Kinetic Roughening of Glucose Isomerase Crystals

Protein crystals grow by the incorporation of growth units from solution into the crystal bulk. At low supersaturation, growth sites are kinks at step edges that are generated through the formation of spiral dislocations and/or two-dimensional (2D) nucleation. At conditions farther away from equilibrium, a critical supersaturation _R can be reached where the critical 2D nucleus equals a single growth unit. As a result, each surface site becomes a potential growth site, making growth fast and the surface rough. This transition from layer-by-layer growth to a faster random addition of solute molecules to the surface, that is, kinetic roughening, was visualized using laser confocal microscopy combined with differential interference contrast microscopy for the case of glucose isomerase. Evidence for continuous growth is presented at _R: the critical nucleus is predicted to be the size of 2 glucose isomerase tetramers and normal face growth velocities switch from a nonlinear to a linear relationship with the supersaturation. Moreover, at supersaturation higher than _R, rounding of crystal facets occurs. From this we conclude that glucose isomerase transitions at _R = 5.0 ± 0.1 to a continuous growth mechanism characterized by an energy barrier of 3.74 ± 0.01 × 10_13 erg/molecule at 20 °C.