Modern EPR spectroscopy: beyond the EPR spectrum

During the last two decades eectron paramagnetic resonance (EPR) spectroscopy (also known as electron-spin resonance (ESR) and electron magnetic resonance (EMR)) has undergone tremendous methodological and instrumental developments that have led to many new applications. The way EPR is currently being practised is very different than what we have been used to two decades ago. The term “modern EPR spectroscopy” refers to a wealth of techniques performed over a very wide range of frequencies, 1–600 GHz. These are carried out in continuous wave, time resolved or pulse modes, exploiting single and multiple resonances carried out as single and multi-dimensional experiments. The good-old, highly reliable X-band (∼9.5 GHz) continuous wave (CW) EPR continues to be the fastest, routine way to measure an EPR spectrum of many paramagnetic systems. However, since the early 1990s a variety of pulse EPR and high field EPR (continuous wave and pulse) techniques have become widespread, providing unprecedented structural (spatial and electronic) as well as dynamic information that cannot be obtained from the conventional CW X-band spectrum. The scope of EPR spectroscopy is large as it is applicable to a broad range of systems—with either intrinsic paramagnetic centers or with paramagnetic centers introduced artificially as probes—that are of interest in physics, chemistry, materials science and biology. The new information obtained, provides further impetus for instrumental and methodological developments.

Pulse EPR spectroscopy consists of high resolution methods that can be grouped into three major types of families:

(i) Hyperfine spectroscopy, aiming at the determination of the local environment of paramagnetic centers such as transition metal ions, clusters or radicals via the interaction with nuclear spins. The techniques include one and two dimensional electron spin echo envelope modulation (ESEEM), electron–nuclear double resonance and electron–electron-double resonance techniques.

(ii) Techniques aiming at measurements of electron–electron distances. These have recently become highly popular in the context of the structure of biomolecules and interactions between them, but have been also applied to polymers, materials and micellar solutions.

(iii) Techniques that provide detailed dynamical information in systems such as membranes. These are carried out at ambient temperatures, as opposed to the upper two where the measurements are usually carried out in the solid state at low temperatures.

These high resolution techniques can in principle be carried out at any operational frequency. In recent years the development of high field/high frequency EPR has opened many new possibilities in EPR applications and new systems have become amenable to EPR investigation, a few examples are: high spin systems with large zero field splittings, either isolated or exchanged coupled (molecular magnets). It allowed resolving g-anisotropy of radicals important in biological systems and revealing new dynamics details in motional regimes that are considered too fast at X-band frequencies and there are many more. The experience gained in high field EPR instrumentation has also had an important impact on dynamic nuclear polarization (DNP) that can now be applied at high fields to enhance NMR signals.

Another important development that has had an important impact on EPR spectroscopy is that of quantum mechanical calculations, particularly density function theory (DFT) calculations. These provide the crucial relation between spin Hamiltonian parameters determined by the EPR experiments and the geometric and electronic structure and are therefore increasingly used in the interpretation of the EPR parameters. This approach, however, has not yet reached maturity and requires further development that relies heavily on the availability of combined experimental/theoretical studies.

This PCCP theme issue groups a collection of manuscripts that give an overview of the state of the art of modern EPR spectroscopy presenting a broad range of applications, new methodologies and advances in data analysis. The contribution of EPR to understanding properties of molecular nanomagnets and magnetic nanoparticles appears in a perspective article (Fittipaldi et al.) and in a regular article on magnetic quantum tunneling (Lawrence et al.). Investigations of dynamics by the multi-frequency approach appear in manuscripts on spin labeled molecules included in cyclodextrines (Dzikovski et al.), in calixarene nanocapsules (Bagryanskaya et al.) and in spin triads of Cu-nitroxides (Fedin et al.). Application of high resolution EPR techniques to chemistry is represented by manuscripts on the reaction of NO with silver clusters in zeolites (Baldansuren et al.), the interaction of vanadyl complexes with an AlF₃ surface (Nagarajan et al.), sulfur-rich Co(II) complexes that serve as a model for some metal binding sites in proteins (Sottini et al.) and enantioselective binding of epoxides to a chiral vanadyl salen complex (Murphy et al.). Studies related to metalloproteins are on [2Fe–2S] clusters in adrenodoxin (Dikanov et al.) and the H-cluster of [FeFe] nitrogenase (Silakov et al.). The use of DFT for extraction of structural information from EPR data is described in manuscripts on the oxygen-evolving complex of photosystem II (Pantazis et al.) and tyrosyl radicals in proteins (Svistunenko et al.).

There are several manuscripts on DEER, concentrating on data analysis in the presence of the exchange interaction (Margraf et al.), orientation selectivity (Lovett et al.) and analyzing three-spin correlations. Applications of DEER appear in the study of conformations of single and double strand DNA (Kuznetsov et al.) and the topology of amphipathic helices in membranes (Böhme et al.). This issue has number of manuscripts on DNP, focusing on theoretical aspects of DNP in liquids at high fields (Sezer et al.) and the use of DNP to follow water dynamics (Pavlova et al.). Studies of spin-correlated radical pairs are also described in this issue, specifically the spin dynamics of the spin-correlated radical pair in photosystem I at high fields (Poluektov et al.) and the effect of radiofrequency polarization on the yield of transient spin-correlated radical pairs at zero field and low field is investigated (Rodgers et al. and Wedge et al.).

On the methodological side the issue includes a description of a method for signal enchantment for high spin systems at high fields (Kaminker et al.) and the presentation of a general and efficient simulation of pulse EPR spectra (Stoll et al.). Simulations are a major and essential tool in EPR spectroscopy for the extraction of the spin Hamiltonian parameters. Finally, solid state EPR imaging with sub-micron resolution is described (Blank et al.) and THz EPR using coherent synchrotron radiation (Schnegg et al.) are presented—bringing new dimensions into the wide world of EPR spectroscopy.

I thank all the authors for their contributions and the PCCP editorial staff for their efforts to put this issue together in a rather tight schedule.

Daniella Goldfarb (Guest Editor), Weizmann Institute of Science, Rehovot, Israel.

---