Page 367 - Organic Electronics in Sensors and Biotechnology

344 Chapter Nine

associated with a distinct amyloid subtype. However, immunohis-
tochemistry is fraught with specific problems. For one thing, most
antibodies penetrate only poorly the compact beta-sheet accumula-
tions of amyloid. Also, many amyloids incorporate nonspecifically
other proteins, which can give rise to false-positive stains with
diagnostic antibodies and––in worst-case scenarios––may lead to
misdiagnoses.
For many years the only structural information about amyloid
fibrils came from imaging techniques such as TEM (transmission
electron microscopy), AFM (atomic force microscopy) and X-ray fiber
diffraction. 103, 104 These experiments revealed that the fibrils predomi-
nantly consist of a number (typically two to six) of protofilaments
that twist together to form the typical amyloid fibrils. X-ray fiber dif-
fraction data have shown that in each individual protofilament, the
protein or peptide molecules are arranged so that the polypeptide
chain forms β-strands that run perpendicular to the long axis of the
fibril. Today novel structural insight and molecular details have been
provided by solid-state NMR spectroscopy, 105, 106 and by single crystal
X-ray diffraction analysis of small amyloid-like peptide fragments. 107
The latter has allowed both the structure of the peptides and the way
the molecules could be packed together to be determined with
unprecedented resolution. A particularly significant aspect of the
structures determined with the different techniques is that they are
strikingly similar even for polypeptides having no sequence homol-
ogy, suggesting that many amyloid fibrils could share similar core
structures. However, the specific nature of the side chain packing,
including characteristics as the alignment of adjacent strands and the
separation of the sheets, provides an explanation for the occurrence
of variations in the details of the structures for specific types of
fibril.
Even though a wide range of techniques have been applied for
studying the amyloid fibrillation event and the pathological mecha-
nism underlying protein aggregation disease, there are still many
questions that remain to be answered regarding these events. Hence,
there is a need for novel tools that provide greater insight into these
events, and next we discuss the utilization of LCPs for monitoring
these events.

9.4 Luminescent Conjugated Polymers
as Amyloid Specific Dyes

9.4.1 Detection of Amyloid Fibrils in Solution
As discussed earlier, novel tools that detect the conformational
changes in proteins, especially the formation of amyloid fibrils, are of
great importance, as this is an extremely complex event and many