Electron Para Magnetic Resonance

8/3/2019 Electron Para Magnetic Resonance

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Electron paramagnetic resonance (EPR)

spectroscopy

McGraw-Hill Science & Technology Encyclopedia:

Electron paramagnetic resonance (EPR) spectroscopy

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The study of theresonantresponse to microwave- or radio-frequency radiation ofparamagneticmaterials placed in a magnetic field. It is sometimes referred to aselectron spin resonance(ESR).

Paramagnetic substances normally have an odd number of electrons or unpaired electrons, but

sometimeselectron paramagnetic resonance(EPR) is observed for ions or biradicals with aneven number of electrons. EPR spectra are normally presented as plots of the first derivative of

the energy absorbed from anoscillating magnetic fieldat a fixed microwave frequency versus

the magnetic field strength. The dispersion may also be detected.

To overcome theintrinsiclow sensitivity of the magnetic dipole transitions responsible for EPR,

samples are placed in resonant cavities. Routine experiments are carried out in the steady state at

a fixed microwave frequency of approximately 9gigahertzby slowly sweeping the magneticfield through resonance. Free electronsresonatein a magnetic field of 3250gauss(325

millitesla) at the microwave frequency of 9.1081 GHz, whereas organic free radicals resonate at

slightly different magnetic fields characteristic of each particular molecule. See alsoElectronspin.

The observation of EPR spectra depends on spin-lattice relaxation, which is the exchange of

magnetic energy with the thermal motion of the crystal or molecule. For transition-metal ions

and rare-earth ions, experiments often require operation at or nearliquid heliumtemperature (4K; 269C; 452F). Organic free radicals can usually be studied successfully at roomtemperature.

Applications

EPR spectroscopy is used to determine the electronic structure of free radicals as well as

transition-metal and rare-earth ions in a variety of substances, to study interactions between

molecules, and to measure nuclear spins and magnetic moments. It is applied in the fields ofphysics, chemistry, biology, archeology, geology, andmineralogy. It is also used in the

investigation of radiation-damaged materials and inradiation dosimetry.
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The basic physics of transition-metal ions and rare-earth ions present in low concentrations in

diamagnetichost crystals has provided a theoretical basis for how electronic structure ismodified by the surrounding atoms. Particular applications include probing phase transitions in

solids and studies of pairs and triads of magnetically interacting ions.

Applications of EPR in chemistry include characterization of free radicals, studies of organicreactions, and investigations of the electronic properties of paramagnetic inorganic molecules.

Information obtained is used in the investigation of molecular structure. EPR is used widely inbiology in the study of metal proteins, for nitroxide spin labeling, and in the investigation of

radicals produced during reaction processes in proteins and other biomacromolecules. EPR has

proved to be an important technique forinterdisciplinaryinvestigations ofphotosyntheticsystems. By means of EPR, more than 20 proteins that function in themitochondrialrespiratory

chains of mammals have been identified, and details regarding their electron transfer processes

have been elucidated.

Solids

EPR spectra from single crystals clearly provide the greatest amount of information. These

include crystals containing small concentrations of paramagnetic ions substituting for the regularions in the crystal or, for organic molecules, small fractions of free radicals produced byionizing

radiation. Spectra which may contain up to several hundred lines are often highlyanisotropic;

that is, they change with the orientation of the magnetic field direction in the crystal. Transition-metal-ion and rare-earth-ion EPR spectra in crystals are generally much more anisotropic than

free radicals due to the intrinsic anisotropy of the electron magnetic moments, and of other

effects that are important when there is more than oneunpaired electron.

The occurrence of many lines is due to interactions of the orbital motions of electrons with the

electric potential of the local surrounding atoms, and to hyperfine interactions between the

paramagnetic electrons and nuclear magnetic moments of the paramagnetic ion and surroundingatoms. In the case of free radicals, symmetric or nearly symmetric characteristic hyperfine

patterns are observed. From knowledge of hyperfine interactions with nuclei whose spins and

magnetic moments are known, the electron distribution throughout a molecule may bedetermined. Since hyperfine interactions vary as thereciprocalof the cube of the distance

between the center of the free radical and the nucleus, structural information may be obtained in

addition to electron densities.

