Electron Paramagnetic Resonance (EPR) Spectroscopy

This technique provides information about the electronic and molecular structure of paramagnetic metal centers. Measurement of the spin state, S, the magnitude of hyperfine interactions with metal and ligand nuclei, and the zero-field splitting of half-integer S > 1/2 electronic states, allows a researcher to identify the paramagnetic center, and to potentially identify ligating atoms. We have multi-frequency (X- and Q-band) continuous-wave EPR capabilities in the MRIL, along with dual-mode cavities to allow us to characterize a large variety of half-integer and integer spin systems [Mike Johnson, UGa].

Acronyms, synonyms

• Electron Paramagnetic Resonance
• Electron Spin Resonance

Measured physical quantities

• Energy separation between different electron spin states
• Nuclear hyperfine coupling constants

Information available

• Spin state, S, of paramagnetic center
• Magnitude of hyperfine interactions
• Zero-field splitting of half-integer S > 1/2 states
• Possible identity of paramagnetic center (free radical; metals in metal cluster)
• Possible identity of ligating atoms

Information NOT available, limitations

• No information for diamagnetic sites
• Integer spin states often unobservable

Examples of questions that can be answered

• What are the different paramagnetic centers in the sample?
• How do these centers change with changes in redox potential and pH or during substrate binding or catalysis?

Major advantages

• Volume (300 mL) and concentration (1.0-0.01 mM) often low
• Recording spectrum can be done quickly (usually no more than 15-20 min)
• Spectrum is only of paramagnetic species
• Spectrum is often simple to interpret
• Spectrum often can be recorded in vivo

Major disadvantages

• Most paramagnetic metal centers require low (20 K or lower) temperatures for detection

Sample constraints

• Sample volume is ca. 0.3 mL
• Concentration can range from 0.01-1.0 mM
• Sample should be free of any adventitious paramagnetic impurities