Binding mode of Congo Red to Alzheimer's amyloid β-peptide studied by UV Raman spectroscopy

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 β-peptide (Aβ) 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 Aβ, 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-Aβ complex. A red shift of the visible absorption is consistent with an expansion of π-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 Aβ amyloid fibrils.