Liquids and motional averaging

Spectra in solution due to free radicals are often quite simple as a result of motional averaging,

and this clearly gives less information than would be obtained from a single-crystal investigation.Linewidths are very narrow (approximately 0.1 gauss or smaller). By varying the temperature

above and below room temperature, EPR spectra range from the frozen solution at low

temperatures, with a powderlike spectrum, to rapid motional averaging at room temperaturewhere anisotropies are averaged out. The intermediate region can provide information about

slow molecular motions, which is especially important for nitroxide spin labels selectively

attached to different parts of macromolecules such as the components of natural and synthetic
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phospholipidmembranes, liquid crystals, and proteins. Such measurements have revealed

important structural and functional information

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EPR: Application

We have highlighted your search term application of electron spin resonance spectroscopy foryou. If you'd like to remove the search term,click here.

This article is about the experimental application of Electron Paramagnetic Resonance

spectroscopy (EPR). For theoretical background on EPR, see alternate articleEPR: Theory. The

most basic application of Electron Paramagnetic Resonance spectroscopy requires the use of a

microwave radiation source, an electromagnet, a resonator, and a detector.

1. 1.Background2. 2.A. Instrumentation1. 2.1.1. History of EPR Instrumentation2. 2.2.2. Microwave radiation source3. 2.3.3. Resonator4. 2.4.4. Detection Methods5. 2.5.5. Electromagnet6. 2.6.6. Optional Components7. 2.7.7. CalEPR Center3. 3.B. Experimental Applications1. 3.1. 1. CW EPR2. 3.2.2. Pulse EPR3. 3.3.3. ENDOR4. 3.4.4. Spin-Spin Labeling Experiments4. 4.References5. 5.Outside links6. 6.Problems7. 7.Contributors

Background

The experimental procedure is to place a paramagnetic sample in the resonator, which isdesigned to resonate at a specific microwave frequency. The resonator is positioned between

two electromagnets, which vary in magnetic field strength depending on how much current is run

through them. Electromagnetic radiation in the microwave frequency range is produced using aklystron and channeled into the resonator. While the microwave radiation is kept at a constant

frequency, the strength of the applied magnetic field will be swept. Resonance is detected by a

decrease in the amount of microwave radiation that is being reflected out of the resonator.Reflected microwaves are channeled specifically into the detector by a special component called
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a circulator. Resonance will be detected at the field strength that corresponds to the energy

splitting of the spin states of the unpaired electron.

Figure 1: Basic experimental setup of an EPR spectrometer.

These are the most basic elements in an experimental setup for the observation of electron

paramagnetic resonance. This article first elaborates on each of these basic elements of EPR

Instrumentation and introduces other advanced elements necessary for collecting moresophisticated spectra, which are used to characterize an unpaired electron and its the

environment. The experimental applications of this spectroscopy are then briefly reviewed.

A. Instrumentation

1. History of EPR Instrumentation

EPR was first detected in 1945 by Yevgeny Zaboisky at Kazan State University in the Soviet

Union. The instrumentation required for EPR was greatly benefited by the development ofRADAR during WWII, which required the use of reliable and tunable microwave sources. After


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the war, the necessary components for an EPR spectrometer were cheap and available. Initially,

the field was advanced with the production of home-built EPR spectrometers. In the 1980s,commercially built EPR spectrometers became available. The German company Bruker leads

the field in the production of commercially available EPR spectrometers, as well as NMR

spectrometers and MRI detectors. Today, EPR spectroscopy is done at a wide range of

frequencies and fields.

2. Microwave radiation source

A microwave radiation source is necessary for any EPR spectrometer, because this the energy

that corresponds with a detectable splitting of the electronic spin states. Microwave radiation istypically produced and amplified by a klystron, which is capable of tuning waves to a precise

frequency, amplitude, and phase. The microwaves are then channeled into the resonant cavity by

use of either waveguides or coaxial cables. Waveguides are the most common method of

microwave propagation. Waveguides are essentially open air, brass rectangular channels withdimensions that correspond to the wavelength of the radiation that is to be propagated. However,

for low power, low frequency microwave radiation, like that often used in EPR, special coaxialcables have been found to be just as effective.

