ENDOR/ELDOR Spectroscopy

ENDOR - Electron Nuclear DOuble Resonance
ENDOR spectroscopy can best be described as a nuclear magnetic resonance experiment on free radicals, paramagnetic metal ions or defect centres, with an EMR spectrometer as the detector. As it's name implies ENDOR is a double resonance technique, using a microwave frequncy (GHz) for the electron resonance and a vhf radiofrequency (MHz) for nuclear resonance, it's cousin ELDOR uses two microwave frequencies. The basic (continuous wave) ENDOR experiment consists of measuring the changes in amplitude of a saturated EMR line, whilst sweeping through the NMR frequency region. At X-band (9.5GHz/0.35T) this is up to 15MHz, or at Q-band (35GHz/1.25T) up to 54MHz. The frequencies for different nuclei depend on the various nuclear magnetic resonance at the field of the EMR experiment.	ENDOR Frequencies for Various Nuclei at X-Band ENDOR and ELDOR Double Resonance Energy Levels and what you see Relaxation ENDOR Research Centers Some Examples
Whilst multiple resonance techniques have been adopted as routine in NMR spectroscopy, the use of ENDOR, let alone ELDOR, for EMR spectroscopy is still relatively uncommon, the situation is changing with the realization, particularly among biochemists, of what ENDOR can tell about their samples. Thus ENDOR is very useful in decoding EMR spectra where there are large numbers of poorly resolved hypefine interactions, or where these interactions are too small to see. The frequency of the ENDOR spectrum also reveals the nuclear spin interacting with the electron spin, and where the nucleus has a quadrupole moment (I>½), this can be measured. In addition, if the ENDOR spectrum can be measured as a function of the EMR spectrum in a solid sample, a glassy or powder sample, or a single crystal, then the spectrum yields structural details as in an X-ray single crystal. This last accomplishment for ENDOR has made it of prime importance in metalloenyzme structure determinations, where a particularly important application is to photochemistry and photobiology. For ELDOR, a recent article by James Hyde points out the potential importance of the technique for the study of complex biological systems based on nitroxide spin labels.	Witch of En-dor. A divining woman consulted by Saul when Samuel was dead. She called up the ghost of the prophet, and Saul was told that his death was at hand. (1 Sam. xxviii)

Whilst multiple resonance techniques have been adopted as routine in NMR spectroscopy, the use of ENDOR, let alone ELDOR, for EMR spectroscopy is still relatively uncommon, the situation is changing with the realization, particularly among biochemists, of what ENDOR can tell about their samples. Thus ENDOR is very useful in decoding EMR spectra where there are large numbers of poorly resolved hypefine interactions, or where these interactions are too small to see. The frequency of the ENDOR spectrum also reveals the nuclear spin interacting with the electron spin, and where the nucleus has a quadrupole moment (I>½), this can be measured. In addition, if the ENDOR spectrum can be measured as a function of the EMR spectrum in a solid sample, a glassy or powder sample, or a single crystal, then the spectrum yields structural details as in an X-ray single crystal. This last accomplishment for ENDOR has made it of prime importance in metalloenyzme structure determinations, where a particularly important application is to photochemistry and photobiology. For ELDOR, a recent article by James Hyde points out the potential importance of the technique for the study of complex biological systems based on nitroxide spin labels.

ENDOR - Electron Nuclear DOuble Resonance