Steric Crowding of the Turn Region Alters the Tertiary Fold of Amyloid-β18-35 and Makes It Soluble

A beta self-assembles into parallel cross-beta fibrillar aggregates, which is associated with Alzheimer's disease pathology. A central hairpin turn around residues 23-29 is a defining characteristic of A beta in its aggregated state. Major biophysical properties of A beta, including this turn, remain unaltered in the central fragment A beta(18-35). Here, we synthesize a single deletion mutant, Delta G25, with the aim of sterically hindering the hairpin turn in A beta(18-35). We find that the solubility of the peptide goes up by more than 20-fold. Although some oligomeric structures do form, solution state NMR spectroscopy shows that they have mostly random coil conformations. Fibrils ultimately form at a much higher concentration but have widths approximately twice that of A beta(18-35), suggesting an opening of the hairpin bend. Surprisingly, two-dimensional solid state NMR shows that the contact between Phe(19) and Leu(34) residues, observed in full-length A beta and A beta(18-35), is still intact in these fibrils. This is possible if the monomers in the fibril are arranged in an antiparallel beta-sheet conformation. Indeed, IR measurements, supported by tyrosine cross-linking experiments, provide a characteristic signature of the antiparallel beta-sheet. We conclude that the self-assembly of A beta is critically dependent on the hairpin turn and on the contact between the Phe19 and Leu34 regions, making them potentially sensitive targets for Alzheimer's therapeutics. Our results show the importance of specific conformations in an aggregation process thought to be primarily driven by nonspecific hydrophobic interactions.