4,4′-dianilino-1,1′-binaphthyl-5,5′-disulfonate: Report on non-β-sheet conformers of Alzheimer's 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.