Research

We want to learn from the best: Nature. Can we build molecular devices good as nature? We want to understand the highly optimized natural systems and recognize their tricks. Therefore, we are working on the following projects:

1. Functional mechanism of natural photosynthetic reaction centres: Application of photo-CIDNP MAS NMR

Using the solid-state photo-CIDNP effect (Figure 1), the photochemical machinery of photosynthetic reaction centers (RCs) can be explored at the atomic resolution with microsecond time-resolution. We have the following questions:

(i) What makes the electron transfer in natural photosynthetic reaction centers (RCs) so efficient? (ii) What is the origin of the extremely high redox power of Photosystem II which allows for water oxidation? (iii) Which construction principles of natural RCs can be adapted for construction of artificial photosynthesis?

Figure 1: The solid-state photo-CIDNP effect. 13C magic-angle spinning (MAS) NMR spectra of a photosynthetic reaction center protein in the dark (top) and under light (bottom). The enhancement factor can be more than 80000.

2. Functional relevance of the Solid-state photo-CIDNP effect: The first order in photosynthesis

All natural RCs which we have investigated, coming from almost all photosynthetic families in the tree of life, show the solid-state photo-CIDNP effect (Figure 2), despite the window for the occurrence of the effect is rather narrow. The effect seems to be conserved in 3000000000 years of evolution. The solid-state photo-CIDNP effect produces spin-order. Is the effect related to functional relevance? To the efficiency of electron transfer even?  Does it, for example, provide a spin-valve preventing back-flow oft he electron? Do we have to implement the effect into artificial photosynthesis to make it more efficient?

Figure 2: Dependence of the DD mechanism of the solid-state photo-CIDNP effect on the lifetime of the radical pair. The value found for RCs of Rb. sphaeroides coincides with the maximum effect. It appears that nature has chosen conditions leading to a maximum nuclear spin polarisation.

3. New hyperpolarisation methods in NMR: More signal and faster measurements

Often application of NMR is limited by low Boltzmann spin polarisation: Therefore, NMR requires large sample volumes and longe measurement times. Production of non-equilibrium spin-states can overcome these limits. Our group works on different of these strategies as photo-CIDNP (Figure 3), optical pumping and the Haupt effect.

Figure 3: Time-resolved photo-CIDNP MAS NMR data. The data allow determining the nuclear spin dynamics during the radical pair evolution at the atomic scale. In addition, transient nuclear polarization occurs (positive features) before the polarization pattern known from steady-state experiments is built up (negative signals). 

4. Natural principles of light control: Photocycle and signal transduction in photosensors

The light-induced change of the geometry of the tetrapyrrole cofactor (Figure 4) is transduced to the surface of the protein. The processes of cofactor-matrix interaction and of signal transduction are not yet understood. Solid-state and liquid-state NMR experiments reveal general contruction principles and inspire synthesis and nanoscience.

Figure 4: Structural formula of protein bound tetrapyrrole chromophore shown in the ZZZssa geometry, as assumed for the Pr state of the natural photosensor phytochrome.

5. Properties of surplexes: Mutual control of substrate and surface

Is a molecule on the surface still the same molecule? Is a surface loaded with substrates still the same surface? No, they form together, as we call it, a surplex. Biological surplexes often show dramatic interrelations (Figure 5) and NMR is a wonderful tool to study both parts of the surplex. Surfaces and surplexes also can be approached by NMR using hyperpolarised gasses.

Figure 5: The photochemistry of the cofactor in phytochrome controls the properties of the protein around it. The photochemistry however is controlled by the protein itself. Both change their properties collectively as for example charge distribution and, as shown, the rigidity.

letzte Änderung: 22.06.2017