1H MAS NMR study of cysteine-coated gold nanoparticles

Article abstract of DOI:10.1021/jp3011298

¹H MAS NMR experiments were performed on gold nanoparticles coated with l-cysteine. The experiments show that l-cysteine molecules are zwitterions and support a structural model of cysteine forming two layers. The inner layer is composed of cysteine molecules chemisorbed to the gold surface via the sulfur atom. The outer layer interacts with the chemisorbed layer. The ¹H NMR suggests that the cysteine in the outer layer exhibits large amplitude motion about specific carbon-carbon bonds.

Full text of DOI:10.1021/jp3011298

Article
pubs.acs.org/JPCB
1H MAS NMR Study of Cysteine-Coated Gold Nanoparticles
Anuji Abraham,† Andrew J. Ilott,‡ Joel Miller,§ and Terry Gullion*,†
†Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
‡Department of Chemistry, SUNY Stony Brook, Stony Brook, New York 11794, United States
§Chemistry Division, Naval Research Laboratory, Code 6122, Washington, DC 20375, United States
S
* Supporting Information
1
ABSTRACT: H MAS NMR experiments were performed on gold
nanoparticles coated with L-cysteine. The experiments show that L-
cysteine molecules are zwitterions and support a structural model of
cysteine forming two layers. The inner layer is composed of cysteine
molecules chemisorbed to the gold surface via the sulfur atom. The
1
outer layer interacts with the chemisorbed layer. The H NMR
suggests that the cysteine in the outer layer exhibits large amplitude
motion about specific carbon−carbon bonds.
protonated thiol groups oriented away from the gold surface.
This NMR-based structural model of nanoparticles is in accord
with structures put forth by metastable deexcitation spectros-
copy8 and X-ray photoelectron spectroscopy10 studies of
cysteine deposited on well-defined gold surfaces under
ultrahigh vacuum conditions.
INTRODUCTION
■
Cysteine can bind to gold via the thiol group, with concomitant
loss of the −SH proton, either in its molecular form or as part
of a peptide.1,2 Adsorption of L-cysteine onto gold surfaces
under various conditions has been studied using a variety of
techniques, and different adsorption modes are observed in the
solid phase. Dimers, self-assembled monolayers and double
layer features have been observed under ultrahigh vacuum
conditions using well-defined Au (111) and Au (110)
surfaces.3−10 Likewise, monolayers have been observed and
double layer models proposed for samples using solution phase
adsorption of cysteine on gold electrodes and surfaces.11−16
Only a few of the aforementioned results report double layer
coverage of the gold surfaces,8,9,12,13 and none of the
experiments were on gold nanoparticles. Since a wide variety
of cysteine structures have been observed on well-defined gold
surfaces, and because of the current interest in biological
applications of nanoparticles, then it is desirable to study the
structure of cysteine and other biological molecules on gold
nanoparticles.
The evidence for two species of cysteine in the gold
nanoparticle system is from the observed 13C and 15N isotropic
chemical shift positions in MAS NMR experiments and from
thermogravimetric experiments.17 A pair of 13C resonances for
each of the C2 and C3 carbons is observed. Each C2 and C3
carbon has a 13C resonance located near their respective
positions observed for polycrystalline cysteine and a resonance
deshielded by approximately 12 ppm. The set of deshielded
resonances is attributed to chemisorbed cysteine molecules, and
the other set of resonances is assigned to cysteine molecules
forming an outer layer to the chemisorbed layer. Contrarily,
only a single 13C resonance is observed for the C1 carbon, and it
is located near the position observed for crystalline cysteine. A
single 15N resonance is observed for cysteine-coated gold
nanoparticles, and its position is shifted only slightly from that
observed for crystalline cysteine. Because the positions of the
13C resonance of the C1 carbon and the 15N resonance are
similar to their observed positions for crystalline cysteine, it
appears that the amino and carboxylate groups do not interact
significantly with the gold surface. Thermogravimetric (TGA)
experiments also show the presence of two types of cysteine,
with one type coming off at 110 °C and the other type coming
Recent 13C and 15N MAS NMR (magic-angle spinning
nuclear magnetic resonance) experiments at ambient con-
ditions on gold nanoparticles coated with uniformly 13C and
15N labeled L-cysteine show the presence of two types of
cysteine for solid phase samples.17 A structural model of the
system from the NMR study shows that one type of cysteine
forms a layer chemisorbed to the gold surface via the sulfur
atom and the other type of cysteine forms an outer layer to the
chemisorbed layer. Stabilization of the two-layer structure
presumably occurs via intermolecular hydrogen bonding
between charged amino and carboxylate groups of the
zwitterions. In this model the outer layer would have
Received: February 3, 2012
Revised: June 15, 2012
Published: June 18, 2012
© 2012 American Chemical Society
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off at 280 °C.17 The TGA result shows the amounts of the two
types of cysteine are close to being equal and is suggestive of a
bilayer.
