Mastering interfaces: advances and challenges in controlling protein nucleation

Interfaces are ubiquitous in nature and are involved in any physico-chemical process. Crystallizing a protein implies the formation of a new interface between the growing crystalline material and the liquid solution. The nucleation step, which is pivotal to the formation of crystals, is extremely sensitive to the presence of interfaces. Such a feature can be exploited to aid the control of this delicate step, improving the chances of getting protein crystals, the reproducibility of the experiment, and the uniformity of the attributes of the crystals, i.e., size, habit, and form. To do this, several approaches have been proposed in recent years, focusing on the use of controlled functionalization of surfaces and nanoparticles, the establishment of diffusion-dominated systems, and, more in general, the use of heteronucleants and additives. Recent advancements and current challenges in the field are here highlighted, including continuous protein crystallization and micro-crystallization.