MIT Chemistry Directory

Stephen J. Lippard

Arthur Amos Noyes Professor

Stephen J. Lippard is the Arthur Amos Noyes Professor of Chemistry at the Massachusetts Institute of Technology. His research activities span the fields of inorganic chemistry, biological chemistry, and neurochemistry. Included are studies to understand and improve platinum anticancer drugs, the synthesis of diiron complexes as models for carboxylate-bridged diiron metalloenzymes, structural and mechanistic investigations of bacterial multicomponent monooxygenases, and investigations of inorganic neurotransmitters and signal transducers, especially nitric oxide and zinc.

The chemistry investigated in Professor Lippard's laboratory explores the interface between inorganic chemistry and biology. Core activities include structural and mechanistic studies of macromolecules as well as synthetic inorganic chemistry. The focus is on the synthesis, reactions, physical and structural properties of metal complexes as models for the active sites of metalloproteins and as anti-cancer drugs. Also included is extensive structural and mechanistic work on the natural systems themselves. A program in metalloneurochemistry devises small molecule probes of inorganic ions and molecules that are involved in cell signaling and applies them to study neurotransmission.

Platinum Anticancer Drug Chemistry and Biology

A major area of work is to understand and improve platinum anticancer drugs. cis-Diamminedichloroplatinum(II), cis-DDP or cisplatin, is an anticancer drug currently in widespread clinical use. We are studying the mechanism of action of cis-DDP in order to be able to design more effective agents in the platinum family of therapeutics. There are four main steps in the molecular mechanism: cell entry; activation of the platinum complex for DNA binding; binding to DNA in the cell nucleus; and the subsequent biological consequences that arise from the damaged genome, specifically, transcription inhibition and repair of the damaged DNA. X-ray and other structural studies of cis-DDP and related platinum anticancer drugs coordinated in a site-specific manner to synthetic double-stranded DNAs, both alone and in complex with proteins that bind or process the platinated DNA, have revealed important features about the stereochemical features. Among the complexes under investigation are the nucleosome core particle and RNA polymerase II in complex with site-specifically platinated DNAs and an RNA leader strand. The information gained in these studies is used to guide the synthesis of novel anticancer drug candidates.

A strategy under active investigation is to devise platinum(IV) pro-drugs in which two additional, axial ligands are employed to attach cis-DDP and its analogues to nanomolecular constructs, such as single-walled carbon nanotubes, gold nanoparticles, and linear polymers, for preserving the lifetime of the platinum payload after administration, for targeting cancer cells, and/or for taking advantage of the unique metabolism of cancer cells to improve the efficacy and selectivity of the prodrug. The redox properties of the compounds are adjusted to release the platinum(II) warhead in the cells and not prematurely in blood, which carries the construct to the tumor site. Animal studies are currently in progress with collaborators to evaluate several platinum(IV) pro-drugs having these characteristics.

We have identified a family of cellular proteins that bind to DNA platinated by cis-DDP. Several genes that encode these proteins have been obtained from both human and yeast cDNA libraries. We demonstrated that the major adducts of cisplatin are removed by excision repair in human cell extracts and that HMG-domain proteins can shield cisplatin-DNA adducts from repair. The resulting adducts block replication and transcription, one possible mechanism for which being to sequester one or more factors that are essential for transcription. Other proteins such as PARP-1 interact with platinated DNA. Knowledge about these interactions is being used in conjunction with chemical synthesis and gene therapy strategies to obtain better platinum-based molecules and protocols for cancer chemotherapy.

Biochemical and Biomimetic Studies of Diiron Hydroxylase and Related Enzymes

Polymetallic centers occur in numerous metalloproteins where they carry out a variety of functions including hydroxylation of methane, the generation of amino acid radicals, hydrolysis of phosphate esters, and dioxygen transport. How do two or more metal atoms in close proximity in these proteins work in concert to carry out their biological function? How do we know the coordination environment of such polymetallic species in biological molecules? To help answer these questions, we synthesize and characterize well-defined model compounds and compare their properties with those of their biological counterparts. Physical and biological studies of the proteins are also undertaken. Recently our attention has focused on systems in which one or more carboxylate groups link two metal ions. Proteins in this category include soluble methane monooxygenase (sMMO), an amazing biomolecular machine that converts methane and dioxygen selectively to methanol and water, and the related protein toluene/o-xylene monooxygenase (ToMO). Related eukaryotic enzymes from mammalian and other sources perform chemistry that is important for mitochondrial metabolism. We have now proved these systems to have analogous carboxylate-bridged diiron centers. The chemistry employed by this superfamily of enzymes involves activation of dioxygen at reduced, diiron(II) centers followed by dioxygen activation and substrate oxidation. Transient diiron(III) peroxo or diiron(IV) oxo species are responsible for the hydrocarbon oxidation step. We use chemical, EPR, Raman, NRVS, and optical spectroscopic, redox, EXAFS, X-ray crystallographic, NMR, Mössbauer and magnetic, and freeze-quench and stopped flow kinetics techniques to determine the how the metal ions activate O2 in the proteins. Intermediates are probed by freeze-quench spectroscopic and low temperature X-ray crystallographic methods. Further information about reaction intermediates and transition states are provided by density functional QM/MM theoretical studies in collaboration with the Friesner laboratory at Columbia University.

