Simultaneous Measurement of Water Desorption Isotherm and Heats of Water Desorption of Proteins Using Perfusion Isothermal Microcalorimetry

The purpose of this work was to study protein–water interactions using a perfusion isothermal calorimetry method by simultaneously measuring the water (de)sorption isotherm and heats of desorption (ΔHdesorption). Lysozyme, bovine serum albumin (BSA), and a monoclonal immunoglobulin (IgG) were studied. Desorption isotherms and ΔHdesorption were calculated using data from two perfusion systems, which measured heat flow resulting from interaction of water vapor with the protein sample and with pure water, respectively. The desorption isotherms calculated from the calorimetry were in good agreement with the gravimetric data. The average ΔHdesorption at high hydration was 54.6 kJ/mol and decreased (approaching heat of water evaporation) with desorption and passed through a minimum at protein specific water content, below which it increased again reaching 59.0 kJ/mol at the lowest hydration levels. The difference between the ΔHdesorption above the minimum and heat of water evaporation has been attributed to conformational changes in the protein. This conclusion is supported with data for lysozyme in which a dynamic glass like transition has been observed at the water content of the minimum in the calorimetric enthalpy data at 293K. This work establishes perfusion calorimetry as a rapid and controlled method to study the thermodynamics of protein–water interaction. © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 96:1974–1982, 2007