Improved flow microcalorimeter for biophysical studies

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Abstract
The flow version of the Beckman model 190 microcalorimeter has been improved by replacing the original platinum tubes with Teflon tubing and a Kel‐F mixing chamber, and introducing a physically identical reference system into the thermopile. This modification abolishes the occurrence of spontaneous and irreproducible heat effects probably due to catalytic reactions of the platinum with sulfhydryl groups of the biochemical systems, and permits operation of the instrument as a differential calorimeter. The time constant of the instrument is 50 sec and the useful sensitivity is 6 μcal/min as demonstrated by calibration experiments employing HCl‐neutralization and sucrose dilution. The calibration constant has been shown to be independent of the magnitude of the heat effects, the flow rate, and the mode of operation by continuous flow or pulsed flow experiments. The accuracy of experiments on biophysical problems has been illustrated by the good agreement of the binding enthalpies for the association of NADH and pig skeletal‐muscle lactate dehydrogenase, and the reaction between isoleucine and isoleucine:tRNA ligase from E. coli, obtained using the flow calorimeter with the ΔH values determined when employing an LKB batch microcalorimeter.
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