4,4′-Dianilino-1,1′-binaphthyl-5,5′-disulfonate: report on non-β-sheet conformers of Alzheimerʹs peptide β(1–40)

The venerable fluorescent probe of protein hydrophobic regions, 4,4′-dianilino-1,1′-binaphthyl-5,5′-disulfonate (bis-ANS), unexpectedly increases in fluorescence with soluble β(1–40) in acidic buffer solutions but reacts weakly with amyloid fibrils while other hydrophobic probes react with the fibrils. CD analysis correlates reaction with the probe with random coil/mixed conformations and α-helical forms of β(1–40) in buffer solutions but less so with soluble β-sheet forms or amyloid fibrils. The kinetics of the fluoroalcohol-induced interconversion of conformers can be followed by changes in bis-ANS fluorescence. Formation of the β-sheet form in aqueous buffer is limited by a slow component (minutes) while fluoroalcohol-promoted changes between β-sheet and α-helix occur over seconds. Variants of β(1–40) such as β(1–42) or the Dutch E22Q mutation of β(1–40) and fragments β(1–28), β(12–28), β(10–20 amide), and β(10–35 amide) react with bis-ANS under conditions that do not support fibril formation. Primary amino acid sequence is important as β(1–11) does not cause bis-ANS fluorescence while β(1–16) does, but hydrophobicity is not as β(25–35) and β(15–20 amide) are unreactive. bis-ANS is a useful biophysical tool for characterizing particular, but not all, soluble Aβ conformations distinct from the fibrillar form of amyloid peptides detected by Thioflavin T.