ESEEM and 2D ESEEM (HYSCORE)measure the magnetic nuclei around an unpaired electron, and you useone-dimensional ESEEM to check which nuclei are present, such as deuterium fromsolvent or nitrogen from a histidine ligand, and 2D ESEEM when signals overlapor you need to work out which frequencies belong to the same nucleus. Electronspin echo envelope modulation (ESEEM) is a pulsed EPR technique, and proteinscientists use it to study metal-binding sites, enzyme active sites,protein–ligand interactions and membrane proteins. This guide is forresearchers in structural biology, biophysics and drug discovery. It explainswhat each experiment measures, how two-pulse, three-pulse and HYSCOREexperiments differ, and what four published protein studies found with them.
What is ESEEM spectroscopy?
ESEEM spectroscopy is a pulsed electron paramagnetic resonance (EPR) method that records how magnetic nuclei near an unpaired electron make the intensity of its spin echo oscillate. These oscillations, or modulations, come from hyperfine interactions between the electron and the nuclei. For nuclei with a spin greater than ½, such asnitrogen-14, nuclear quadrupole interactions also shape the signal.
The frequencies, amplitudes andpatterns of the modulations tell you which nuclei sit near the paramagneticcentre and how strongly each one couples to it. ESEEM is one of several pulsedEPR spectroscopy methods. It focuses on the nuclei within a few ångströms ofthe electron, so it reports on local chemistry rather than overall proteinshape.
What information does ESEEM provide?
ESEEM provides nuclearfrequencies that identify which nuclei are nearby, hyperfine couplings thatshow how strongly each one interacts with the unpaired electron, and from bothof these, a picture of the local nuclear environment.
What are nuclear frequencies in ESEEM?
Nuclear frequencies are theresonance frequencies at which each type of nucleus responds in thespectrometer's magnetic field, and ESEEM peaks at or near these frequenciesidentify the nucleus. Hydrogen and nitrogen, for example, have differentgyromagnetic ratios, so they resonate at different frequencies in the samefield. Depending on the pulse sequence and the spin system, the spectrum canalso contain sums and differences of these frequencies.
What is hyperfine coupling?
Hyperfine coupling is themagnetic interaction between an unpaired electron and a nearby nucleus. Itsstrength depends on the electron–nucleus distance, the electronic structure,how the unpaired electron is spread over the molecule and the orientation ofthe interaction. Because electronic structure plays a part, a hyperfinecoupling cannot be read directly as a distance.
What does ESEEM show about the local environment?
ESEEM shows which groupssurround a paramagnetic centre and how accessible that centre is to solvent.Nitrogen signals characterise metal coordination, such as histidine bound tocopper. Proton and deuterium signals report on solvent accessibility and hydrogenbonding. Comparing spectra before and after a ligand binds shows how bindingchanges the site.
Which nuclei can ESEEM detect?
In biological samples, ESEEMmost often detects nitrogen-14, nitrogen-15, hydrogen, deuterium andphosphorus-31. Which of these appear in a given experiment depends on theparamagnetic centre, how close the nuclei are, the strength and orientation oftheir couplings and the experimental conditions.
How does ESEEM work?
ESEEM works by applying a timedsequence of microwave pulses to a paramagnetic sample in a magnetic field,recording the spin echo intensity as one pulse delay is stepped, and convertingthe resulting oscillations into a frequency spectrum with a Fourier transform.A typical experiment, using a hypothetical enzyme with a paramagneticiron–sulfur cluster as the example, runs as follows.
1. Prepare the protein in an EPR-compatible sample andcool it, usually to cryogenic temperatures, so the electron spins stay coherentlong enough to form an echo.
2. Set the magnetic field and microwave frequency to aposition on the EPR spectrum of the cluster.
3. Apply the pulse sequence. The pulses tip the electronspins, let them lose phase alignment and then bring them back into phase, whichproduces a spin echo.
4. Step the delay between pulses and record the echointensity at each step. Nearby nuclei change how the electron spins evolve, sothe echo intensity oscillates as the delay changes. This record is thetime-domain signal.
5. Apply a Fourier transform to turn the time-domainsignal into a frequency spectrum, with peaks at nuclear transition frequenciesand, depending on the experiment, their combinations.
