Two polymorphs of lysozyme nitrate: temperature dependence of their solubility

Two crystallographic forms of lysozyme nitrate are known, namely monoclinic and triclinic. Having previously determined the temperature dependence of the solubility of the monoclinic form (0.2 M NaNO₃ solutions at pH = 4.5) [Legrand et al. (2001). J. Crystal Growth 232, 244-249], we focus here on the solubility of the triclinic form. The temperature dependence of the solubility of this crystallographic form has been measured with a static light device developed in our laboratory. This device allows to observe of the dissolution of one phase and/or the occurrence of a new one by varying the temperature with a sweep rate as low as 0.6 degree/hour. The new solubility data are complemented with crystallographic data of the triclinic form for the sake of completeness. The faces of a triclinic crystal are indexed. The crystallisation enthalpy of the triclinic form is deduced from these new results. These new solubility data allow us now to discuss (1) the publishedprotocols used to obtain the monoclinic and triclinic forms of lysozyme nitrate and (2) the phase transformation.