(655b) Two-Step Crystal Nucleation Is Selected Because of Lower Surface Free Energy Barrier

Nucleation of crystals from solution underlies multitudinous natural and engineered processes. The formation of a crystal nucleus in a supersaturated environment creates an energy-rich interface. According to classical theory, the associated surface free energy barrier is overcome by random fluctuations leading to direct assembly of monomers into a crystalline embryo. Discrepancies between classical predictions and experimental observations suggested a two-step mechanism of crystal nucleation, whereby nuclei are hosted by disordered solute-rich precursors. A fundamental controversy of two-step nucleation is that whereas precursor populations often occupy a minor volume, nucleation confined to precursors is faster than direct nucleation. Here we demonstrate that nucleation of insulin crystals, which occurs with a highly regulated rate in mammalian pancreases as a part of insulin biosynthesis, is assisted by protein-rich clusters of radius 160 â?? 200 nm that occupy ca. 10^-6 of the solution volume. We show that the surface free energy encountered by newly nucleated crystals is ca. 80-fold lower than the independently determined surface free energy of the crystal-solution interface; the latter value dictates the barrier for direct crystal nucleation in the solution. This comparison indicates that two-step nucleation is faster than the direct pathway and is selected as a phase transformation route due to a lower surface free energy barrier and suggests that nucleation of crystals can be controlled by manipulating the properties of the disordered precursors.