J. Zhang et al. / Carbohydrate Research 384 (2014) 102–111
107
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complex formation, by comparing the UV–vis absorption spectra of
HSA and S TdR–HSA complex at the same concentration, it can be
1224 cm at pH 2.23 shifts to 1247 cm at pH 7.09, and moves
4
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to 1245 cm at pH 12.23. The band at this range is attributed to
found that they could not be superposed in the range of 280–
the Kk stretch. It may be due to the deprotonation of Kk stretch
or it is likely that the bond S–N of Kk plane is perpendicular to
the Au colloid.
3
50 nm, thus, this result offers further support to the proposed sta-
tic mechanism, by which the fluorescence of HSA is quenched. In
addition, the intermolecular interaction corresponding to the fluo-
rescence quenching can also be observed from the absorption
spectra.
4
In addition, Figure 10b shows the SERS of S TdR–HSA com-
plexes at different concentrations. From Figure 10b(A) to
Figure 10b(E), the concentration of HSA is constant, while the
4
concentration of S TdR is gradually increased. Except for the vibra-
4
4
4
2
.7. SERS of S TdR at different pH and SERS of S TdR–HSA
tions of DMSO, the SERS spectra of S TdR are mainly located at
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complexes at different concentration
1146, 1246, 1291, and 1477 cm . The most characteristic peak
4
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embodying the orientation of S TdR is the line at 1146 cm , which
SERS Spectra of S4TdR at different pH are shown in
Figure 10a(A–C). From Figure 10a, it can be found that the band
accounts for the ring breath and C@S stretch. This band is hardly
4
enhanced in the low concentration (S TdR–HSA = 1:2), while in
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1
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at 1612 cm at pH 2.23 shifts to 1619 cm at alkaline pH level.
the high concentration (1:1, 2:1, 4:1), the intensity increases grad-
ually. This confirms that no matter what the concentration of
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The band 1347 cm at pH 2.23 disappears at pH 7.09, and shifts
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4
4
to 1349 cm at pH 12.23 which undergo an upward shift. This ef-
fect is a consequence of protonation and the electronic resonance
S TdR is, the plane of S TdR molecule is almost perpendicular to
the surface of Au nanoparticles. As the spontaneous binding of
anti-tumor molecules with HSA takes place initially, then, the
4
32
4
increase in the S TdR. Consequently, the S TdR species can
undergo a nonradiative de-excitation by interaction with the metal
or by the formation of an ionic pair. In addition, the band at
4
S TdR–HSA complex adsorbs on the surface of Au nanoparticles.
In accordance with the result of fluorescence, with the addition
4
of S TdR solution, the fluorescence intensity decreases gradually.
4
On the basis of the phenomenon, S TdR has been embodied into
the hydrophobic cavity present in HSA. Thus, we can indirectly
4
suggest that when the automatic interaction between S TdR and
4
HSA takes place, the plane of S TdR is always perpendicular to
the hydrophobic site of HSA. In addition, the bands at 1246, 1291
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and 1477 cm are weak or even disappear at high concentrations
4
of S TdR (2:1, 4:1). At the concentration of 1:1, 1:2, the intensity
of the heterocyclic ring and C5-Me is slightly enhanced, it can be
interpreted that the heterocyclic ring is parallel to the Au nanopar-
ticle at high concentrations, while at lower concentration, the
plane of the heterocyclic ring is almost perpendicular to Au nano-
particle, when the automatic binding takes place. This phenome-
non may be that the angle between heterocyclic ring and C@S
stretch is related to the protein local circumstance and amino acid
4
consequence nearby. In addition, the UV spectra of S TdR–HSA
complexes are displayed in Figure 9. Some bands change evidently
at different ratios. From the Figure 10a and b, it can be found that
the multiple SERS spectra of Figure 10b show very high similarity
4
to the SERS of S TdR at different pH.
4
The SERS spectra of the 1:4 and 1:1 S TdR–HSA complexes are
4
very similar to the SERS of S TdR at alkaline pH 12.23. At the ratio
4
of 1:2, 2:1, 4:1 the SERS of S TdR–HSA complexes is very similar to
4
the SERS of S TdR at acidic pH 2.23. This phenomenon confirms
4
that the S TdR molecule has the characteristic binding to HSA
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which undergo the formation of C–S or C–O in subdomain II.
The spectra of Figure 10a and b indicate that there are two binding
4
sites between S TdR and HSA, one is the fatty acid in HSA, while
the other is defatty acid. With respect to this, it is important to
know some aspects of the binding site of protein.
It is generally accepted that the initial binding of small aromatic
20
ligands in HSA settled in subdomains II A and III A, which are
3
3
4
consistent with the I and II Sudlow sites. So, S TdR may interact
with HSA at one or two of these sites. On the other hand, HSA’s ini-
tial binding sites corresponding to fatty acid are placed in subdo-
2
0
34
main I B, II A and III B, with III A being the most important.
4
Hence, fatty acid and S TdR have a common binding site in site
II. Since the presence of fatty acid in HSA probably changes the
4
4
structure of S TdR in the S TdR–HSA complexes, it may suggest
that the initial binding site of S TdR in defatty HSA corresponds
to site II, where S TdR may interact through electrostatic interac-
4
4
tion with the basic aminoacid residues localized in the pocket en-
4
4
trance. Since the S TdR molecule undergoes the dianionic form in
Figure 10. (a) SERS spectra of S TdR (1 mM) at pH 2.23 (A), 7.09 (B), 12.23 (C); (b)
4
the complex (Fig. 10a(A) and (B), Fig. 10b(C), the intensity of the
SERS spectra in Figure 10a(A and B) is weak. This may suggest that
SERS spectra of S TdR–HSA complex at different concentration ratios in the region
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of 300–1850 cm
.