1H MAS NMR experiments performed on polycrystalline
cysteine and on solid-phase cysteine-gold nanoparticles are
described in this paper. The 1H NMR results support a
structural model having two layers of cysteine coating the gold
and provide additional details about molecular dynamics of the
cysteine in the outer layer that were not evident in an earlier
13C NMR study. A 1H resonance for the thiol proton is
observed in the cysteine-gold nanoparticle sample, providing
direct evidence that not all cysteine molecules are chemisorbed.
1
The H spectrum of the gold nanoparticle sample shows that
the cysteine molecules are in zwitterionic form. In addition, the
cysteine-gold nanoparticle system showed two sets of proton
resonances; one set consisted of broad resonances and the
other set was made of sharp resonances. We postulate that the
set containing the broad 1H resonances arises from
chemisorbed cysteine molecules, and the set containing the
1
sharp H resonances comes from an outer layer of cysteine.
1
Furthermore, the set of sharp H NMR resonances provides
evidence of large amplitude motions about specific carbon−
carbon bonds of molecules in the outer layer.
EXPERIMENTAL SECTION
■
Sample Preparation. Gold nanoparticles were coated with
L-cysteine by a previously described method that produced 6.6
2.7 nm diameter particles.17 Cysteine-coated gold nano-
particle samples were prepared with [1-13C, 99 at. %]L-cysteine
and are referred to as 13C-CysAu. The 13C labeled cysteine was
purchased from Cambridge Isotope Laboratories, Inc. Briefly,
120 mL of 0.5 mM gold(III) chloride trihydrate solution was
reduced with 0.015 g of sodium borohydride to produce a ruby
red solution of gold nanoparticles. A total of 400 mL of 1 mM
isotopically labeled L-cysteine solution was added to the gold
nanoparticle solution. The solution was stirred for 30 min and
then left standing overnight. The cysteine-coated nanoparticles
were separated from solution by centrifugation at 20 000g for
30 min. The supernatant was removed and the nanoparticles
were washed several times with water and then dried overnight
at 333 K. Sample weights for the NMR measurements were
approximately 1.5 mg.
2
2
Figure 1. H MAS NMR spectrum of H-Cys is shown in the top
spectrum. An expansion of the centerband of the 2H MAS NMR
spectrum with peak assignments is shown in the middle spectrum. The
1H MAS NMR spectrum of 2H-Cys is shown in the bottom spectrum
along with peak assignments. Sixty-four transients were taken for each
of the 2H and H spectra. All spectra were obtained with a MAS
spinning speed of 18 kHz.
1
In addition to the gold nanoparticle samples, solid-state
NMR experiments were performed on 13C labeled cysteine and
on 2H labeled cysteine. [1-13C, 99 at. %] L-cysteine was
recrystallized from water and is referred to as 13C-Cys. [3,3′-2H,
98 at. %] L-cysteine was purchased from Cambridge Isotope
Laboratories, Inc. and recrystallized from D2O having a small
1
Figure 2. H MAS NMR spectrum of 13C-Cys is shown in the top
+
spectrum. The 1H resonances for the −NH3 , −CH, and −CH2
2
protons appear at 8.7, 4.3, and 2.0 ppm, respectively. The −SH proton
amount of added HCl and is named H-Cys.
