The application of electron paramagnetic resonance (EPR) spectroscopy to the study of biomolecules frequently is at the limit of the signal/noise of existing instruments. For modern pulsed and multifrequency EPR techniques it is important to separate microwave pump power and observed spin resonance signals in space and or in time. This requires sophisticated microwave matching of the sample-containing resonator to the microwave source and to the detector. All of this must be accomplished in a way that flexibly adapts to the types of samples and temperatures encountered in the study of biological systems. It is proposed to solve these problems with an innovative crossed-loop loop gap resonator (CLLGR), closely coupled with a cryogenically cool able low noise amplifier. The CLLGR will accept standard 4 mm OD quartz sample tubes. A combination of mechanical tuning to ensure orthogonality of the microwave modes, and electrical tuning to vary the degree of coupling, will result in a very convenient (user friendly) spectrometer system. Initial implementation will be at S-band (2-4 GHz). Each of these features: the CLLGR, the type of mechanical tuning of orthogonality, the closely-coupled low noise amplifier, and the electronic tuning of impedance match, is a departure from standard (and commercial) EPR spectrometer design. Proof of performance will be the application to pulsed EPR of high-spin cobalt(II)-substituted metalloproteins.
