Estimating Lysozyme Crystallization Growth Rates and Solubility from Isothermal Microcalorimetry

A microcalorimetric technique has been developed to measure crystal-growth kinetics and enthalpies of crystallization. The enthalpy of crystallization of hen egg-white lysozyme in 0.1 M acetate buffer at pH 4.6 was determined at 287 K using this technique. The enthalpies were directly measured to be −14.3 ± 2.0 and −14.6 ± 1.3 kcal mol⁻¹ (1 kcal mol⁻¹ = 4.184 kJ mol⁻¹) for 3 and 5% NaCl solutions, respectively, which is in good agreement with values estimated from previous solubility measurements. Non-linear regression of the transient heat flow allowed measurement of the crystal growth rate as a function of protein supersaturation as well as the solubility. The crystal growth rates determined by this method were found to agree with those in the literature under the same solution conditions.