high school is underway.

The synthesis of a new family of polydentate ligands and their related nonheme iron complexes is under investigation for the binding and activation of NOx species. The oxidative and reductive transformations of several forms of NOx are examined. The observed reactivity is related to enzymatic processes that occur in iron-containing nitric oxide reductases, nitrite reductases, and nitric oxide dioxygenases. In addition, the fundamental reactivity of NOx species, which are potent signaling molecules, is important to a range of processes in the brain and nervous system. Through ligand design, the steric and electronic properties at the metal center are controlled. The study of these complexes may provide insights regarding how structure relates to function in iron-mediated NOx reactivity. Specific efforts will focus on the reduction of NO to NO-/HNO and N2O, the oxidation of NO to NO2- and NO3-, and the reduction of nitrite (NO2-) by biologically relevant reducing agents. The educational plans include an outreach program to a local Baltimore City high school that provides a hands-on laboratory experience.