Helix-dipole effects in peptide self-assembly to amyloid.

The formation of amyloid fibrils is associated with incurable diseases including Alzheimer's, Parkinson's, and type 2 diabetes. Important mechanistic details of the self-assembly are unknown partly because of the absence of a clear structural characterization of intermediates. There is experimental evidence, however, for alpha-helical intermediates that has come primarily from circular dichroism spectroscopy. Here, we strengthen the evidence for helical intermediates by demonstrating helix-dipole effects in the early events of self-assembly. Previously, we showed that capped peptides containing the part of the islet amyloid polypeptide that may be responsible for the initial intermolecular contacts (Acetyl-R(11)LANFLVHSSNNFGA(25)-NH2 and Acetyl-R(11)LANFLVHSGNNFGA(25)-NH2 which contains the S20G mutation associated with early onset type 2 diabetes) self-assemble via helical intermediates [Liu et al. (2010) J. Am. Chem. Soc. 132, 18223-18232]. We demonstrate here that when the peptides are uncapped, they do not self-assemble as indicated primarily by circular dichroism and nuclear magnetic resonance data. Self-assembly is restored when the charge on alpha-NH3+ of Ar11 is eliminated but not when the charge on alpha-COO- of Ala2S is removed, consistent with the helicity of the peptides skewed toward the N-terminus. Our results strengthen the hypothesis that a-helical intermediates are on pathway to amyloid formation and indicate that the helix dipole is an attractive target for inhibiting the formation of alpha-helical assemblies.