In order to observe resonance, the frequency of the microwaves must correspond to the splitting

of the spin states of the electron, which is determined by the strength of the magnetic field.Thus, the strength of the magnetic field required is dependent on the frequency of radiation used,

and vise versa. In addition, the dimensions of the waveguides and the resonator are specific to

the microwave frequency. In this light, an experimental setup requires a correspondingcombination of microwave energy, magnetic field strength, and resonator/waveguide

dimensions. Experimental setups are classified by the frequency of microwave energy utilized,

by the table below:

Letter Designation Frequency Range

L Band 1-2 GHz

S Band 2-4 GHz

C Band 4-8 GHz

X Band 8-12 GHz

Ku Band 12-18 GHz

K Band 18-26.5 GHz

Ka Band 26.5-40 GHz


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Q Band 30-50 GHz

U Band 40-60 GHz

V Band 50-75 GHz

E Band 60-90 GHz

W Band 75-110 GHz

F Band 90-140 GHz

D Band 110-170 GHz

Table 1: Letter designations of various microwave frequency ranges.

This naming system is a vestigial organ of WW2 RADAR use. The letters assigned to a given

range are random and were used as code to prevent the axis powers from knowing which

frequency was being used to detect incoming bombers. The first observation of electronparamagnetic resonance was seen at S-band. The majority of EPR spectrometers operate at X-

band, a mid-range frequency, due to the fact that equipment from this range was most easily

available after WW2. In addition, this is a suitable range to observe many EPR transitions and

does not require and exceptionally high magnetic field strength. However, in the past fewdecades, spectrometers operating at higher frequency ranges have become widely used, thus

requiring greater applied field strengths. Different energy level experimental setups allow the

observation of different transitions that are hidden at one applied field strength, but clearly

visible at another. This is because at a higher magnetic field strength, the spin states will be splitfurther, spreading apart transitions that are close in energy.

3. Resonator

A resonant cavity is the most common form of resonator used in EPR spectroscopy. Theresonant cavity of an EPR spectrometer is designed with dimensions corresponding to the

specific wavelength of microwave radiation that is to be used. For example, at X-band

frequencies (~9.5 GHz), the wavelength of the electromagnetic wave is ~3 cm. In this case, the

resonant cavity has dimensions of ~3 cm. This allows the microwaves to resonate in the cavity.In a resonant cavity, the level of microwave energy is 1000s of times greater than that in the

waveguide. The sample is located in the cavity at a location that allows maximum absorption of

magnetic energy from the microwaves, and minimum absorption of electric energy. This is

because it is the magnetic field component of the microwave energy that excites and EPRtransition, while the electric field component will be absorbed in a non-resonant manner.

Typically, the sample is placed in a central location in the cavity, where it will absorb the

greatest level of energy from the magnetic field of the microwaves.


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Figure 2: An X-band resonant cavity, with waveguide attached. Note the dimensions of ~3 cm.

To fine-tune the range and level of resonating microwaves, different procedures are used. The

most common technique is by controlling the amount of microwaves allowed to enter the cavity

with an iris screw. The iris is the hole by which the microwaves enter the cavity and a screw isused to precisely vary the size of this whole. "Tuning" the microwaves to resonate in the cavity

must be done prior to each experiment. This is done by first finding the frequency that results in

the least amount of reflection out of the cavity, and then fine-tuning the cavity to the optimum

range or resonance desired for the experiment. The frequency and coupling required will changedepending on sample and its position (this changes the dimensions of the cavity), and hence,must be done whenever either of these parameters is changed.

An optional characteristic of a resonance cavity is the ability to operate in either "parallel mode,"

or "perpendicular mode." In this case, the dimensions of the cavity will vary just slightly, so that

a slightly different frequency of radiation will resonate parallel to the magnetic field as compared

with that which resonates parallel to the magnetic field. Hence, the sample will absorb energyfrom the electromagnetic radiation either parallel or perpendicular to its induced magnetic dipole

moment. The purpose of this component is not to change the frequency of radiation, but to

change the direction that the electromagnetic radiation approaches the sample. This changes the

number and nature of transitions that are allowed by quantum mechanical rules. These differenttransitions produce spectra that can provide different structural insights into the electronic nature

of the molecule. Typically, a spectrometer operates in perpendicular mode.


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Figure 3: the resonator of a high field EPR spectrometer.

This is a custom built resonator designed to resonate microwaves at a frequency of 130 GHz.

This corresponds to a wavelength of ~2mm. Thus, the size of the cavity in this resonator has

those dimensions. The probe that this resonator is attached to has coaxial cables to propagateradio frequency waves for ENDOR experiments (see optional section below), as well as a fiber

optic cable to illuminate light-sensitive samples and observe changes in their spectra. When in

the spectrometer, then resonant cavity sits directly on top of a waveguide, which propagates the

microwaves into the cavity. This resonator has specially designed components that allow one tochange the dimensions and orientation of the cavity, in order to tune the cavity to resonate.

