144
G. Compagnini et al.
Many structural characterizations are not satisfactory either for oligomers and
fibrils. Indeed, since the oligomers show a fast-transient state, X-ray diffraction can
merely assign amorphous structures, rich in β-sheet [47]. This especially in the early
stage of fibril self-assembling pathway [39, 48].
A possible alternative is the use of highly sensible and chemical-specific techniques such as SERS [49, 50]. This has already been proposed for the detection of
prefibrillar assemblies [51–53].
The introduction of SER active surfaces during the amyloid fibril formation
can also alter (inhibit/promote) the nucleation-dependent fibrillogenesis mechanism.
Recent investigations have in fact highlighted the influence of metal and polymeric
nanoparticles in the amyloid fibril formation.
In this respect, Zhang et al. [54] reported that gold nanoparticles (NPs) catalyze
the aggregation and growth of lysozyme, while Liao et al. [55] demonstrated that
negatively charged gold NPs inhibit Alzheimer’s amyloid-β fibrillation. Moreover,
Brancolini et al. [56] showed that the interaction between proteins and citrate-capped
gold NPs is weak in the physiological condition due to citrate presence.
Of course, chemical reduction of metal salts is one of the most frequently used
methods for preparing colloidal metals in order to perform SERS in water protein
solutions. Copper, Platinum, Palladium, Silver, and Gold colloids have been prepared
by this method. The latter two are most often prepared by reduction of AgNO 3 or
HAuCl 4 . Sodium borohydride and sodium citrate are common reductants, although
a number of other compounds have been used.
As previously mentioned, these procedures leave into the ambient a large number
of unwanted impurities which limit the interaction and reactivity of the analyte with
the naked surface and then the sensitivity of the surface-enhanced technique.
The fabrication procedures that are based on pulsed laser ablation in liquid (PLAL)
are particularly interesting, because they permit preparation of stable and unprotected
nanostructures even in pure solvents [10, 57]. Therefore, having in mind the abovementioned applications, an investigation on the surface of PLAL-synthesized AgNPs,
and on the possible means to control its characteristics, is important in view of proper
tailoring of the colloid properties.
Figure 4.6a shows the Raman spectra for hIAPP in the prefibrillar structures and
after the formation of amyloid fibrils at a protein concentration of 0.4 µM. The
positions of the vibrational bands characterizing the secondary structure depend
on inter and intramolecular protein interactions, including peptide-bond angles and
hydrogen-bonding patterns.
Typically, a protein Raman spectrum is the result of three major vibrational modes
in the range 1000–1800 cm
−1 :
(a) amide bands (amide I, II, III) of the polypeptide backbone
(b) vibrations of aromatic amino acid residue
(c) vibrations of nonaromatic side chains.
As expected, at the beginning of the fibrillogenesis process, no protein signal is
detected. At the end of the growth process, fibrils are well visible spectroscopically.
Atomic Force Microscopy in tapping mode (Fig. 4.6b) determines the morphology
G. Compagnini et al.
Many structural characterizations are not satisfactory either for oligomers and
fibrils. Indeed, since the oligomers show a fast-transient state, X-ray diffraction can
merely assign amorphous structures, rich in β-sheet [47]. This especially in the early
stage of fibril self-assembling pathway [39, 48].
A possible alternative is the use of highly sensible and chemical-specific techniques such as SERS [49, 50]. This has already been proposed for the detection of
prefibrillar assemblies [51–53].
The introduction of SER active surfaces during the amyloid fibril formation
can also alter (inhibit/promote) the nucleation-dependent fibrillogenesis mechanism.
Recent investigations have in fact highlighted the influence of metal and polymeric
nanoparticles in the amyloid fibril formation.
In this respect, Zhang et al. [54] reported that gold nanoparticles (NPs) catalyze
the aggregation and growth of lysozyme, while Liao et al. [55] demonstrated that
negatively charged gold NPs inhibit Alzheimer’s amyloid-β fibrillation. Moreover,
Brancolini et al. [56] showed that the interaction between proteins and citrate-capped
gold NPs is weak in the physiological condition due to citrate presence.
Of course, chemical reduction of metal salts is one of the most frequently used
methods for preparing colloidal metals in order to perform SERS in water protein
solutions. Copper, Platinum, Palladium, Silver, and Gold colloids have been prepared
by this method. The latter two are most often prepared by reduction of AgNO 3 or
HAuCl 4 . Sodium borohydride and sodium citrate are common reductants, although
a number of other compounds have been used.
As previously mentioned, these procedures leave into the ambient a large number
of unwanted impurities which limit the interaction and reactivity of the analyte with
the naked surface and then the sensitivity of the surface-enhanced technique.
The fabrication procedures that are based on pulsed laser ablation in liquid (PLAL)
are particularly interesting, because they permit preparation of stable and unprotected
nanostructures even in pure solvents [10, 57]. Therefore, having in mind the abovementioned applications, an investigation on the surface of PLAL-synthesized AgNPs,
and on the possible means to control its characteristics, is important in view of proper
tailoring of the colloid properties.
Figure 4.6a shows the Raman spectra for hIAPP in the prefibrillar structures and
after the formation of amyloid fibrils at a protein concentration of 0.4 µM. The
positions of the vibrational bands characterizing the secondary structure depend
on inter and intramolecular protein interactions, including peptide-bond angles and
hydrogen-bonding patterns.
Typically, a protein Raman spectrum is the result of three major vibrational modes
in the range 1000–1800 cm
−1 :
(a) amide bands (amide I, II, III) of the polypeptide backbone
(b) vibrations of aromatic amino acid residue
(c) vibrations of nonaromatic side chains.
As expected, at the beginning of the fibrillogenesis process, no protein signal is
detected. At the end of the growth process, fibrils are well visible spectroscopically.
Atomic Force Microscopy in tapping mode (Fig. 4.6b) determines the morphology