Biomedical Engineering Reference
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where only a small three-stranded sheet is formed by 6.2% of the sequence, the
negative ROA band at
1240 cm
−
1
in the spectrum of hen lysozyme is able to
clearly monitor its presence. Conventional Raman and IR spectra of
∼
-sheet
proteins also contain bands in the amide III region but they are typically too
weak or obscured by other bands for diagnostic use. The backbone skeletal
stretch region from
β
1000 to 1100 cm
−
1
∼
also contains
β
-sheet ROA marker
bands as observed for both
-lactoglobulin and hen lysozyme. Therefore, the
sensitivity of ROA to the manner in which a vibrational mode interacts with
the local chirality gives rise to marker bands, such as those for
β
-sheet, that
are dicult to observe in other spectroscopies, thus opening up a wider and
more informative spectral window.
As with
β
-helix, other signature bands exist within the ROA spectrum
that are diagnostic of
α
-sheet [3]. The characteristic amide I couplet is again
negative at lower wave number and positive at higher wave number, though it
appears 20-30 cm
−
1
higher than for
β
-helix. This is sucient to differentiate
between the two structural motifs, as is evident in the hen lysozyme spectrum.
The corresponding backbone skeletal stretch marker bands for
α
β
-sheet appear
∼
1000-1050 cm
−
1
.
in the region
7.4.3 ROA Signatures of Disordered Structure
It is now obvious that disordered structure, from wholly natively unfolded
proteins to flexible loops between structured domains, plays a critical role
in the behaviour and function of many proteins. Such flexible regions have
proven dicult to resolve at atomic resolution and various optical spec-
troscopies have proven useful in characterizing and studying disordered se-
quences. Protein unfolding is also closely related to the phenomenon of protein
misfolding,which is critical to the formation of fibrils by some proteins. The
large number and complexity of disordered protein structures are indicated
by the DisProt Database [43] (http://www.disprot.org) and it is thought that
unfolded proteins, or proteins with long unfolded sequences, may comprise at
least 25% of all proteins so far discovered [44]. NMR studies are able to recog-
nize the presence of disordered structure but the flexibility of these sequences
and the slow relaxation times of NMR (on the order of milliseconds) reduce the
sensitivity of these measurements compared to those on folded proteins. ROA,
in common with other Raman techniques, operates on a much faster timescale
allowing a 'snapshot' of the whole ensemble of inter-converting conformers to
be taken. The sensitivity of ROA also generates a more information-rich spec-
trum for disordered proteins than other Raman techniques [45-49], allowing
the complexity of disordered structures such as polyproline II helix to be
characterized.
7.5 Glycoproteins
Though there have been few studies performed on glycoproteins so far, these
indicate that intact glycoproteins provide high-quality ROA spectra with clear
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