Page 374 - Organic Electronics in Sensors and Biotechnology

Luminescent Conjugated Polymers for Staining and Characterization of Amyloid Deposits 351

further studies of complexes between in vitro produced protein
aggregates with defined conformations and LCPs with distinct ionic
side chain functionalities or different chain lengths will likely be nec-
essary to understand the correlation of the spectroscopic readout
from the LCP and the molecular structure of the protein aggregate.
Although the achievement of obtaining certain spectroscopic LCP
signatures from heterogenic populations of protein aggregates is ben-
eficial compared to conventional amyloid specific dyes, correlating
this spectroscopic signature to a specific form of the aggregated pro-
tein is still necessary to gain novel insight into the pathological pro-
cess of the disease. Nevertheless, the LCPs can be useful for compari-
son of heterogenic protein aggregates in well-defined experimental
systems.
Heterogenic protein aggregates can also be found in other protein
aggregation disorders, such as the infectious prion diseases. As men-
tioned previously, prion disease is caused by a proteinaceous agent
Sc
called PrP , a misfolded and aggregated version of the normal prion
protein. In addition, prions can occur as different strains, and the
prion strain phenomenon is most likely encoded in the tertiary or
quaternary structure of the prion aggregates. This belief was also
verified when protein aggregates in brain sections from mice infected
112
with distinct prion strains were being stained by LCPs. The LCPs
bound specifically to the prion deposits and different prion strains
can be separated due to alternative staining patterns of LCPs with
distinct ionic side chains. Furthermore, the anionic LCP, PTAA, emits
light of different wavelengths when bound to distinct protein depos-
its associated with a specific prion strain (Fig 9.10a). As the emission
profiles of LCPs are associated with geometric changes of the poly-
mer backbone, 36, 43, 44 ratios of the intensity of the emitted light at cer-
tain wavelengths can be used as an indicator of the geometry of the
polymer chains. 36, 86 Nonplanar and separated LCP chains emit light
around 530 to 540 nm, whereas a planarization of the thiophene back-
bone will shift the emission maximum E toward longer wave-
max
lengths. A planar backbone might also give rise to an aggregation of
LCP chains, seen as an increase of the intrinsic emission around
640 nm. When plotting the ratio 532/E and the ratio 532/639 nm in
max
a correlation diagram, prion aggregates associated with distinct prion
strains, chronic wasting disease (CWD) and sheep scrapie, were easily
distinguished from each other, verifying the usefulness of spectral
properties of LCPs for classification of protein deposits (Fig. 9.10b).
These conformation dependent spectral characteristics can only be
afforded by LCPs and provide the opportunity to get an optical fin-
gerprint for protein aggregates correlating to a distinct prion strain.
Although it was shown that the emission profile of LCPs could be
used to characterize protein deposits, further evidence was necessary
to enable relating the geometric alterations of the LCPs to a structural