B.S. Chemistry, North Carolina State University

PhD. Chemistry, University of North Carolina

post-doctoral research, Michigan State University

post-doctoral research, Amherst College

I began my training as a chemist at North Carolina State University.  My research experience as an undergraduate concerned the interaction of small metal complexes with DNA.  In that work, we used the characteristic optical properties of transition metal complexes to study the mechanism of binding and strength of interaction of small ruthenium coordination complexes with nucleic acids.  The binding of such complexes has been used to determine structure of nucleic acids and in some cases to create new therapies for disease.  My involvement in this research convinced me that I should attend graduate school and study bioinorganic chemistry.
 

Polypyridal ligands can be used to synthesize Ruthenium complexes.  These complexes can bind to DNA, either covalently or electrostatically.  The binding effects the optical properties of the metal complexes and can be used to determine the mechanism and strength of binding. Smith et al., New J. Chem., 1994, 18, 397.

The emission of dppz complexes of ruthenium is quenched by water.  Binding to the hydrophobic interior of DNA protects the complex from water, "turning on" the emission.  These complexes have been referred to in the literature as "molecular light-switches."

I completed my PhD training in 1997 at the University of North Carolina, under the guidance of Professor Holden Thorp.  My doctoral research focused on the reduction of small molecule substrates like nitric oxide catalyzed by an iron-containing class of biomolecules called siderophores.  This work has fascinating implications in biological and environmental systems.  NO has long been recognized as a pollutant, and much of the focus on NO reseach has been aimed toward developing systems which can mediate the contribution of NO to environmental problems. The discovery in recent years of NO's role as a biological signalling agent has shifted the focus somewhat towards the mechanisms of production, regulation and recognition of NO in biological systems.  My graduate research fell at a point somewhere in between these two fields of NO research.  The discovery that ferrioxamine B, a biologically-derived iron complex, mediated mortality caused by excess NO production during septic shock,  led to an exploration of the suitability of complexes similar to ferrioxamine B as reduction catalysts for NOx compounds.  This project will be ongoing in my lab at UMD.
 

Ferrioxamine B belongs to a class of natural compounds known as siderophores (sider=iron; phore=lover).  These ligands have been evolved to bind ferric ion (Fe3+) tightly and preferentially over ferrous ion (Fe2+).  Organisms produce siderophores in response to low iron levels in the environment.  Iron is necessary for the production of various cellular machinery.

Reduction of the ferric complex by one electron results in "unraveling" of the complex, by the release of one or more of the chelate arms.  This process creates open binding sites on the ferrous ion, where NO can attach.  binding of NO allows the iron to transfer the extra electron out onto the NO ligand.  The siderophore ligand  closes around the ferric ion, forcing the ejection of the reduced NO and completing the catalytic cycle.  Dr. Sheila Smith, Thesis, UNC, 1997.

My first post-doctoral position involved the determination of the structure and role of the iron sulfur cluster(s) in an anaerobic metalloenzyme called pyruvate-formate lyase activase.  This enzyme plays a key role in the ability of certain bacteria to survive in the absence of oxygen.  Following anaerobic overexpression and purification of the enzyme from e. coli, the metal centers were studied by biochemical assay and spectroscopic methods.
 

Iron sulfur clusters play important roles in biological systems such as photosynthesis, electron transport, and iron regulation.  These clusters can undergo conversion between oxidation states.  More recently, it has been shown that cluster interconversion,  i.e. [2Fe2S] ---> [4Fe4S], may be an important factor in the chemistry of enzymes containing FeS clusters.

Resonance Raman Spectroscopy is one of several physical methods that can be used to determine cluster content in biological samples.  In the spectrum above, the frequencies observed were consistent with vibrations of a [3Fe4S] center in anarobically isolated enzyme.  S. Smith, J. Broderick, unpublished results

My second post-doctoral position has focused again on the structure and role of metals in metalloenzymes, particularly focusing on the magnetic properties resulting from the presence of unpaired electrons on the metal.
 

The active site copper in amine oxidases is coordinated in a distorted square pyramid.  The base of the pyramid ids composed of three hystidyl ligands from the protein and a water molecule.  The top of the pyramid is a second water molecule.  In an unactive form of the enzyme, the copper is bound to a modified tyrosine residue from the enzyme (shown in green). Murray et al., Biochemistry, 1999, 38, 8217.

Continuous wave EPR (Electron Paramagnetic Resonance) spectroscopy indicates that the coordination around the  Cu ion becomes more symmetric upon substrate binding.  This change can be seen in the increased sharpness of the signal at 3150 G.   We think this change may indicate a shift in the electronic structure and possibly in the reduction potential of the copper center. Smith et al., unpublished results

The point of all this is to say that the study of metals in biological systems encompasses a broad range of problems and the presence of metal ions opens up an extensive array of biophysical techniques for use in the study of the structure and role of these metals.

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