Dismukes Group Research
Solar Hydrogen via Water Oxidation Catalysts
Synthesis and Structure of the Mn-oxo cubanes
We are using the photosynthetic water oxidizing enzymes as "blueprints" for designing durable abiotic catalysts for the generation of oxygen and hydrogen from water by photolytic water splitting. Remarkably, only a single blueprint has been discovered for photosynthetic water splitting in all oxygenic phototrophs. Our synthetic work was highlighted in 2001 by discovery of oxygen production from water by a synthetic mimic of the photosynthetic enzyme. This complex contains a unique manganese-oxo core comprised of a Mn4O4 "cubane" topology that was unprecedented in the chemical literature. We have gone on to show that the cubane class of manganese oxo clusters exhibits novel reactivity not seen in any other class of manganese-oxo complexes. Yagi, M., et al. Angew. Chem Int. 2001 40(15)
This chemistry includes selective oxygen atom transfer to various organic substrates under catalytic turnover conditions, and the catalytic condensation of water and O2 to make hydrogen peroxide. We have studied in atomic detail the mechanism of hydrogen atom transfer reactions involving the manganese-oxo cubanes.
In collaboration with Dr. Roberto Car and Fillippo DeAngelis we have examined the energetics and dynamics of O2 production using dynamic Density Functional Theory. This pdf file describes the energetics of the reaction coordinate for O2 production following removal of a phosphinate ligand. DeAngelis DFT Mn_O_Cubane.pdf
This mpg file depicts the molecular dynamics of O2 production following removal of a phosphinate ligand
Molecular dynamics of O2 production from the Mn-oxo cubane_01 Deangelis .mpg
These calculations have identified new cubane targets for improved catalytic activity. We seek to develop this promising system as a practical catalyst for use in renewable hydrogen generation via light-driven water splitting.
Biophysical Studies of Enzyme Structure and Mechanism
We employ magnetic resonance and biophysical methods to examine the structures
and mechanisms of action of several metallo-enzymes, particularly metal
clusters. Among them include enzymes required for light energy utilization by
photosynthetic organisms (PSII reaction center), anti-oxidant enzymes (manganese
catalases), hydrolysis of amino acids (arginase) and RNA processing, and
oxidases/dehydrogenases (nicotinic acid hydroxylase and formate dehydrogenase).
Photosynthetic Water Splitting Enzyme: The photosynthetic process of water oxidation produces all of the O2 in our atmosphere. The enzyme responsible for this process and is the only durable enzyme or man-made catalyst known that is capable of sustained water splitting/O2 evolution. We are investigating this process at the atomic level using biochemical, molecular biological and spectroscopic methods. Electron spin resonance and electron-nuclear double resonance spectroscopies are used to identify the paramagnetic intermediates produced during the photochemical charge separation events. Structure of the Inorganic Core by EPR ENDOR 2002 JBIC.pdf
"Inorganic mutants" of the inorganic core of the water splitting enzyme have been prepared in which each inorganic cofactor has been substituted by other surrogate cofactors as a means to test their role in enzymatic activity. 2001 Photoactivation of apo-WOC-PSII.pdf
Paleobiochemistry. The innovation of water splitting/oxygen production by the oxyphotobacteria in the archean geological era is a major conundrum in the evolution of life. We are attempting to identify how evolution created the first photosynthetic organism that was capable of using water as a reductant circa 2.7-3.5 billion years ago and thus lead to the oxygenation of earth's atmosphere. The creation and evolution of stable biogeochemical cycles is being studied as a model for understanding the creation of self-replicating chemical systems and the creation of the first biological life-forms as primitive organisms. 2002 Origin Evolution of Cyanobacteria.pdf
