Water sorption isotherms and enthalpies of water sorption by lysozyme using the quartz crystal microbalance/heat conduction calorimeter

Abstract

The water sorption isotherm and the water vapor activity dependence of the enthalpy of water sorption ΔsorpH° of lysozyme have been measured at 25°C. A thin film of lysozyme of mass 250 μg was exposed to H2O/N2 mixtures in a quartz crystal microbalance/heat conduction calorimeter (QCM/HCC). The QCM/HCC is a new gravimetric/calorimetric method that measures simultaneously and with high precision the mass change and the corresponding heat flow in a thin film exposed to a gas. ΔsorpH° for lysozyme agrees with previous determinations, although hysteresis effects are evident in the data. No van't Hoff analysis is necessary because sorption enthalpies are measured calorimetrically. The water vapor activity dependence of ΔsorpH° agrees with that measured previously by Bone. As the water content of the lysozyme film drops below 10 mass%, ΔsorpH° becomes more exothermic, indicating that water is being bound to the charged or highly polar groups of the solvent-accessible surface of lysozyme. The dynamics of water uptake and release from lysozyme thin films are much slower than in polymer films of comparable thickness. Because the QCM/HCC operates with sub-milligram samples, any protein is now amenable to study by this technique. © 2002 Elsevier Science B.V. All rights reserved.