Constraints On The Structure Of Fibrils Formed By A Racemic Mixture Of Amyloid-Beta Peptides From Solid-State Nmr, Electron Microscopy, And Theory

Previous studies have shown that racemic mixtures of 40- and 42-residue amyloid-beta peptides (D,L-A beta 40 and D,L-A beta 42) form amyloid fibrils with accelerated kinetics and enhanced stability relative to their homochiral counterparts (L-A beta 40 and L-A beta 42), suggesting a "chiral inactivation" approach to abrogating the neurotoxicity of A beta oligomers (A beta-CI). Here we report a structural study of D,L-A beta 40 fibrils, using electron microscopy, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations. Two- and three-dimensional solid-state NMR spectra indicate molecular conformations in D,L-A beta 40 fibrils that resemble those in known L-A beta 40 fibril structures. However, quantitative measurements of C-13-C-13 and N-15-C-13 distances in selectively labeled D,L-A beta 40 fibril samples indicate a qualitatively different supramolecular structure. While cross-beta structures in mature L-A beta 40 fibrils are comprised of in-register, parallel beta-sheets, our data indicate antiparallel beta-sheets in D,L-A beta 40 fibrils, with alternation of D and L molecules along the fibril growth direction, i.e., antiparallel "rippled sheet" structures. The solid-state NMR data suggest the coexistence of D,L-A beta 40 fibril polymorphs with three different registries of intermolecular hydrogen bonds within the antiparallel rippled sheets. DFT calculations support an energetic preference for antiparallel alignments of the beta-strand segments identified by solid-state NMR. These results provide insight into the structural basis for A beta-CI and establish the importance of rippled sheets in self-assembly of full-length, naturally occurring amyloidogenic peptides.