Biomolecular EPR Meets NMR at High Magnetic Fields

Abstract

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 orientations—even for molecules with very small g-anisotropies. Unique structural, functional, and dynamic information about molecular systems is thus revealed that can hardly be obtained by other analytical techniques. On the other hand, the limitation to systems with unpaired electrons means that EPR is less widely used than NMR. However, this limitation also means that EPR offers greater specificity, since ordinary chemical solvents and matrices do not give rise to EPR in contrast to NMR spectra. Thus, multifrequency EPR spectroscopy plays an important role in better understanding paramagnetic species such as organic and inorganic radicals, transition metal complexes as found in many catalysts or metalloenzymes, transient species such as light-generated spin-correlated radical pairs and triplets occurring in protein complexes of photosynthetic reaction centers, electron-transfer relays, etc. Special attention is drawn to high-field EPR experiments on photosynthetic reaction centers embedded in specific sugar matrices that enable organisms to survive extreme dryness and heat stress by adopting an anhydrobiotic state. After a more general overview on methods and applications of advanced multifrequency EPR spectroscopy, a few representative examples are reviewed to some detail in two Case Studies: (I) High-field ELDOR-detected NMR (EDNMR) as a general method for electron–nuclear hyperfine spectroscopy of nitroxide radical and transition metal containing systems; (II) High-field ENDOR and EDNMR studies of the Oxygen Evolving Complex (OEC) in Photosystem II, which performs water oxidation in photosynthesis, i.e., the light-driven splitting of water into its elemental constituents, which is one of the most important chemical reactions on Earth.

Selective 13C labelling reveals the electronic structure of flavocoenzyme radicals

“…In this contribution we exploit the potential of high-field ENDOR 25 – 28 in combination with selective isotopologue editing to overcome the problem of measuring and assigning 13 C hfcs of a flavin cofactor in its native protein environment. This facilitates a thorough mapping of the hyperfine structure and hence will ultimately allow theoreticians to draw quantitative conclusions on unpaired electron spin densities of flavocoenyzmes in their functional surroundings.…”

Direct enantiomeric discrimination through antisymmetric hyperfine coupling

“…The hyperfine coupling (HFC) between the spins of an unpaired electron and a nucleus provides important information about the molecular structure and interactions. 1,2 However, the techniques used in electron paramagnetic resonance (EPR) spectroscopy can so far only indirectly be applied to distinguish the enantiomers of chiral molecules, e.g., by transforming the molecule to a diastereoisomeric entity by chiral environment such as a gel or a chiral solvent, or by conducting a reaction with a chiral derivatizing agent. 3,4 In this letter we propose a new effect that allows direct chiral discrimination from the vector of antisymmetric HFC, A*.…”

“…with the appropriate initial conditionq(0). At high field, the Hamiltonian [eqn(1)] can be approximated aŝ…”

Special Issue: Electron Paramagnetic Resonance

“…Recent achievements in both theory and instrumentation have made EPR central in order to study new interdisciplinary topics. Such achievements are found in the collection of papers from the Special Issue, devoted to EPR, with contributions in technical improvement [1,2], theory in physics [3], biology [2,4,5], and biochemistry [6]. This issue is articulated around three original articles and three specific reviews.…”

“…They demonstrated a challenging low concentration protein labeled by Gd(III) distance measurements. Staying in high field EPR, Klaus Möbius and Anton Savitsky and co-workers [6] presented an impressive review on complementary high field EPR and NMR (nuclear magnetic resonance) in advance biomolecular spectroscopy. More than 600 papers were presented and classified, showing the entire history as well as recent improvements and state of the art in the domain.…”