Multi-Quantum Spin Counting and Long Range Distance Measurements by Pulsed EPR

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Pulsed EPR methods allow to determine distances in the 1-10 nm range on macromolecules. This is done by measuring the magnetic dipole-dipole interactions between two covalently attached paramagnetic spin labels. In this project we want to extend these methods for the most common used nitroxide spin labels in two respects:1) Extending the measurable distance range by developing Carr-Purcell-SIFTER pulse sequences with broadband amplitude and frequency modulated microwave pulses. Such refocusing sequences decouple the electron spin from closeby nuclear spins and allow to prolong the observation time window. This is important for applications to large macromolecular systems with high internal flexibility (as for example intrinsically unfolded proteins, membrane protein complexes or protein-nucleic acid complexes). 2) Counting of the dipolar coupled electron spins in a complex by Multi-Quantum EPR experiments with broadband amplitude and frequency modulated microwave pulses.This would allow to determine for example oligomeric states of membrane protein complexes in their native lipid environment. The new type of experiments will be tested and optimized on model compounds and then applied to biological systems.