A Metastable Prerequisite for the Growth of Lumazine Synthase Crystals

TLDR

Dense liquid phases that are metastable relative to solid crystals form in protein solutions and act as a prerequisite for crystal nucleation. The study aims to show that, for lumazine synthase crystals, growth layers arise solely from the association of submicrometer dense‑liquid droplets with the crystal surface over a range of supersaturations. Using dynamic light scattering and crystal growth observations, the authors found that these droplets are short‑lived (seconds), decay into solution unless they encounter a crystal surface where they transform into crystalline matter, thereby ensuring perfect registry. The results reveal that doubly metastable, short‑lifetime dense phases play a crucial role in the growth of ordered solid phases, highlighting a novel mechanism of phase transformation.

Abstract

Dense liquid phases, metastable with respect to a solid phase, form in solutions of proteins and small-molecule materials. They have been shown to serve as a prerequisite for the nucleation of crystals and other ordered solid phases. Here, using crystals of the protein lumazine synthase from Bacillus subtilis, which grow by the generation and spreading of layers, we demonstrate that within a range of supersaturations the only mechanism of generation of growth layers involves the association of submicrometer-size droplets of the dense liquid to the crystal surface. The dense liquid is metastable not only with respect to the crystals, but also with respect to the low-concentration solution: dynamic light scattering reveals that the droplets' lifetime is limited to several seconds, after which they decay into the low-concentration solution. The short lifetime does not allow growth to detectable dimensions so that liquid-liquid phase separation is not observed within a range of conditions broader than the one used for crystallization. If during their lifetime the droplets encounter a crystal surface, they lower their free energy not by decay, but by transformation into crystalline matter, ensuring perfect registry with the substrate. These observations illustrate two novel features of phase transformations in solutions: the existence of doubly metastable, short-lifetime dense phases and their crucial role for the growth of an ordered solid phase.