Double Electron-Electron Resonance
DEER, also known as PELDOR or Pulsed ELDOR, uses two separate microwave frequencies to examine the coupling between two electron spins in order to make a distance measurement, typically between two nitroxide spin labels.
In the experiment, a Hahn Echo sequence is used to produce an echo for the spins in resonance with the probe microwave frequency. At a time τ2 after the first echo, a second π pulse is applied to form a refocused echo. At a separate microwave frequency, the pump frequency, a π pulse is applied to invert the population of a separate set of spins. This π pulse is then incremented in time and the intensity of the refocused echo is monitored. The set of spins affected by the probe frequency experiences a different magnetic environment before and after the pump π pulse. The change in the magnetic environment affects the ability of the magnetization to be called back with the second probe π pulse. The net effect is a modulation in the intensity of the refocused echo with a periodicity that is a function of the strength of the coupling and therefore, the distance, between the two electron spins.
Time Domain DEER Data from a Doubly Nitroxide Spin-labeled Protein
An FT of the time domain data gives a spectrum in the frequency domain, indicating the coupling between the two electron spins.
Frequency Domain DEER Data from a Doubly Nitroxide Spin-labeled Protein
From the frequency domain data, a distrubution of electron-electron distances can be constructed indicating the distance between the two site directed spin labels.
Distance Domain DEER Data from a Doubly Nitroxide Spin-labeled Protein
Here at the Ohio Advanced EPR Laboratory, we have the fortunate capability of conducting DEER measurements at both X-Band and Q-Band. The incredible signal to noise enhancement that is achieved at Q-band is evident from the figure below. What would typically be a 12 hour experiment at X-band can be done in only a couple of hours at Q-band with better results.
Comparison of X-band and Q-Band DEER