Repurposing Triphenylmethane Dyes to Bind to Trimers Derived from Aβ

Soluble
oligomers of the β-amyloid peptide, Aβ, are
associated with the progression of Alzheimer’s disease. Although
many small molecules bind to these assemblies, the details of how
these molecules interact with Aβ oligomers remain unknown. This
paper reports that crystal violet, and other C3 symmetric
triphenylmethane dyes, bind to C3 symmetric trimers
derived from Aβ17–36. Binding changes the
color of the dyes from purple to blue, and causes them to fluoresce
red when irradiated with green light. Job plot and analytical ultracentrifugation
experiments reveal that two trimers complex with one dye molecule.
Studies with several triphenylmethane dyes reveal that three N,N-dialkylamino substituents are required
for complexation. Several mutant trimers, in which Phe19, Phe20, and Ile31 were mutated to cyclohexylalanine,
valine, and cyclohexylglycine, were prepared to probe the triphenylmethane
dye binding site. Size exclusion chromatography, SDS-PAGE, and X-ray
crystallographic studies demonstrate that these mutations do not impact
the structure or assembly of the triangular trimer. Fluorescence spectroscopy
and analytical ultracentrifugation experiments reveal that the dye
packs against an aromatic surface formed by the Phe20 side
chains and is clasped by the Ile31 side chains. Docking
and molecular modeling provide a working model of the complex in which
the triphenylmethane dye is sandwiched between two triangular trimers.
Collectively, these findings demonstrate that the X-ray crystallographic
structures of triangular trimers derived from Aβ can be used
to guide the discovery of ligands that bind to soluble oligomers derived
from Aβ.