4. Detection Methods

Various detection methods exist for EPR spectroscopy. The majority of EPR spectrometers arereflection spectrometers, meaning they measure the amount of radiation that is reflected back out

of the resonator. Changes in the level of reflected microwave energy at various field strengthsallow the observation of spectroscopic transitions. A special channeling device called a

"circulator" is used to insure that radiation from the microwave source is sent only into the cavityand that reflected radiation is sent only to the detector. A diode is used to measure the power of

reflected energy. To achieve an accurate quantitative measurement of absorbed microwave

energy, the diode must sometimes be "biased," or supplied with a steady level of current which

insures that the diode is reading in the correct linear range.


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CW spectrometers (those which have been described up to this point) make use of a "field

modulator," which creates an oscillating magnetic field around the sample, in addition to theapplied magnetic field created by the electromagnet. This serves to fix the range of

electromagnetic energy that reaches the detector, increasing resolution. Because the absorption

of microwave energy is measured relative to this the modulating field, CW EPR spectra show up

as a derivative line shape relative to actual energy absorption.

5. Electromagnet

An electromagnet is used in an EPR experiment because it allows one to vary the strength of the

applied magnetic field precisely, quickly, and consistently. As mentioned previously, in anyexperimental setup, the strength of the applied magnetic field must correspond to the frequency

of the electromagnetic radiation being applied to a sample. For example, if the experiment is

being performed at X-band (~9.5 GHz), the field strength should be swept from a range of 0-

4000 gauss (or 0-0.4 Tesla) in order to see all transitions. At higher frequencies, the fieldstrength must be higher to see the same frequencies, because the degenerate spin states are be

split more.

In some experiments, the magnetic field is kept constant for continuous measurement of a single

transition. In spin labeling experiments, where the paramagnetic species are bound to specific

positions on a molecule (usually, a protein), the field strength of the electromagnet only needs tobe swept a couple hundred Gauss. This small sweep range makes EPR spectrometers specially

designed for spin labeling experiments much smaller than a traditional EPR spectrometer. Spin-

label specific EPR spectrometers can sit on a lab bench, while a typical EPR spectrometer willtake up a good 25 square feet of lab space including computer and power source.

As the field is swept, a significant amount of current is run through the electromagnetic coils.

This creates a large amount of heat. For this reason, an EPR spectrometer must have some kindof cooling apparatus to keep it from getting hot. This is usually accomplished by continuously

flowing water around the electromagnet.


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Figure 4: Ka-band EPR spectrometer.

The two copper-colored discs in the Ka-band spectrometer pictured in figure 4 are the

electromagnet. The sample cavity sits directly in the center of these two discs, as the center of

the magnetic field. This is the conventional design for the applied magnetic field of an EPRspectrometer. The pictured magnet can be swept from 0-1.5 Tesla, allowing observation of EPR

transitions excited with 31 GHz microwave energy.


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Figure 5: D-band spectrometer.

Figure 5 pictures an alternate experimental setup for the applied magnetic field of an EPR

spectrometer. This instrument utilizes a superconducting electromagnet that is kept at a constant

temperature of ~4 Kelvin by a closed-loop liquid helium pump. This setup contains 2 magneticcoils, and can sweep an applied field of 0-8 Tesla. Powerful superconducting magnets like this

are necessary to split paramagnetic spin states when high frequency microwaves are to be used to

excite the transitions. This instrument operates at D-band frequencies of 130 GHz.

6. Optional Components

Though optional, the following components are commonly used in EPR experimental setups:

1) A cryogenic cooling device is used to cool samples down to ultra-cold temperatures. Thisleads to a greater population difference in the split spin states, increasing the signal level. Many


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transitions can only be seen at ultra-cold temperatures, especially in biological samples. A

temperature as low as 4 Kelvin is often necessary to gain a resolved spectrum. Liquid heliummust be used to maintain these ultra-cold temperatures. Two types of cryogenic cooling setups

are common: A) A reservoir dewar, which maintain a constant level of liquid helium and/or

liquid nitrogen surrounding the sample, or B) A continuously flowing liquid helium pump, which

continuously pumps liquid helium into the space surrounding the sample cavity. The latter is thesetup commonly utilized on commercially available spectrometers. Naturally, these setups

require the use of high-powered vacuum pumps to evacuate the areas insulating the lines and

reservoirs that transfer and contain the cryogen.

