Auramine‑O as a
Fluorescence Marker for the
Detection of Amyloid Fibrils
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Abstract
There is an indispensable need for a fluorescence marker
for the
detection of amyloid fibrils, where, at present, the most used marker
is thioflavin-T (ThT). Here, we present the use of auramine-O (AuO)
as a possible alternative to ThT. As with ThT, the increase in the
emission of AuO upon binding to amyloid fibrils is the result of inhibition
of the free rotation of the two dimethylamino arms of the molecule.
This inhibition prevents the excited-state electronic wave function
from moving from the emissive locally excited state to the dark charge-transfer
state. We further show that not only AuO is comparable to ThT as a
fluorescent marker for amyloid fibrils but also it has a unique spectroscopic
signature. AuO has distinct two modes that are characterized by a
large shift in the absorption and emission peak positions between
its unbound and bound states (before and after the fibrils formation,
respectively). In this context, we show that, whereas the emission
band position is red-shifting, the absorption peak shifts to the blue
and the spectrum exhibits an isosbestic point. The large shifts in
emission and absorption peak positions can be explained by the photoacid
activity of AuO exhibiting an excited-state proton-transfer process