6. Assign the peaks, often with spectral simulations, toidentify the nuclei and estimate their couplings.
What is the difference between two-pulse and three-pulse ESEEM?
Two-pulse ESEEM records themodulation of a primary echo made by two pulses, whereas three-pulse ESEEMrecords a stimulated echo made by three pulses, which gives better frequencyresolution but can suppress some frequencies. HYSCORE extends three-pulse ESEEMto two dimensions with a fourth pulse.
What is two-pulse ESEEM?
Two-pulse ESEEM is the simplestESEEM experiment: a 90° pulse and a 180° pulse separated by a delay, τ, producea primary spin echo. The first pulse moves the electron magnetisation into thetransverse plane, where the spins dephase during τ. The 180° pulse reversestheir evolution so they rephase and form the echo. The experiment is repeatedwith increasing τ, and the echo modulation is Fourier transformed into aspectrum.
What is three-pulse ESEEM?
Three-pulse ESEEM adds a thirdpulse so the experiment measures a stimulated echo, and the delay between thesecond and third pulses, T, is the one that is stepped. Between those pulses,part of the magnetisation is stored along the magnetic field, where it decaysmore slowly. The signal can therefore be followed for longer, which narrows thepeaks and separates frequencies that overlap in a two-pulse spectrum. Thetrade-off is blind spots: depending on the fixed delay τ between the first twopulses, some nuclear frequencies are suppressed. Recording spectra at several τvalues recovers them.
What is HYSCORE spectroscopy?
HYSCORE (hyperfine sublevelcorrelation) spectroscopy is a four-pulse, two-dimensional ESEEM experimentthat shows which nuclear frequencies belong to the same nucleus. It adds a 180°mixing pulse between the second and third 90° pulses of the three-pulsesequence. This pulse transfers nuclear coherence between the two electron-spinmanifolds. Two delays, t1 and t2, are stepped independently, and a Fouriertransform along both gives a 2D spectrum. Cross peaks in that spectrum linkfrequencies from the two manifolds, which separates nuclear environments thatoverlap in one-dimensional ESEEM.
How is ESEEM used in protein research?
In protein research, ESEEM andHYSCORE are used to identify the ligands around paramagnetic metal centres, towork out which protein supplies which ligand in a complex, and to measure howexposed spin-labelled sites are to water and lipids. The four studies belowshow each of these.
How is ESEEM used to study metalloproteins?
ESEEM identifies the ligands andsolvent exposure of paramagnetic metal sites, as in a 2015 Journal of PhysicalChemistry B study by Albertiniand colleagues on the iron–sulfur cluster of HydF. HydF is thought to actas a scaffold and carrier for the two-iron ([2Fe]) subcluster of the[FeFe]-hydrogenase H-cluster. Its [4Fe–4S] cluster is held by three conservedcysteines and a fourth, non-cysteine ligand. The team combined three-pulseESEEM and HYSCORE with hydrogen–deuterium exchange to examine that fourthligand and how open the cluster is to solvent. They proposed that the iron atomwith the non-cysteine ligand is where the [2Fe] precursor is anchored andprocessed. A 2025 Journalof the American Chemical Society study from Montana State University laterconcluded that the precursor binds next to the cluster rather than directly toit, so the anchoring site is still debated.
How does ESEEM show copper binding between two proteins?
ESEEM and HYSCORE show whichprotein supplies each copper ligand when one protein is labelled withnitrogen-15 and its partner is not, the approach used by Smart and colleagues in theJournal of Biological Chemistry in 2025. They asked whether a copper ioncould bridge the cellular prion protein (PrPᶜ) and amyloid-beta (Aβ), thepeptide that forms Alzheimer's plaques, since both bind copper with highaffinity. Uniformly ¹⁵N-labelled PrPᶜ was mixed with Aβ at natural ¹⁴Nabundance, so histidines from each protein gave different signals. The resultssupported a copper-stabilised complex in which one histidine from Aβ and threefrom PrPᶜ coordinate the copper. The method gives local coordination detail forproteins that lack a well-defined overall structure.
How is ESEEM used to study membrane proteins?
