Peptides - Beta-Sheet Folding from Hairpins to Aggregates

Abstract

Selected examples of turn sequence and hydrophobic contact stabilized β-hairpin peptides were previously studied using ECD, fluorescence, IR and VCD spectroscopies to assess stability of β-hairpin formation. Extending this, two three-stranded β-sheet peptides, based on modified Trpzip sequences, using D-Pro-Gly (B3pG) or Asn-Gly (B3GN) turn sequences gave ECD spectra reflecting cross-strand Trp-Tyr aromatic interactions, and indicated both were partially multi-stranded. Both showed initial IR spectra at low temperatures indicative of extended β-sheet structure that were more characteristic of an aggregate than a small oligomer structure. Thermal variation of their IR spectra gave strikingly different behaviors. B3pG reversibly unfolded from aggregated β-like structure at low temperature to disordered at high temperature. B3GN formed aggregates at low temperature, became disordered with heating, but upon re-cooling gave typical soluble β-sheet peptide spectra, which was could be reversibly unfolded. ThT binding to dilute B3GN, but not B3pG, caused a fluorescence enhancement, consistent with fibril formation. These results suggest that turn sequence mutation leads to different micro- and macro-structures, resulting in tuning their structurally related properties. Modifying the sequences to reduce hydrophobicity (aromatic residues) but incorporate Aib-Gly turns gave partially folded peptides with reversible folding but less stability. Taking another view of β-sheet based peptide aggregation, we studied fibril formation in glutamic acid oligomers at low pH (β2 form). We had shown that poly-Glu IR is relatively insensitive to mixing of D and L isomers, which causes a loss of long range order (fibrilization), as seen in EM, but VCD is hypersentive to this change, providing a new way of detection of long range ordering (chirality) in fibril formation as opposed to aggregate forms. The oligo-Glu peptide models add isotopic labeling to provide new insight into the structure of the fibrils formed.