ENDOR Spectroscopy

The technique of electron-nuclear double resonance (ENDOR) complements the EPR technique and is especially useful for identifying nuclei that are weakly interacting with the electron spin. It can give detailed information about the atoms involved in a paramagnetic site [Vincent Huynh, Emory].

Acronyms, synonyms

• Electron Nuclear DOuble Resonance

Measured physical quantities

• Hyperfine coupling constants for nuclei at paramagnetic center

Information available

• Magnitude of hyperfine coupling constants
• Identity of different nuclei coupled to unpaired electron

Information NOT available, limitations

• To be detected, center must be paramagnetic
• Integer spin states often unobservable
• Nuclei must have I > 0
• The number of identical nuclei (i.e., nuclei with the same coupling constant) cannot be determined

Examples of questions that can be answered

• What kind of nuclei interact with the unpaired electron?
• How do the hyperfine coupling constants for these different nuclei change with changes in solvent, pH, or during substrate binding or catalysis?

Major advantages

• Volume (300 µL) is low
• ENDOR lines are sharper than EPR lines; therefore, hyperfine coupling constants, undetectable by ordinary EPR spectroscopy, can often be determined
• Spectrum is only of paramagnetic species without interference from diamagnetic molecules

Major disadvantages

• Need more specialized and more expensive equipment than an ordinary EPR spectrometer
• Recording a "satisfactory" spectrum often much more difficult than recording EPR spectrum
• Samples often need very high concentrations and low temperatures (< 4 K)

Sample constraints

• Sample volume is ca. 0.3 mL
• Signal intensity must be strong and signal must be easily saturated