ESEEM combined withsite-directed spin labelling shows which parts of a membrane protein touchwater and which touch lipid headgroups, as Cieslak, Focia and Gross showed forthe KcsA potassium channel in Biochemistry in 2010. Using D₂O, deuteriumESEEM detected water along the lipid-exposed surface of the channel's outerhelix, with less water deeper in the membrane. Phosphorus-31 ESEEM identifiedresidues in contact with the phosphate headgroups of the bilayer. Combined withX-ray crystallography, these data defined the surface of the protein that facesthe membrane.
How do you measure membrane protein depth with ESEEM?
Membrane depth is measured withdeuterium ESEEM by placing spin labels at chosen residues and comparing theircontact with deuterated water and with deuterated lipid chains, the method Morris, McCarrick and Loriganapplied to the SARS-CoV-2 envelope (E) protein in Biophysica in 2025. Theirquestion was where the protein's C-terminal domain sits relative to themembrane. Labels along the transmembrane helix showed close contact with lipidchains and also substantial water exposure. Labels on the C-terminal domainshowed substantial but lower lipid contact, with comparable water exposure.These results support a C-terminal domain that rests on the surface of thebilayer.
How is ESEEM used in drug discovery?
In drug discovery, ESEEM is usedto see how a compound changes the environment of a paramagnetic metal centre orspin label when it binds nearby. A compound that displaces a metal-bound water,for example, removes that water's proton or deuterium signal, and a compoundthat changes a histidine ligand changes the nitrogen signal. These measurementscharacterise binding sites in metalloenzymes and other metalloproteins wherecrystal structures are missing or incomplete. ESEEM needs a paramagnetic centreor a spin label, and samples are usually frozen, so it reports on the localsite under those conditions rather than on the whole protein in solution.
Key takeaways
• ESEEM is a pulsed EPR method that identifies magneticnuclei near an unpaired electron from oscillations in its spin echo.
• Hyperfine coupling depends on distance and electronicstructure, so ESEEM gives local environment information rather than directdistances.
• Three-pulse ESEEM gives better frequency resolutionthan two-pulse ESEEM but has τ-dependent blind spots.
• HYSCORE is a four-pulse 2D experiment that correlatesnuclear frequencies and separates overlapping signals.
• Isotope labelling with ¹⁵N, D₂O or deuterated lipidslets ESEEM assign signals to a specific protein, solvent or membrane region.
Frequently asked questions
How close does a nucleus need to be for ESEEM?
A nucleus needs to be within afew ångströms of the unpaired electron, because ESEEM modulation weakenssharply with distance. In a 2011 Protein Science study, Mayo and colleagues reported thatthe modulation depth from a weakly coupled nucleus scales with r⁻⁶, and thatdeuterium modulation from a labelled amino acid is seen when it is within about8 Å of a nitroxide spin label. Exact limits depend on the nucleus, theparamagnetic centre and the signal-to-noise ratio.
Why is ESEEM measured at low temperature?
ESEEM is usually measured atcryogenic temperatures because cooling slows electron spin relaxation, whichkeeps the spin echo strong enough to record its modulation over a useful timewindow. The best temperature depends on the paramagnetic centre and itsrelaxation rate. The 2025 Journal of Biological Chemistry study of copper-boundPrPᶜ and Aβ by Smart and colleagues ran its pulsed experiments at 18 K.
What are blind spots in three-pulse ESEEM?
Blind spots are nuclearfrequencies that a three-pulse ESEEM experiment suppresses because of the fixeddelay, τ, between the first and second pulses. A given frequency disappearswhen τ matches a whole number of periods of the related frequency in the otherelectron-spin manifold. Recording the experiment at several τ values, andsumming or comparing the spectra, brings the missing peaks back. HYSCORE hasthe same τ dependence, so it is also often recorded at more than one τ.
What is the difference between ESEEM and DEER?
ESEEM measures the interactionbetween an unpaired electron and nearby magnetic nuclei, whereas DEER (doubleelectron–electron resonance) measures the interaction between two unpairedelectrons to give the distance between them. ESEEM therefore reports on localchemistry, such as metal ligands and water or lipid contact. DEER givesdistance distributions between two spin labels, which are used to followconformational changes, flexibility and protein–protein interactions. Manyprojects use both.
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