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Marking a
Conformational Change in Amyloid Beta Protein
by Gold Colloidal Nanoparticles
Kazushige Yokoyama, Daniel R. Welchons, and Bradley
Johnson
Department of
Chemistry, The State University of New York, Geneseo College,
Geneseo, New York, 14454 U.S.A.
Denaturization and the subsequent
self-organization of amyloid beta-peptides (AB) is
a critical
event leading to the onset of Alzheimer’s disease. Our study aims to design a sensitive
indicator for
probing this process.
We investigated the ability of gold colloidal nanoparticles
to mark the self
-organization process of AB. Absorption spectroscopy was conducted on AB
conjugated gold
colloidal nanoparticles
with the size of 20 nm diameter as a function of pH ranging from pH 2 to
pH 10. The reddish
color of the unconjugated colloidal nanoparticle exhibited a band centered at
520
nm. However, a remarkable color change
to blue was observed at pH 5 and lower, and had an
absorption peak shift to ca. 700 nm. We speculated that a cluster of unfolded AB
proteins
conformed to an oligomer on the
colloidal surface at pH 5. The similar
pH dependence of the shift
was also confirmed for segments of AB proteins. In order to determine the reversibility of
the color
change between blue and red, the pH of the solution was
repeatedly varied between pH 4 and 10.
Quite
interestingly, reversibility was only observed in a full sequence of AB but
not in segments of
AB.
Furthermore, among tested gold particle size ranging from 10 to 100 nm
in diameter, only gold
nanoparticle with 20nm diameter
exhibited reversibility. Therefore, we
concluded that oligomers
that
undergo a reversible change need to be comprised of full AB sequences, and
this structure takes
place at a specific size of gold nanoparticle.
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