Fibrillar amyloid beta-protein forms a membrane-like hydrophobic domain.

Microviscosity of the biological membranes is determined by measuring the fluorescence polarization of diphenylhexatriene (DPH). DPH, a hydrophobic probe, has negligible fluorescence in the solution. When DPH is incorporated into the membrane, it is localized in the membrane hydrophobic core and fluoresces strongly. We report here that DPH also fluoresces in the presence of fibrillar Abeta (fAbeta). However, it does not fluoresce when it is added to the soluble Abeta (sAbeta). DPH inserts into Abeta fibrils in a time-dependent manner, and upon centrifugation, it is sedimented along with fibrils. The steady state fluorescence polarization of DPH with fAbeta1-40 and fAbeta 1-42 was 0.4592 and 0.4898 respectively. These results suggest that fAbeta (but not sAbeta) forms a hydrophobic domain similar to that of membrane.