1 / 26

The “Blue” Dimer : Water Oxidation Catalyst

The “Blue” Dimer : Water Oxidation Catalyst. Presented By: Margo Roemeling Mentor: Dr. James K. Hurst. The Basis for Life. Photosynthesis is the basis for all aerobic life on Earth. The process uses water and carbon dioxide as a source of electrons to make sugar and oxygen as a bi-product.

tanner
Download Presentation

The “Blue” Dimer : Water Oxidation Catalyst

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The “Blue” Dimer: Water Oxidation Catalyst Presented By: Margo Roemeling Mentor: Dr. James K. Hurst

  2. The Basis for Life • Photosynthesis is the basis for all aerobic life on Earth. • The process uses water and carbon dioxide as a source of electrons to make sugar and oxygen as a bi-product. • It involves the use of biocatalysts and energy from sunlight.

  3. Photosynthesis and the Oxygen Evolving Complex Electron Acceptor Oxygen Evolving Complex (O.E.C.) Pigments

  4. Oxygen Evolving Complex Mechanism • The O.E.C. is the water oxidation center of PSII • Has a 4 Mn metal active center • Di-oxo bridges • 2 terminal waters • Uses 4 photons to lose 4 e- and 4 H+ from waters • Upon reaching the S4 state, O2 is given off and 2 waters are taken up to bring it back to its most reduced state.

  5. Light is flashed on the photo reaction center, and every four flashes, O2 is given off.

  6. “Artificial Biosynthesis” • “Artificial biosynthesis” attempts to mimic photosynthetic reactions in simpler systems • In the growing energy crisis, it has become exceedingly important that we find new alternative fuels to replace fossil fuels. • Using these systems, we could mimic the way photosynthesis converts sunlight to energy, and convert sunlight into usable fuel energy. • Artificial biosynthesis could yield hydrogen fuel as well as alcohol fuels.

  7. A Simpler System hn 2H2O 2H2 (or CH2O + H2O) 4[e-] WOC photo- active element O2 + 4H+ 4H+ (+ CO2)

  8. The “Blue Dimer” • The “blue dimer” is a very effective catalyst for water oxidation (like the O.E.C.) • Water oxidation mechanisms of blue dimer are analogous to the water oxidation mechanisms in the O.E.C.

  9. Structure • 2 Ruthenium metal centers • 2 Terminal waters • Oxygen • 3 Bipyridineligands

  10. “Blue Dimer” vs. O.E.C. {3,3}-[(bpy)2Ru(OH2)]2O4+ PSII

  11. “Blue Dimer” Mechanism {3,3} e-, H+ • 2 Ru metal active center • Mono-oxo bridge • 2 terminal waters • Loses 4 e- and 4 H+ from water • Upon reaching {5,5} oxidation state, gives off an O2 and takes up 2 waters bringing it back to its most reduced state. {3,4} {5,5} e-, H+ e-, H+ {4,5} {4,4} e-, H+

  12. The Crucial Step • The two ruthenyl groups are structurally situated to allow joint addition of water to form the peroxo-bound intermediate • Once formed, the intermediate species is unstable and internal electronic rearrangements lead directly to the final products ({3,3} and O2).

  13. A Similar System “Pigment”, Photoreaction center Water Oxidation Center Electron acceptor

  14. The Reaction: Persulfate • S2O8 is often used as a reactant (electron acceptor) to study catalyzed water oxidation by redox-active metal ions. • S2O82- 2SO42-

  15. Surprising Results • Persulfate reacts thermally with the blue dimer and partially oxidizes it. • 2{3,3} + S2O82- 2{3,4} + 2SO42- • We need to understand this reaction and its relationship to the overall photocatalytic system.

  16. Question • Does this reaction involve direct reaction between persulfate and {3,3}? • S2O82- + {3,3} {3,4} + SO42- + SO4.- • SO4.- + {3,3} fast 2{3,4} + 2SO42- Net: S2O8 + 2{3,3} 2{3,4} + 2SO42- • Or is it indirect? • S2O82- 2SO4.- • SO4.- + {3,3} fast {3,4} + SO42-

  17. Hypothesis • We can use kinetics to distinguish between these reactions. • If direct, Rate = K[S2O8][{3,3}] • If indirect, Rate = K[S2O8]

  18. Methods • To study the kinetics of the reaction, a special instrument is used. • Because the reactions are very fast in basic solution, we use a stopped-flow machine. • 2 Syringes, one for each solution. • Solutions are quickly mixed and absorption is measured for 100 seconds as reaction is progressing.

  19. Stopped-Flow Trace • This trace from the stopped-flow machine shows the exponential decay of the reaction which tells us that it is first order.

  20. pH Dependence • The reaction is very fast in basic solution and very slow in acidic solution.

  21. THE RATE LAW • From the stopped-flow data, we can get the rate law. • If Rate = k[{3,3}][S2O8], this means that it is first order in {3,3} • So, we can treat [S2O8] as a constant making the new rate law: Rate = k’[{3,3}]

  22. First Order Reaction • Using Rate = k’[{3,3}], we can graph k’ vs. [S2O8] • And if the reaction is first order in {3,3} like we predicted, we should see a straight line.

  23. Further Tests • To further test our hypothesis of direct reaction, we tested the reaction at various ionic strengths. • If there was direct reaction between S2O8 and {3,3} we would see that as ionic strength increases, the rate of the reaction decreases.

  24. Temperature Dependence • Testing the temperature dependence allowed us to look at the activation energy barrier of the reaction

  25. Future • In the future, we plan to use computer modeling simulations to further study the kinetics of the reaction.

  26. Acknowledgements Howard Hughes Medical Institute Dr. James K. Hurst Dr. Kevin Ahern The Beckman Lab Group

More Related