Perturbing the Central Hydrophobic Cluster (CHC) of Aβ(10-35) by Incorporation of Fluorinated Phenylalanine Derivatives

Abstract

One of the putative causes of Alzheimer's disease involves aggregation of misfolded amyloid β (Aβ), a 39-42 residue polypeptide chain, and its subsequent deposition as amyloid plaques. The aggregation process proceeds via a nucleated polymerization mechanism where disordered peptide monomers interact with each other through hydrophobic interactions and rapidly extend and aggregate to eventually form larger fibrils with a highly ordered cross-strand β-sheet structure. It has also been suggested that the aromatic amino acid residues, tyrosine Y10 and phenylalanines (F19 and F20) in the central hydrophobic cluster (CHC) of the peptide play an important role in fibril assembly. In particular, F19 and F20 are suspected to be the drivers of the aggregation mechanism because of their hydrophobicity and aromaticity. In this context perturbation of the CHC through the introduction of non-natural (fluorinated) amino acids is expected to affect the aggregation process. Fluorinated amino acids in particular demonstrate distinct properties dictated by the presence of highly electronegative and hydrophobic fluorine atoms. However such fluorination is known to potentially eliminate the favorable interaction of aromatic hydrogens with the π-electron cloud, which can affect protein-protein interactions. In the present study the introduction of a pentafluoro-Phe in the hydrophobic core of the 26 residue Aβ peptide (Aβ10-35) and its effect on fibril formation has been investigated using circular dichroism (CD) and fluorescence methods which indicate a sequential conformational transition of the peptide from random coil → antiparallel β-sheets → parallel β-sheets. Transition time points have been obtained from these methods and compared to those obtained for the non-fluorinated peptide. UV resonance Raman (UVRR) studies have been performed to probe and characterize the vibrational modes of the fluoro-phenylalanines in the peptide and to explore their effect on the Phe-Phe π-stacking interactions.