2) Most EPR experimental setups do not only operate in the "continuous wave" (CW) format

described above, during which the sample is continuously bathed in microwave energy. Instead,advanced EPR experiments utilize pulsed microwave energy to selectively excite different

transitions of a paramagnetic sample. These experiments have the same basic components as

previously described, but require different amplifying and detecting equipment specifically

designed for pulse creation and detection. Experiments that utilize pulse microwaves include

Electron Spin Echo Envelope Modulation (ESEEM) and Hyperfine Sublevel CorrelationSpectroscopy (HYSCORE).

3) Many EPR experimental setups contain a radio frequency amplifier, which is used to excite

transitions between the spin states of magnetic moments of nuclei surrounding the paramagnetic

nuclei. These advanced EPR experiments are called Electron Nuclear Double Resonance(ENDOR) experiments, and provide valuable information about nuclei in the environment

surrounding an unpaired electron.

7. CalEPR Center

The CalEPR center on campus contains 5 EPR experimental setups, located in the basement ofthe Chemistry building. These were assembled and are maintained and used by the lab Dr.

David R. Britt. The range of frequencies and the overall experimental capability makes the

CalEPR center a truly world-class EPR lab. Of the 5 spectrometers, 3 are home-built by Dr. Britt

and his graduate students, and 2 are commercially built by Bruker.


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Figure 6: X-band CW spectrometer.

Figure 6 pictures the ECS-106, a Bruker instrument. This is an X-band instrument configured

only for CW EPR at ~9.5 GHz. The resonant cavity of this instrument is capable of performing

in parallel or perpendicular mode. The sample cavity of this instrument is cooled withcontinuous flowing liquid helium, pumped straight from the helium dewar.

The power supply is sitting to the left of the instrument, which can be identified by the large

circular electromagnets. To the right of the instrument is the console, which contains thenecessary electrical components. Above the magnet is the cryostat and bridge, which containsimportant components like the microwave amplifier, attenuators, and the detector.


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Figure 7: Home-built pulsed X-band spectrometer.

Figure 7 pictures the oldest instrument in the lab, a home-built spectrometer capable of a wide

frequency range (8-18 GHz). It is configured for pulsed EPR spectroscopy, including ESEEM

and ENDOR. The sample of cavity of this spectrometer is cooled with a reservoir dewar whichmust be filled with liquid helium and/or nitrogen. This instrument is currently not in use because

it is going through some necessary upgrades to increase its speed (as of March 2009).


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Figure 8: X-band and Q-band CW and pulse spectrometer.

Figure 8 pictures the other commercially built spectrometer, the E560 ELEXYS EPR system

built by Bruker. This spectrometer is capable of doing CW and pulsed EPR at both X-band

(~9.5 GHz) and Q-band frequencies (~35 GHz). The sample cavity of this instrument is cooledwith continuous flowing liquid helium, pumped straight from the helium dewar. The line to the

helium dewar flowing into the sample cavity can be seen in this picture.


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Figure 9: Home-built pulse Ka-band spectrometer.

Figure 9 pictures the home-built Ka-Band spectrometer, which performs experiments at ~31

GHz using a 100 W traveling wave tube amplifier. This spectrometer is configured for pulsed

EPR spectroscopy, and has primarily been used for ESEEM studies. The sample of cavity of thisspectrometer is cooled with a reservoir dewar which must be filled with liquid helium and/or

nitrogen. This spectrometer was built by current Britt lab group member Michelle Dicus andformer Britt lab group member Gregory Yeagle.


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Figure 10: Home-built pulse D-band spectrometer.

Figure 10 pictures the home-built pulsed D-band spectrometer, which performs high-field pulsed

EPR experiments at ~130 GHz. It utilizes a superconducting electromagnet to produce fields of

up to 8 Tesla. The magnet is constantly kept at ~4 Kelvin by a closed helium loop pumpsystem. This spectrometer is configured for ESEEM and ENDOR studies. This spectrometer

was built and is maintained by current Britt lab group member Alex Gunn.

