Structural Organization of Insulin Fibrils Based on Polarized Raman Spectroscopy: Evaluation of Existing Models

Many different proteins undergo misfolding and self-assemble into amyloid fibrils, resulting in a range of neurodegenerative diseases. The limitations of conventional methods of structural biology for fibril characterization have led to the use of polarized Raman spectroscopy for obtaining quantitative structural information regarding the organization of amyloid fibrils. Herein, we report the orientation of selected chemical groups and secondary structure elements in aligned insulin fibrils, including beta-sheets, which possess a high level of orientation in the cross-beta core, and alpha-helices in the disordered portions of the fibrils. Strong orientation of disulfide bonds in amyloid fibrils was also revealed, indicating their association with the fibril core. The determined orientation of chemical groups provides strong constraints for modeling the overall structure of amyloid fibrils, including the core and disordered parts. The developed methodology allows for the validation of structural models proposed in the literature for amyloid fibrils. Specifically, the polarized Raman data obtained herein strongly agreed with two insulin fibril models (Jimenez et al., Proc. Natl Acad. Sci. U.S.A. 2002, 99, 9196-9201 and Ivanova et al., Proc. NatL Acad. Sci. U.S.A. 2009, 106, 18990-18995) yet revealed significant qualitative and quantitative differences. This work demonstrates the great potential of polarized Raman spectroscopy for structural characterization of anisotropic biological species.