Spherulitic growth of hen egg-white lysozyme crystals

In protein crystallography, spherulites are considered the result of a failed crystallization experiment. Understanding the formation of these structures may contribute to finding methods to prevent their formation. Here, we present an in situ study on lysozyme spherulites grown from sodium nitrate and sodium thiocyanate solutions, investigating their morphology and growth kinetics using optical microscopy. In a morphodrom, we indicate the conditions at which spherulites form for the lysozyme-nitrate system, showing that liquid-liquid phase separation is not a prerequisite to form sheaflike spherulites and that supersaturation is not the only factor determining their creation. Despite their sheaflike morphology, the spherulites all appear to be formed through heterogeneous nucleation. The spherulites are of a new polymorphic form and are less stable than the monoclinic form. For a single needle, growth kinetics indicate surface processes to be the rate-limiting step during growth, but for an entire spherulite volume, diffusion still plays a role. Spherulites simulated by using a time-dependent, tip-splitting model are found to compare well to experimentally observed spherulites.