Ac nanocalorimeter for measuring heat capacity of biological macromolecules in solution

Abstract

A precise calorimeter has been developed to measure the heat capacity of a small amount of liquid using a novel ac calorimetric method in which the effect of heat loss from a sample cell is corrected using the phase of the ac temperature. The sample cell is made of a fine glass tube, whose outer surface is plated with a nickel film as an ac heater. The ac temperature of the sample is detected precisely with a microbead thermistor attached to the middle of the tube. The resistance of the thermistor is measured with an ac Wheatstone bridge which is composed of resistors with a low temperature coefficient of ±1 ppm/K. The unbalance ac signal of the bridge is measured with a lock-in amplifier. To reduce the drift of measured values caused by the variation of room temperature, the amplifier and measuring instruments with temperature coefficients of >1 ppm/K are kept at constant temperature. Moreover, the gain of the amplifier is calibrated at every measuring point. Consequently, the heat capacity of 10 μl liquid can be determined with an extremely high sensitivity of ±0.001%, which corresponds to heat capacity changes of ±300 nJ/K. A test of the performance was made of the heat capacity measurement during thermal denaturation of lysozyme dissolved in buffered solution. This method is particularly useful for studying thermal properties of biological macromolecules in solution, since the heat capacity of macromolecules can be measured with a 10 μg sample.