  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.