An hypothesis that we are investigating concerns the unique role for inorganic carbon (CO2 bicarbonate) during biogenesis/assembly of the inorganic core of the water splitting enzyme. One approach we are investigating is to characterize native populations of oxyphotobacteria that thrive in strange environments, like soda lakes near volcanos, within coral reefs, and the nutrient-depleted ocean. We predict that these organisms may contain unusual catalytic sites for water splitting. We are examining the DNA sequences in these "weirdophiles" to search for novel water splitting genes and transformation into photosynthetic hosts (cyanobacteria) to examine function. 2002 Role of Bicarbonate in Photoassembly.pdf
"Co-evolution of nitrogen fixation and oxygenic photosynthesis in the marine cyanobacterium Trichodesmium" AY 2000-2002: Co-PIs, G. C. Dismukes and P. Falkowski (Marine and Coastal Sciences Dept. at Rutgers University). Joint studies are under investigation with collaborators from the Princeton Center for Environmental BioInorganic Chemistry. A marine photosynthetic organism that is responsible for fixing the largest amount of atmospheric nitrogen in the oceans on a global scale is being studied as a model for understanding the co-evolution with water oxidation, since the two reactions are generally incompatible in the same organism. Members of the genus Trichodesmium, which are undifferentiated nonheterocystous cyanobacteria, are unique in their ability to fix dinitrogen during the diurnal portion of the diel light cycle when photosynthetic O2 formation is maximum. Most cyanobacteria separate these two antagonistic systems either spatially by localizing dinitrogen fixation in specialized cells known as heterocysts, or temporally by fixing dinitrogen at night at the expense of carbon reserves synthesized in the light. We investigated how trichodesmium permits both photosynthesis and dinitrogen fixation in the light. The main scientific conclusion: Trichodesmium uses the Mehler reaction and possibly photorespiration to consume oxygen and protect nitrogenase from the O2 produced by PSII. It achieves this by utilizing PSI to photoreduce O2 in a light-dependent reaction that utilizes electrons derived from water splitting and photorespiration. As a result of this protective reaction trichodesmium is inefficient in its light utilization, requiring a large number of photons to fix CO2. But light is cheap in the open ocean. Since O2 is both produced by splitting water and consumed by the Mehler reaction, the level of intracellular O2 can be reduced, while still generating energy photosynthetically via classical coupled electron/proton transfer.
"Trace metals, photosynthetic capacity and oxygenic photosynthesis in marine cyanobacteria" Pending CEBIC proposal, Jan-Dec 2004; Co-PIs: G. C. Dismukes & G. M. Ananyev. The purpose of this new project is to characterize at the molecular and cellular levels how marine cyanobacteria and oxyphotobacteria generate energy by photosynthesis for their life cycle and how this differs from freshwater and terrestrial algae and plants. We are also interested in understanding the molecular adaptation that occurs in these organisms that can exist both as free living cells in the open oceans and symbiotically, for example in corals. We have three specific goals. First, we seek to determine the trace metal requirements for oxygenic photosynthesis in the marine oxyphotobacterium Prochlorococcus marinus. Second, we seek to identify/compare/contrast the photosynthetic genes required for oxygen production in Prochlorococcus marinus and marine cyanobacteria in general to other prokaryotic oxygenic phototrophs, e.g. the cyanobacteria and contrast this to the anoxygenic marine phototrophs whose genomes are known or under investigation. Third, we seek to measure the total photosynthetic capacity for CO2 fixation in Prochlorococcus marinus versus other cyanobacteria having different carbon concentrating mechanisms for intracellular CO2 enrichment.
Manganese Catalases: Nature has provided us with different classes of enzymes that protect us against the destructive effects of reactive oxygen species. One of these, hydrogen peroxide, forms during normal cellular biochemistry and is destroyed by a metalloenzyme called catalase. We have studied the non-heme catalases produced by a thermophillic bacterium using magnetic resonance methods and "inorganic mutagenesis". Our research is aimed at developing a fundamental understanding of catalysis so that the factors which distinguish catalases from peroxidases, oxygenases and hydrolases can be defined and, ideally, reproduced in completely synthetic molecules for practical applications in catalysis.
Manganese catalase