B. Experimental Applications

EPR spectroscopy can be applied to any sample that contains a paramagnetic electron. Thisincludes a range of samples from basic organic and inorganic compounds to complex

biomolecules such as proteins. Spectra are used to identify molecules within a sample and more

importantly, to characterize the environment of the unpaired electron. EPR spectroscopy has

advantages in that it is very sensitive (1000x more sensitive than NMR) and in that it has very

good specificity (it looks only at the region that contains the unpaired electron). Thesecharacteristics make EPR especially useful for the study of metallic cofactor-containing proteins,

which perform the most important enzymatic reactions on the planet (photosynthesis,nitrogenase, etc.). How substrates bind to the metallic cofactors in these enzymes can be

elucidated best using EPR.


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1. CW EPR

Continuous Wave EPR (CW) is an experiment during which a sample is continuouslyilluminated with microwave radiation at a fixed frequency and the strength of the applied

magnetic field is swept, observing changes in microwave absorption. CW EPR is the most basic

experiment performed. The instrumentation section of this article primarily describes theinstrumental setup utilized to obtain CW spectra. Figure 11 displays a sample CW spectrum

taken at X-band. This is a sample of isolated photosystem II, a complex of proteins involved inphotosynthesis. Note that the X-axis displays magnetic field strength in Gauss. Also note that

the spectrum displays the derivative of absorption of microwaves. The Y-axis displays the

derivative of units of absorption in arbitrary units.

Figure 11: Sample CW EPR spectrum


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Each peak of this spectrum represents a transition between the spin states of the unpaired

electron at a specific energy. In a system as complex as PSII, each transition corresponds to anentirely different molecule, be it a metallic cofactor or a biomolecular cofactor such as

chlorophyll. This spectrum is interpreted in light of theory which describes how the unpaired

electron's spin states are affected by its environment, most importantly, coupling to the

surrounding nuclei. A thorough interpretation of this spectrum would require comparisons withspectra of similar samples. Such an analysis can be used to determine precisely the orientation

of the surrounding nuclei and the unpaired spin density on each.

2. Pulse EPR

Pulse EPR varies from CW in that microwaves are applied to the sample as a series of

nanoseconds-long pulses. The magnetic field strength is kept constant, and the sample is pulsed

with microwave energy. The strategic pattern of pulse length and spacing allows the creation of

aspin echo.The parameters of the pulse pattern can be changed and utilized in such a way thatthe hidden features of a transition observed at a specific field strength in a CW EPR spectrum

can be observed. For example, figure 12 presents an ESEEM (electron spin echo envelopemodulation) spectrum taken on the same PSII sample see in figure 11. The ESEEM spectrum istaken at the field strength corresponding to the largest (in amplitude) transition in figure 11

(~3450 G).

Figure 12: This sample ESEEM spectrum demonstrates the advanced capabilities of pulse EPR.
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ESEEM spectra provide direct information about the frequencies that make up the less energetic

transitions of an EPR transition. While CW EPR more bluntly shows the transition between onespin state to another. Pulse EPR spectra have the capability of showing the transitions that occur

within a spin state, which are caused by the interaction of the unpaired electron with the

surrounding nuclei. The peaks seen in figure 12 correspond to specific quantitative parameters

that describe the coupling of the electron and the nucleus, thus corresponding to the distancebetween the nuclei and the electron. In short, pulse EPR allows the observation of transitions

within the transitions seen in CS spectra.

3. ENDOR

Electron nuclear double resonance spectroscopy (ENDOR) further advances pulsed EPR

techniques by including pulses in the radiofrequency region to the pulse patterns utilized to

produce spin echoes. The result is the ability to measure the resonance energies of the specific

nuclei that are coupled to the unpaired electron. Most importantly, it allows one to observe howthese resonance energies are perturbed with respect to the coupling of the nuclear and electronic

magnetic moments. The direct measurement of these coupling parameters allows understandingof physical relationship of the electron with its surrounding nuclei.

4. Spin-Spin Labeling Experiments

An entirely different application of EPR spectroscopy is done with spin-spin labeling

experiments. In these, a protein is tagged with paramagnetic groups (usually NO2) that can beeasily detected by EPR. The distance between parts of the molecule which contain the

paramagnetic groups can observed by the interaction of their respective transitions in spectra.

The most valuable information is gained from these experiments by altering the molecule and

observing the whether the paramagnetic groups interact with each other. For example, this

technique has been utilized to determine whether membrane-bound ion channels open or close inresponse to a stimulus.

Documents

Electron Para Magnetic Resonance