Acetone-induced structural variant of insulin amyloid fibrils.

Self-propagating polymorphism of amyloid fibrils is a distinct manifestation of non-equilibrium conditions under which protein aggregation typically occurs. Structural variants of fibrils can often be accessed through physicochemical perturbations of the de novo aggregation process. On the other hand, tiny changes in the amino acid sequence of the parent protein may also result in structurally distinguishable amyloid fibrils. Here, we show that in the presence of acetone, the low-pH fibrillization pathway of bovine insulin (BI) leads to a new type of amyloid with the infrared features (split amide I' band with the maximum at 1623 cm-1) bearing a striking resemblance to those of the previously reported fibrils from recombinant LysB31-ArgB32 human insulin analog formed in the absence of the co-solvent. Insulin fibrils formed in the presence ([BI-ace]) and absence ([BI]) of acetone cross-seed each other and pass their infrared features to the daughter generations of fibrils. We have used dimethyl sulfoxide (DMSO) coupled to in situ infrared spectroscopy measurements to probe the stability of fibrils against chemical denaturation. While both types of fibrils eventually undergo DMSO-induced disassembly coupled to a β-sheet→coil transition, in the case of [BI-ace] amyloid, the denaturation is preceded by the fibrils transiently acquiring the [BI]-like infrared characteristics. We argue that this effect is caused by DMSO-induced dehydration of [BI-ace]. In support to this hypothesis, we show that, even in the absence of DMSO, the infrared features of [BI-ace] disappear upon drying. We discuss this very peculiar aspect of [BI-ace] fibrils in the context of recently accessed in silico models of plausible structural variants of insulin protofilaments.