Introduction: The phage shock protein (Psp) system is essential for maintaining membrane integrity under stress conditions in bacteria. Determining its three-dimensional structure requires well-ordered crystals, typically obtained through controlled crystallization methods. In this manuscript we present a range of crystal morphologies, reflecting variations in nucleation and growth conditions.
Methods: Crystallization was performed using the sitting drop vapor diffusion technique. A drop containing purified phage shock protein mixed with precipitant solution was equilibrated against a reservoir with higher precipitant concentration. Vapor diffusion gradually increased protein supersaturation within the drop, promoting nucleation and subsequent crystal growth. Experimental parameters such as protein concentration, precipitant composition, pH, and temperature were systematically optimized.
Results: Multiple crystals of the same protein appear under different conditions. Well-defined hexagonal crystals indicate highly ordered growth under near-equilibrium conditions. Rod-shaped crystals suggest anisotropic growth along a preferred axis, while dendritic and clustered structures are indicative of rapid nucleation at high supersaturation. Regions containing numerous microcrystals suggest excessive nucleation with limited crystal growth.
Discussion: Crystal formation can be interpreted using thermodynamic principles, where the Gibbs free energy change is given by:
Δ?=Δ???+??
Here, ?? represents the bulk free energy change, γ is the interfacial surface tension, ? is the nucleus volume, and A is the surface area. The presence of well-formed hexagonal crystals suggests minimized free energy and stable growth conditions, whereas dendritic and clustered morphologies indicate kinetically dominated regimes. These observations highlight the importance of controlling supersaturation to balance nucleation and growth, ultimately improving crystal quality for structural analysis