CD studies on films of amyloid proteins and polypeptides: quantitative g-factor analysis indicates a common folding motif.

Irrespective of the constituent protein, all amyloid fibrils show similar morphology in the electron microscope and x-ray diffraction patterns characteristic of a "cross-beta" structure, with extended beta-strands perpendicular to the fibril's long axis. Little is known about the amount or type of this structure. I have measured CD spectra of films formed from a number of amyloid proteins and polypeptides, and estimated their contents of extended secondary structure, by analysis of their g-factor spectra, the ratio of the CD and absorbance signals (P. McPhie, Analytical Biochemistry, 2001, Vol. 293, pp. 109-119). Amyloid films of Abeta-(1-40) peptide, beta-2-microglobulin, insulin, and three homopolypeptides show very intense CD spectra, compatible with the presence of a beta-helix-like structure, arranged in a common framework in the fibrils. The extent of this structure was estimated as 45-80% in the protein fibrils and 30-80% in the polypeptide fibrils.