Infrared Spectroscopic Characterization of Alzheimer Plaques

Abstract

Neuritic plaques in the brains of victims of Alzheimer's disease are primarily composed of a 42 amino acid polypeptide, the β-amyloid peptide (βA4), the neurotoxicity of which is related to its aggregation. Fourier transform infrared spectroscopy has been used to study the conformational properties of two synthetic analogues of βA4 peptides known to be involved in the formation of the neuritic plaques formed in patients with Alzheimer's disease and the influence of a single, naturally occurring point mutation upon the tendency of the peptide to aggregate. Peptides from both “normal” Alzheimer's and the more severe Dutch variant of the disease were found to form aggregated antiparallel strands. However, the replacement of a single, negatively charged amino acid (glutamic acid) with an uncharged amino acid (glutamine) in the Dutch-type peptide results in significant differences in the strength and stability of these aggregates and the microenvironment of a number of amino acids. The differences in the strength and the stability of the aggregates are attributed to the presence of varying (small) proportions of the classical secondary structures and differences in net charge. Environmental (solvent) effects were shown to significantly affect the strength of the inter-molecular hydrogen bonding in the aggregates, solvent systems mimicking the membrane/water interface resulting in more strongly hydrogen bonded aggregates. Infrared spectra of material from autopsied human Alzheimer's brains show spectral features indicative of the formation of similar aggregates, which may be related to plaque formation. This observation suggests that IR spectroscopic methods may in the future be useful in the diagnosis and monitoring of Alzheimer's disease.