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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.

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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
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.
photosynthesis and the oxygen evolving complex
Photosynthesis and the Oxygen Evolving Complex

Electron Acceptor

Oxygen Evolving Complex (O.E.C.)

Pigments

oxygen evolving complex mechanism
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.
artificial biosynthesis
“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.
a simpler system
A Simpler System

hn

2H2O

2H2 (or CH2O + H2O)

4[e-]

WOC

photo-

active

element

O2 + 4H+

4H+ (+ CO2)

the blue dimer
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.
structure
Structure
  • 2 Ruthenium metal centers
  • 2 Terminal waters
  • Oxygen
  • 3 Bipyridineligands
blue dimer vs o e c
“Blue Dimer” vs. O.E.C.

{3,3}-[(bpy)2Ru(OH2)]2O4+

PSII

blue dimer mechanism
“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+

the crucial step
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).
a similar system
A Similar System

“Pigment”, Photoreaction center

Water Oxidation Center

Electron acceptor

the reaction persulfate
The Reaction: Persulfate
  • S2O8 is often used as a reactant (electron acceptor) to study catalyzed water oxidation by redox-active metal ions.
  • S2O82- 2SO42-
surprising results
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.
question
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-
hypothesis
Hypothesis
  • We can use kinetics to distinguish between these reactions.
    • If direct,

Rate = K[S2O8][{3,3}]

    • If indirect,

Rate = K[S2O8]

methods
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.
stopped flow trace
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.
ph dependence
pH Dependence
  • The reaction is very fast in basic solution and very slow in acidic solution.
the rate law
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}]
first order reaction
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.
further tests
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.
temperature dependence
Temperature Dependence
  • Testing the temperature dependence allowed us to look at the activation energy barrier of the reaction
future
Future
  • In the future, we plan to use computer modeling simulations to further study the kinetics of the reaction.
acknowledgements

Acknowledgements

Howard Hughes Medical Institute

Dr. James K. Hurst

Dr. Kevin Ahern

The Beckman Lab Group