Effect of amyloid β-peptide on the fluidity of phosphatidylcholine membranes: Uses and limitations of diphenylhexatriene fluorescence anisotropy

There is accumulating evidence that peptide-induced perturbations in the order and dynamics of cellular membranes may play a role in the neurotoxicity of amyloid β-peptide (Aβ). Several studies have reported that Aβ decreases fluidity of membranes based on an Aβ-induced increase in the fluorescence anisotropy of diphenylhexatriene (DPH). However, the effect of Aβ on the membrane fluidity is still a subject of controversy, because other studies that employed pyrene as a fluorescent probe have shown that Aβ has the opposite effect. To reveal the reason for this discrepancy, we have examined the effect of Aβ on the fluidity of phosphatidylcholine membranes using spectroscopic methods. The fluorescence anisotropy of DPH is dramatically increased on addition of Aβ to DPH-containing phosphatidylcholine membranes. However, Aβ does not affect the Raman spectrum of the membrane, which is sensitive to the packing order of the hydrocarbon chains of lipids. We have also found that circular dichroism (CD) bands of DPH appear during incubation of DPH-containing membranes with Aβ, whereas DPH is an achiral molecule. The observed CD bands of DPH are induced by a chiral environment of Aβ but not by that of the lipids, because positive CD bands appear regardless of the d/l-chirality of phosphatidylcholine. The findings obtained from CD measurements provide evidence that DPH molecules translocate from the membrane to Aβ. The peptide-mediated extraction of DPH from the membrane may cause changes in the fluorescence anisotropy of DPH, even though Aβ does not affect the fluidity of membranes.