DEVELOPMENT OF A RARE EARTH CRYSTALLINE THERMOMETER FOR FROZEN METALLOPROTEIN EPR SAMPLES

Electron Paramagnetic Resonance (EPR) spectrometers are widely used in biophysical and biochemical research, particularly in the study of metalloproteins at low temperatures. Precise knowledge of the temperature of frozen metalloprotein samples in commercially available EPR spectrometers is not possible due to the inherent limitations of the liquid helium flow cryostats employed. Single crystals of the trivalent ion cerium, Ce3+, doped in the diamagnetic host yttrium ethylsulfate, Ce:Y(C2H5SO4)3.9H2O or Ce:YES, are grown in this laboratory and can be used as thermometers to accurately determine the temperature of such samples. These crystals can also serve a dual function as quantitation standards. The Ce3+ ion in these crystals is at a site of C3h symmetry with small C3v distortions, experiencing a crystal field that splits the 2F5/2 ground state manifold into three Kramers doublets. The energy separation of the two lowest Kramers doublets is very well known. Both doublets give rise to very strong EPR signals at liquid helium temperatures with relative intensities that depend on the doublet separation and the temperature of the system. Measurement of these intensities yields precise information on the temperature of the crystal and the metalloprotein sample containing the crystal. The purpose of this research is to develop such a novel crystalline thermometer and quantitation standard that will provide a solution to a long-standing problem in metalloprotein EPR spectroscopy and make available a valuable tool to researchers involved in biophysical/biochemical research.

Metalloprotein sample inside a TE102 X-band cavity with continuous-flow liquid helium cryostat: