Amyloid fibril formation by A beta(16-22), a seven-residue fragment of theAlzheimer's beta-amyloid peptide, and structural characterization by solidstate NMR

The seven-residue peptide N-acetyl-Lys-Leu-Val-Phe-Phe-Ala-Glu-NH2 called A beta (16-22) and representing residues 16-22 of the full-length beta -amyl oid peptide associated with Alzheimer's disease, is shown by electron micro scopy to form highly ordered fibrils upon incubation of aqueous solutions. X-ray powder diffraction and optical birefringence measurements confirm tha t these are amyloid fibrils, The peptide conformation and supramolecular or ganization in A beta (16-22) fibrils are investigated by solid state NMR me asurements. Two-dimensional magic-angle spinning (2D MAS) exchange and cons tant-time double-quantum-filtered dipolar recoupling (CTDQFD) measurements indicate a beta -strand conformation of the peptide backbone at the central phenylalanine. One-dimensional and two-dimensional spectra of selectively and uniformly labeled samples exhibit C-13 NMR line widths of <2 ppm, demon strating that the peptide, including amino acid side chains, has a well-ord ered conformation in the fibrils. Two-dimensional C-13-C-13 chemical shift correlation spectroscopy permits a nearly complete assignment of backbone a nd side chain C-13 NMR signals and indicates that the <beta>-strand conform ation extends across the entire hydrophobic segment from Leu17 through Ala2 1. C-13 multiple-quantum (MQ) NMR and C-13/N-15 rotational echo double-reso nance (REDOR) measurements indicate an antiparallel organization of beta -s heets in the A beta (16-22), fibrils, These results suggest that the degree of structural order at the molecular level in amyloid fibrils can approach that in peptide or protein crystals, suggest how the supramolecular organi zation of beta -sheets in amyloid fibrils can be dependent on the peptide s equence, and illustrate the utility of solid state NMR measurements as prob es of the molecular structure of amyloid fibrils, A beta (16-22) is among t he shortest fibril-forming fragments of full-length beta -amyloid reported to date, and hence serves as a useful model system for physical studies of amyloid fibril formation.