Biomolecular EPR Meets NMR at High Magnetic Fields

• Published in: Magnetochemistry 2018-12, Vol.4 (4), p.50
• Main Authors: Möbius, Klaus, Lubitz, Wolfgang, Cox, Nicholas, Savitsky, Anton
• Format: Article
• Language: English
• Subjects: Biomolecules; Case studies; Chemical reactions; Coordination compounds; Cryomagnetic properties; Crystal structure; desiccation tolerance by anhydrobiosis; Distance measurement; EDNMR; Electrons; ENDOR; Heat stress; high-field EPR; Magnetic fields; Magnetic resonance imaging; Mathematical analysis; Molecular biology; nitroxide radicals; NMR; NMR spectroscopy; Nobel prizes; Nuclear magnetic resonance; Oxidation; PELDOR; Photosynthesis; Physics; Physiology; Proteins; Radicals; Reference systems; Resonance; Selectivity; short-lived radical intermediates; Signal processing; Single crystals; site-directed spin labelling; Solid state; Structural analysis; Superhigh frequencies; Tensors; Transition metal compounds
• ISSN: 2312-7481; 2312-7481
• DOI: 10.3390/magnetochemistry4040050

In this review on advanced biomolecular EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR regarding the measurement of interactions and dynamics of large molecules embedded in fluid-solution or solid-state environments. Our focus is on the characterization of protein structure, dynamics and interactions, using sophisticated EPR spectroscopy methods. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy to new horizons reaching millimeter and sub-millimeter wavelengths and 15 T Zeeman fields. Expanding traditional applications to paramagnetic systems, spin-labeling of biomolecules has become a mainstream multifrequency approach in EPR spectroscopy. In the high-frequency/high-field EPR region, sub-micromolar concentrations of nitroxide spin-labeled molecules are now sufficient to characterize reaction intermediates of complex biomolecular processes. This offers promising analytical applications in biochemistry and molecular biology where sample material is often difficult to prepare in sufficient concentration for NMR characterization. For multifrequency EPR experiments on frozen solutions typical sample volumes are of the order of 250 μL (S-band), 150 μL (X-band), 10 μL (Q-band) and 1 μL (W-band). These are orders of magnitude smaller than the sample volumes required for modern liquid- or solid-state NMR spectroscopy. An important additional advantage of EPR over NMR is the ability to detect and characterize even short-lived paramagnetic reaction intermediates (down to a lifetime of a few ns). Electron–nuclear and electron–electron double-resonance techniques such as electron–nuclear double resonance (ENDOR), ELDOR-detected NMR, PELDOR (DEER) further improve the spectroscopic selectivity for the various magnetic interactions and their evolution in the frequency and time domains. PELDOR techniques applied to frozen-solution samples of doubly spin-labeled proteins allow for molecular distance measurements ranging up to about 100 Å. For disordered frozen-solution samples high-field EPR spectroscopy allows greatly improved orientational selection of the molecules within the laboratory axes reference system by means of the anisotropic electron Zeeman interaction. Single-crystal resolution is approached at the canonical g-tensor orient