Using light polarisation to unravel the structure of insulin fibril polymorphs

Amyloid fibrils lie at the core of major human degenerative diseases, such as Alzheimer's, Parkinson's, and diabetes. The structural polymorphism of amyloid fibrils has been linked to disease severity and many polymorph structures of proteins such as Aβ or α-synuclein are now known in detail. Amyloid fibril polymorphism has also been observed for insulin amyloid fibrils, but very little is known about the underlying structural differences. Here, we investigate the structure of insulin amyloid fibril polymorphs created in different solvents by Raman, infrared and fluorescence spectroscopy. We show that insulin polymorphs have different twists of their β-sheets which impacts alignment of backbone carbonyls relative to the fibril axis, changes β-turn structure and disulfide bond conformation, changes alignment of tyrosines and the hydrogen-bonding state of Gln and Glu side chains. These different structures arise because solvent composition changes hydrophobic and hydrogen-bonding interactions between insulin monomers. This work is a first step towards understanding how different environmental conditions create specific insulin fibril structures.