Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimer's disease. Zn(II) induces the Abeta aggregation at acidic-to-neutral pH, while Cu(II) is an effective inducer only at mildly acidic pH. We have examined Zn(II) and Cu(II) binding modes of Abeta and their pH dependence by Raman spectroscopy. The Raman spectra clearly demonstrate that three histidine residues in the N-terminal hydrophilic region provide primary metal binding sites and the solubility of the metal-Abeta complex is correlated with the metal binding mode. Zn(II) binds to the N(tau) atom of the histidine imidazole ring and the peptide aggregates through intermolecular His(N(tau))-Zn(II)-His(N(tau)) bridges. The N(tau)-metal ligation also occurs in Cu(II)-induced Abeta aggregation at mildly acidic pH. At neutral pH, however, Cu(II) binds to N(pi), the other nitrogen of the histidine imidazole ring, and to deprotonated amide nitrogens of the peptide main chain. The chelation of Cu(II) by histidine and main-chain amide groups results in soluble Cu(II)-Abeta complexes. Under normal physiological conditions, Cu(II) is expected to protect Abeta against Zn(II)-induced aggregation by competing with Zn(II) for histidine residues of Abeta.

No abstract

No abstract

Congo Red (CR) has long been used for staining amyloid fibrils and recently been found to inhibit amyloid formation. We examined the binding mode of CR to Alzheimer's amyloid b-peptide (Ab) by UV Raman spectroscopy. The 244 nm-excited Raman spectrum of CR exhibits five bands in the 1620-1550 cm −1 region, where C C stretching vibrations of the naphthalene and biphenyl moieties are expected. Among the C C stretching bands, one at 1615 cm −1 becomes weaker and another at 1598 cm −1 gains intensity upon binding to Ab, resulting in a reversal of the relative intensity of the two bands. Concomitant with the intensity reversal, a band at 1158 cm −1 assignable to the N-C stretch becomes stronger. These UV Raman spectral changes are in line with those observed when the fraction of the planar CR molecule is increased by crystallization. Furthermore, opposite spectral changes were observed when the central biphenyl group was methylated to force the two benzene rings in an orthogonal orientation. These observations suggest an increase in planarity of the biphenyl group in the CR-Ab complex. A red shift of the visible absorption is consistent with an expansion of p-conjugation in the planar CR molecule. The torsional mobility of the biphenyl group permitting a planar conformation may be essential for the binding of CR to Ab amyloid fibrils.

No abstract