x-fel experiments on organic and biological systems

X-FEL Workshop, Dresden-Rossendorf, Sep. 5th to 10th 2011 Karim Fahmy, Div. Biophysics

X-FEL Experiments on Organic and Biological Systems

Karim FahmyDivision of Biophysics

Institute of RadiochemistryHelmholtz-Zentrum Dresden-Rossendorf



Talking about biological systems in the context of HED physics is challenging

High energies and densities do not necessarily help to reveal

the secrets of life



Two fundamental processes govern organic and biological chemistry:

1) formation and breaking of chemical bonds

=> involves electron transfer reactions

2) conformational freedom / restriction

=> involves isomerization / protein folding, aggregation...

Problem-solving applications of X-FEL should address:

photo-inducedelectron transfer

metal organic systems

photo-inducedconformational transitions

in proteins


Three systems to be discussed for X-FEL experiments

1) Uranyl photochemistry: excited state electron transfer affects redox state - light-dependent solubility changes, relevance to environmental mobility - technological applications (separation / photon-induced partitioning)

structural basis: ligand to metal charge transfer LMCT

photo-induced electron transfer metal organic systems

photo-induced conformational transitions in proteins

3) Photoreaction of rhodopsin (basis of vision and molecular model for hormone reception)

2) Dye-sensitized photovoltaics - ligt-induced injection of electrons from an organic chromophore into the conductance band of a semiconductor

structural basis: metal to ligand charge transfer MLCT


Aqueous coordination chemistry and photochemistry of uranyl(VI) oxalate revisited: a density functional theory studySatoru Tsushima, Vinzenz Brendler and Karim Fahmy 2010, 39, 10953–10958

Photo-induced electron transfer metal organic systems1) Uranyl photochemistry

Observation: Complexes of UO22+ with organic acids decompose under light

reaction products CO2, CO, UVI -> UIV , depends on pH, stoichiometry...

Zhang et al. Radiochimica Acta (2010). Uranyl photochemistry: decarboxylation of gluconic acid

How does the coordination structuredefine the chemical reaction pathway?

hydrogen abstraction / charge transfer

Can it be predicted from first principles?

solution complexes of Uranyl oxalate


Photo-induced electron transfer metal organic systems1) Uranyl photochemistry

Reaction coordinate predicted from DFT calculations based on spin density in the excited triplet state of UO2

2+

Suggestion:

optical pump / X-ray probe experiment in (poly)crystalline state

requires a) strong pump beam (optical transition in Uranyl is forbidden)

b) single shot probe X-ray: pumping is chemically destructive


Photo-induced electron transfer metal organic systems2) Dye-sensitized photovoltaics

An organic dye transfers an electron into the conductance band of a semi onductor

DSP is beyond prototype and on the market


Principle of Dye-Sensitized Solar Cells

nano-crystalline TiO2 electrode

Michael Grätzel, EPFL



Optical transition has the character of a Metal-to-Ligand-Charge Transfer (MLCT), large cross-section, e-injection into TiO2 within fs-ps.

Suggestion:Structural and dynamic properties of dye to semiconductor electron transfer.Studies on selfassembled 2D arrays may profit from high intensity beams byenlarging the pumped surface

More general field for X-FEL studies: photocatalysis at liquid / solid interfaces



Photo-induced conformational transitions in proteins3) Time-resolved protein conformational changes

The central goal in modern Structural Biology:resolve the 3D structure of large proteins

identify the structural basis of biological function

rational design of drugs, which enhance or inhibit function by interfering with key structural elements of their protein targets


1) The classical approach: crystallization and isomorphic replacement

- Crystallization difficult for proteins residing in the cell membrane

However: 50% of pharmaceuticals target membrane proteins

Limitations and challenges

low T trapping of intermediates in the crystal


2) Obtained structures are static

However: structural transitions are the basis of biological function

- dynamics not resolved, crystal contacts lock flexible domains

Bacteriorhodopsincurrently Rhodopsin



Suggestion: Follow structural changes in rhodopsin, a photosensitive membrane protein

Primary photoreaction (200 fs)followed by slow thermally activated conformational changes



Rhodopsin can be prepared in a variety of states:

3D and 2D crystals, micelles, liposomes, nanodiscs

a = 44 Åb = 131 Å

Davies et al., 1996 JStrBiol

functionality in these states is well characterized


high energy pump and probe may allow sampling large spot sizesin lateraly extended samples

visualize photoisomerization in real-time (fs-ps) at atomic resolution

alternatively: small angle X-ray scattering may resolve helical movements

visualize large domain movements over ~100 µs


The longer time scales are more informative for pharmacology


SUMMARY

There is large interest in studying biological photoreaction mechanismsat atomic resolution and in real time:

- metal organic systems in photosynthesis and photocatalysis are attractive

- proteins should be studied for which crystal structures have been solved

- fs to ps data are relevant to quantum yields (primary photoreaction)

- longer time scales are required to elucidate protein function

- rhodopsin has become a paradigm for membrane proteins and will surely find its way into time-resolved X-ray studies

- but where: at synchrotron or at X-FEL?

- many aspects have not been addressed but may become crucial: liquid sample handling, flow through systems, hydration control... a.o. efforts to maintain nativeness

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x-fel experiments on organic and biological systems

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