Solvational tuning of the unfolding, aggregation and amyloidogenesis of insulin

Solvational perturbations, accomplished by the addition of the three model cosolvents glycerol, ethanol and trifluoroethanol, exert pronounced and diversified effects on the unfolding, non-native assembly and fibril formation of the amyloidogenic protein insulin. Fluorescence, CD and UV-spectroscopic methods as well as atomic force microscopy imaging have been employed to reveal distinct structural and kinetic features upon the aggregation of insulin under different solvational perturbations, which ultimately manifest in morphological variations of mature aggregates and fibrils. In particular, fluorescence anisotropy studies proved to be very valuable in characterizing the corresponding aggregation nuclei. Glycerol stabilizes, through enhanced hydration, native oligomerization and retards fibrillar aggregation at all concentrations studied (up to 40% (w/w)). In contrast, both monoalcohols facilitate the formation of aggregation-prone intermediates by destabilization of the native assembly. The reversal from a kosmotropic to a merely chaotropic solvational behaviour can explain the accelerating effect on ordered fibrillation of low concentrations and the inhibitory nature of high concentrations of ethanol and trifluoroethanol, ultimately leading to amorphous aggregate structures. Mechanistically, dimer dissociation under stabilizing and nucleation under destabilizing conditions have been identified to be the rate-limiting steps that account for the non-monotonic concentration effects of the monoalcohols on the aggregation kinetics. A rationale as to how solvational constraints can tune the stability of the species on the native self-assembly and non-native aggregation pathway, and the energetic barriers that need to be overcome for the required structural interconversions has been put forward. We may propose that the concept of perturbed solvation is generally applicable to phenomena that are related to pathogenic amyloidogenesis of proteins and, in general, solvational effects, besides other aspects of the cellular environment, may play a significant role in a reshaping of the folding/aggregation funnel of proteins.