Cholesterol Crystallization Tests the Limits of Microscopic Reversibility for Molecular Pathways of Growth and Dissolution.

A subject often debated in the field of crystallization is whether opposing processes of growth and dissolution abide by microscopic reversibility under conditions far from equilibrium. This is particularly relevant for nonclassical growth pathways such as crystallization by particle attachment (CPA), which is an irreversible process owing to the unique sequence of steps involving precursor attachment to crystal surfaces followed by their disorder-to-order transitions as they integrate into the underlying crystal. The reverse process of crystal dissolution is frequently associated with the retraction of unfinished layers, which is the opposite of classical growth and distinctly different than CPA. Here, we selected cholesterol as a model system based on its prevalence in physiological environments that can switch between conditions of supersaturation (crystal growth) and undersaturation (crystal dissolution). We leverage our recent observations that cholesterol nucleation and growth occur by nonclassical and classical pathways, respectively. Our findings reveal the limits of microscopic reversibility when working in moderately undersaturated solutions where dissolution occurs by a unique nonclassical pathway involving the formation of cholesterol-rich clusters on crystal surfaces. Attempts to regenerate crystal surface growth after clusters are formed reveal they have an irreversible effect on crystallization in ways that potentially inhibit pathological cholesterol precipitation.