Researchers have developed a high-frequency electron paramagnetic resonance (EPR) system for studying transition metal ions in chemical and biological samples. The instrument operates at 263 GHz and was tested on manganese-containing samples, including two metal-binding proteins.
EPR spectroscopy examines substances containing unpaired electrons, such as free radicals and some metal ions. It can provide information about a metal’s chemical environment and its interactions with nearby molecules. In medical and biological research, these transition metals contribute to processes ranging from enzyme activity to immune defense.
Most EPR instruments operate at lower frequencies. However, the signals produced by high-spin metal ions can become broadened at these frequencies, making important features difficult to distinguish. Very high-frequency pulse EPR could provide clearer measurements, but its use has been restricted by the limited availability of powerful amplifiers. The researchers addressed this limitation with a 10 W traveling-wave tube amplifier. It delivered high-power pulses across a bandwidth of 5.6 GHz.
The system does not require a resonant cavity, a component normally used to strengthen EPR signals. Removing this component allows researchers to examine larger samples, use different sample shapes, and operate across a wider range of frequencies. The study used samples of approximately 12 µL, compared with around 30 nL in a commercial 263 GHz system fitted with a small resonant cavity.
The team first analyzed trace amounts of manganese(II) in magnesium oxide powder. Under comparable conditions, the 263 GHz instrument produced a signal-to-noise ratio of approximately 1,540, compared with 123 using a commercial X-band system. This represented a 12.5-fold improvement. The amplifier itself increased the signal by approximately ninefold.
The researchers then tested manganese in frozen water, bound to EDTA, and incorporated into the proteins calprotectin and PsaA. Calprotectin is involved in the immune response and can withhold essential metals from microorganisms, while PsaA helps certain bacteria acquire manganese.
The spectra revealed differences in how manganese was coordinated within each sample. These differences reflect the symmetry and chemical environment around the metal, information that can help researchers investigate how metal-binding proteins function.
The system could support studies of metalloproteins, catalysts, molecular magnets, and interactions between electron and nuclear spins. It may also contribute to methods that use electron polarization to improve the sensitivity of nuclear magnetic resonance.
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