T HE B LUE D IMER : W ATER O XIDATION C ATALYST Presented By: Margo Roemeling Mentor: Dr. James K. Hurst

T HE B ASIS FOR L IFE 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.

P HOTOSYNTHESIS AND THE O XYGEN E VOLVING C OMPLEX Pigment s Oxygen Evolving Complex (O.E.C.) Electron Acceptor

O XYGEN E VOLVING C OMPLEX M ECHANISM 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, O 2 is given off and 2 waters are taken up to bring it back to its most reduced state.

Light is flashed on the photo reaction center, and every four flashes, O 2 is given off.

A RTIFICIAL B IOSYNTHESIS 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.

A S IMPLER S YSTEM 2H 2 O O 2 + 4H + WOC photo- active element 4[e - ] 2H 2 (or CH 2 O + H 2 O) 4H + (+ CO 2 ) h

T HE B LUE D IMER 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.

S TRUCTURE 2 Ruthenium metal centers 2 Terminal waters Oxygen 3 Bipyridine ligands

B LUE D IMER VS. O.E.C. {3,3}-[(bpy) 2 Ru(OH 2 )] 2 O 4+ PSII

B LUE D IMER M ECHANISM 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 O 2 and takes up 2 waters bringing it back to its most reduced state. {3,3} {3,4} {4,4}{4,5} {5,5} e-,H+e-,H+ e -, H +

T HE C RUCIAL S TEP 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 O 2 ).

A S IMILAR S YSTEM Electron acceptor Pigment, Photoreaction center Water Oxidation Center

T HE R EACTION : P ERSULFATE S 2 O 8 is often used as a reactant (electron acceptor) to study catalyzed water oxidation by redox-active metal ions. S 2 O SO 4 2-

S URPRISING R ESULTS Persulfate reacts thermally with the blue dimer and partially oxidizes it. 2{3,3} + S 2 O {3,4} + 2SO 4 2- We need to understand this reaction and its relationship to the overall photocatalytic system.

Q UESTION Does this reaction involve direct reaction between persulfate and {3,3}? (1) S 2 O {3,3} {3,4} + SO SO 4.- (2) SO {3,3} fast 2{3,4} + 2SO 4 2- Net: S 2 O 8 + 2{3,3} 2{3,4} + 2SO 4 2- Or is it indirect? S 2 O SO 4.- SO {3,3} fast {3,4} + SO 4 2-

H YPOTHESIS We can use kinetics to distinguish between these reactions. If direct, Rate = K[S 2 O 8 ][{3,3}] If indirect, Rate = K[S 2 O 8 ]

M ETHODS 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.

S TOPPED -F LOW T RACE This trace from the stopped-flow machine shows the exponential decay of the reaction which tells us that it is first order.

P H D EPENDENCE The reaction is very fast in basic solution and very slow in acidic solution.

THE RATE LAW From the stopped-flow data, we can get the rate law. If Rate = k[{3,3}][S 2 O 8 ], this means that it is first order in {3,3} So, we can treat [S 2 O 8 ] as a constant making the new rate law: Rate = k[{3,3}]

F IRST O RDER R EACTION Using Rate = k[{3,3}], we can graph k vs. [S 2 O 8 ] And if the reaction is first order in {3,3} like we predicted, we should see a straight line.

F URTHER T ESTS To further test our hypothesis of direct reaction, we tested the reaction at various ionic strengths. If there was direct reaction between S 2 O 8 and {3,3} we would see that as ionic strength increases, the rate of the reaction decreases.

T EMPERATURE D EPENDENCE Testing the temperature dependence allowed us to look at the activation energy barrier of the reaction

F UTURE In the future, we plan to use computer modeling simulations to further study the kinetics of the reaction.

A CKNOWLEDGEMENTS Howard Hughes Medical Institute Dr. James K. Hurst Dr. Kevin Ahern The Beckman Lab Group