Substituted tryptophans at amyloid-β(1-40) residues 19 and 20 experience different environments after fibril formation.

Amyloid-β protein (Aβ) is the principal component of the neuritic plaques found in Alzheimer's disease. The predominant Aβ morphology in the plaques is fibrillar which has prompted substantial in vitro work to better understand the molecular organization of Aβ fibrils. In the current study, tryptophan substitutions were made at Aβ(1-40) position 19 (F19W) or 20 (F20W) to ascertain environmental differences between the two residues in the fibril structure. Kinetic studies revealed similar rates of fibril formation between Aβ(1-40) F19W and F20W and both peptides formed typical amyloid fibril structures. Aβ(1-40) F19W fibrils displayed a significant tryptophan fluorescence blue-shift in λ(max) (33nm) compared to monomer while Aβ(1-40) F20W fibrils had a much smaller shift (9nm). Fluorescence quenching experiments with water-soluble acrylamide and KI demonstrated that both W19 and W20 were much less accessible to quenching in fibrils compared to monomer. Lipid-soluble TEMPO quenched the fluorescence of Aβ(1-40) F19W fibrils more effectively than F20W fibrils in agreement with the fluorescence blue-shift results. These findings demonstrate distinct environments between Aβ(1-40) residues 19 and 20 fibrils and indicate that while W20 accessibility is compromised in Aβ fibrils it resides in a much less hydrophobic environment than W19.