1. Light-induced hyperpolarization in solid-state 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?
Figure 1: The solid-state photo-CIDNP effect. 13C magic-angle spinning (MAS) NMR spectra of reaction centers (RCs) of Rhodobacter sphaeroides R26 in the dark (top) and under light (bottom). The enhancement factor is more than 10000.
(ii) What is the origin of the high redox power of Photosystem II of plants?
(iii) How are radical pairs stabilized? Which role plays electrical polarization?
(iv) What is the functional variability of natural RCs? Photo-CIDNP MAS NMR is, in particular in connection with selective isotope labelling, an excellent method to characterize radical pairs in photosynthetic RCs.
2. The origin of the Solid-state photo-CIDNP effect: Principles of efficient electron transfer
|Efficient artificial electron pumps are needed. The photochemically induced electron transfer in natural reaction centers (RCs) is highly optimized, the quantum yield for the charge separation is almost 100%. 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. On the other hand, the solid-state photo-CIDNP effect has not yet been observed in systems having lower quantum yield.|| |
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 solid-state photo-CIDNP effect.
|The solid-state photo-CIDNP effect seems to be an intrinsic property of natural RCs. It even appears that the effect has been conserved in evolution, suggesting some correlation to functional relevance. The window of parameters of the effect is rather limited, and natural systems stay nicely around the optimum of this window. Until now, it was impossible to observe the solid-state photo-CIDNP effect in artificial RCs in stationary experiments. The solid-state photo-CIDNP effect and efficient electron transfer seem to be correlated. It is possible that both are based on the same principles. Knowledge of these principles would provide a blue print for artificial photosynthesis. Currently, we understand the mechanism of the occurrence of the solid-state photo-CIDNP effect at rather artificial conditions, as high fields and blocked electron transfer, but we do not know why nature has chosen a certain set of parameters. We also do not know whether the effect is preserved at natural conditions and we do not have a theory able to treat systems under such conditions.|
3. New optical solid-state NMR methods: More enhancement, time-resolved experiments, spin-torch experiments
Currently, the maximum signal enhancement due to the solid-state photo-CIDNP effect has been determined to be a factor of more than 10000. At lower magnetic fields, even more enhancement is expected.
Using flash lasers, the evolution of the radical pair can be studied with microsecond time resolution and electron spin densities can be observed at the atomic resolution. Processes of stabilizing the charge separation, which are relevant for the high efficiency, can be measured.
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).
|The high signal intensity of the solid-state photo-CIDNP effect can be used to explore the surroundings. Transfer of the signal intensity from the “spin-torch” allows for exploration of protein pockets, membranes and other surfaces.|
4. Principles of natural light control: Photocycle and signal transduction in phytochrome
|The light-induced change of the geometry of the tetrapyrrole cofactor (Figure 4) is transduced to the surface. The processes of cofactor-matrix interaction and of signal transduction are not yet understood. Solid-state and liquid-state NMR experiments in conjunction with selective isotope labelling can solve these problems. Comparison with other light switch proteins may reveal general principles and may inspire synthesis and nanoscience.|| || |
Figure 4: Structural formula of protein bound tetrapyrrole chromophore shown in the ZZZssa geometry, as assumed for the Pr state.