Insulin Amyloid Fibrils Form an Inclusion Complex with Molecular Iodine:  A Misfolded Protein as a Nanoscale Scaffold

The study describes formation of an intensely violet inclusion complex of insulin amyloid fibrils and molecular iodine. Resonance Raman spectra of complexes formed by staining mature insulin fibrils with iodine and by seeding fibrils in the presence of iodine imply similar topologies of entrapped iodine and oligoiodide species. Iodine captured by growing fibrils remains accessible to a bulk chemical reagent. In light of its small size and the fact that iodine can partition into polar as well as nonpolar media, the data suggest that intrafibrillar structure of insulin amyloid is densely packed with no appreciable void volumes capable of accommodating iodine atoms. The complex is stable: only drastic perturbation of the beta-pleated fibrous scaffold by dimethyl sulfoxide (rather than of the beta-sheet conformation) leads to the release of iodine atoms from surface moieties. While the reaction between iodine and in vivo amyloid deposits was first described by Virchow in the 19th Century [Virchow, R. (1854) Virchows Arch. 6, 268-271], the underlying molecular mechanism has not been thoroughly explored since then. This work shows how the long-forgotten concept can be utilized as a probe of void volumes in protein fibrils, providing a new tool for structural studies on amyloids, and a model for design of protein-based drug delivery media.