Studies with synthetic models focus on the use of sterically hindered carboxylate, macrocyclic, and preorganized ligands to afford complexes that best mimic the stoichiometric and functional properties of the enzyme active sites. With the use of the models we now have carboxylate-bridged diiron systems that can perform O2 activation, C–H bond hydroxylaton, and catalytic oxo transfer reactions. Important design features are the positioning of two N-donor ligands syn to the Fe–Fe vector and sufficient flexibility to allow for carboxylate shifts during the reaction chemistry.

Apart from this chemistry at the hydroxylase components of the enzymes we are investigating the structures and functions of the reductase, coupling, and other proteins in the systems alone and in complexes with the other components. Solution NMR spectroscopy has been used to determine the structures of the coupling protein, and the ferredoxin and flavin components of the sMMO reductase. The electron transfer reactions are also being studied. Access of four substrates, namely, protons, electrons, dioxygen, and a hydrocarbon, to the active sites of these enzymes are being extensively investigated. Site-directed mutagenesis is employed to interrogate functions of amino acid side chains in the second and third coordination shells of the hydroxylase active sites and to investigate protein-protein interactions important for function.

Metalloneurochemistry

Several projects have been launched in a relatively new area that we term metalloneurochemistry. Novel sensors for zinc have been prepared that append metal-chelating units to fluorescein and to a fluorescein/rhodamine framework that designated as rhodafluors. Zinc has been imaged in the dentate gyrus cells of the hippocampus in rat brain slices, and collaborative studies in progress have associated that release of zinc with long-term potentiation. The design and utilization of novel zinc-specific extracellular chelating agents have facilitated these experiments. Sensors programmed to visualize extracellular zinc or zinc in damaged neuronal cells have revealed selectively damaged neurons in the brains of animals that suffered blunt head trauma or drug-induced seizures. These same probes have identified new zinc-rich cells in brain. Work in progress includes several strategies to synthesize ratioable zinc sensors, which shift their wavelength upon zinc binding, and programmable sensors that can be attached to specific extra- and intracellular compartments and organelles. Experiments are being conducted with live brain slices to use zinc release to track neural network formation and communication. The ultimate goal is to understand zinc release in brain and to use it to investigate neural networks, including both hippocampal and olfactory bulb loci. Sensors capable of quantitating mobile zinc have also been devised and are being applied to study both prostate and pancreatic functions. In vivo microscopic imaging has revealed the loss of zinc during prostate cancer formation in the TRAMP, or transgenic adenocarcinoma of the mouse prostate model, mouse. Related sensors based on fundamental transition metal chemistry have been devised to detect biological nitric oxide. The focus is currently on the synthesis and characterization of transition metal complexes containing a ligand that becomes fluorescent and is released upon addition of NO to signal the presence of this neurotransmitter by light emission. We also study biomimetic chemistry of metal-sulfur clusters with nitric oxide to guide the interpretation of biological experiments in which NO released in cells signals a function or evokes pathology. Included are iron-sulfur cluster reactions with nitric oxide, the chemistry of zinc thiolates with NO and its metabolites, and nitrosation reactions involving metal-carbon and metal-nitrogen bonds.

 

PUBLICATIONS

Contact Information

t: 6172531892
e: lippard@mit.edu

Office

18-498

Education

B.A. 1962, HAVERFORD COLLEGE
Ph.D. 1965, MASSACHUSETTS INSTITUTE OF TECHNOLOGY (MIT)

Administrative Assistant

 

Richard Girardi

Email: girardi@mit.edu

Tel: (617) 253-1848

Room 18-493
77 Massachusetts Ave.
Cambridge, MA 02139