Observation of Protein Thermodynamics in Ice by Passive Millimeter-Wave Microscopy

The study of protein functions attributed to the conformation and fluctuation that are ruled by both the amino acid sequence and thermodynamics, requires thermodynamic quantities given by calorimetry using thermometric techniques. The increased need for protein function in different applications requires improvements of measurement systems assessing protein thermodynamics to handle many kinds of samples quickly. We have developed a passive millimeter-wave microscope that allows near-field imaging of thermal radiation, even in the low temperature range under room temperature where passive infrared imaging systems are ineffective. This advantage of our microscope system in combination with low thermal emission property of water ice in the millimeter-wave region enables the characterization of the thermal radiation from the proteins themselves in aqueous solution at a temperature range low enough to freeze water and to trap conformation intermediates in the proteins. Experiments performed at a millimeter-wave frequency of 50 GHz in a temperature range from 130 K to 270 K for a 20 % bovine serum albumin (BSA) aqueous solution showed a displacement between two conformational states of BSA at a temperature of approximately 190 K as a boundary. Our microscope system using this freeze-trapping method is expected to provide noninvasive thermal images to enable novel high-throughput calorimetry useful for the analysis of protein functions.