1
resonance is part of the 2.0 ppm peak. H MAS NMR spectrum of
Solid-State NMR. Two solid-state NMR spectrometers
were used to generate the results presented in this paper.
The 1H and 2H NMR spectra in Figure 1 were obtained on a
9.4 T Varian Inova spectrometer with a proton frequency of
399.89 MHz using a 3.2 mm Varian/Chemagnetics T3 solid-
13C-CysAu is shown in the bottom spectrum along with peak
assignments. The broad resonances centered at 7.6 ppm, 4.3 ppm and
+
2.0 ppm are from the −NH3 , −CH, and −CH2 groups, respectively.
The sharp 1H resonances at 2.1 ppm and 2.5 ppm are from the −CH2
protons. The sharp 1H resonance at 1.9 ppm is from the −SH protons,
and the sharp resonance at 5.5 ppm is from water. Sixteen and 385
transients were obtained for the 13C-Cys and 13C-CysAu spectra at
MAS speeds of 60 and 53 kHz, respectively.
1
2
state probe-head. The H and H 90° pulse lengths were 5.0
and 4.0 μs, respectively. Recycle delay times were 10 s. The
spectrometer was located at the State University of New York
at Stony Brook.
The spectra in Figures 2 and 4 were obtained on a Varian
spectrometer located at the Naval Research Laboratory
(Washington, DC). The proton operating frequency was
500.1765 MHz. A Varian 1.2 mm double-resonance solid-
were 0.76 μs and 1.0 μs, respectively. A recycle delay of 6 s was
used for both 1H and {1H}13C (CP) HETCOR (Heteronuclear
Correlation) experiments. The 1H−13C cross-polarization (CP)
time for the {1H}13C (CP) HETCOR experiment was set to
400 μs in order to correlate (proton-carbon) only with the
1
state NMR probe was used. The H and 13C 90° pulse lengths
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closest protons. Frequency switched Lee−Goldburg (FSLG)
decoupling18 was used on the proton channel during the t1
(1H) evolution time and heteronuclear TPPM decoupling was
used on the 13C channel during the data acquisition time.19
Table 1. Approximate Full-Width-Half-Maximum Values of
1H Resonances, in Hz
+
sample\functional group
−NH3
−CH
−CH2
−SH
H2O
13C-Cys
13C-CysAu (broad)
13C-CysAu (sharp)
800
1000
2000
630
RESULTS AND DISCUSSION
1960
■
1H and H MAS NMR Spectroscopy of H-Cys. The H
resonance of the thiol proton was not uniquely identifiable in
the spectrum of 13C-Cys (see Figure 2). Since the two-layer
model for cysteine-coated gold nanoparticles suggested that
thiol protons would be present in the outer layer, it was
deemed important to clearly determine where thiol protons
would contribute in the 1H MAS NMR spectrum. Con-
2
2
1
70
100
80
sharp resonances) is assigned to C3 protons. The sharp set of
1
resonances observed in the H spectrum of 13C-CysAu have
linewidths an order of magnitude smaller than the proton
resonances observed in 13C-Cys (see Table 1). The pair of
intense sharp peaks at 2.1 ppm and 2.5 ppm (frequency
difference of 200 Hz) is assigned to the C3 protons. The C3
protons are directly bonded to a 13C nucleus (only the C3
2
sequently, H-Cys was examined with the assumption that the
1
C3 protons and thiol protons have H NMR resonances that
1
carbon is 13C labeled), and a solution H NMR spectrum (see
either overlap or are very close to one another. Deuteration of
1
1
Figure S1 in the Supporting Information) showed a H−13C
the C3 carbon eliminated the contribution to the H spectrum
of the C3 protons. 2H-Cys was recrystallized from D2O that had
a small amount (nonstoichiometric drop) of HCl; conse-
quently, the amino and thiol groups would at least be partially
deuterated. The acquired 2H MAS NMR spectrum is shown in
Figure 1 (top). An expansion of the centerband is shown in
Figure 1 (middle) and clearly shows the presence of −CD2 and
scalar coupling of 144 Hz between the C3 protons and the 13
C
spin at the C3 carbon position. Accordingly, this pair of peaks is
1
the result of the H−13C scalar coupling and is assigned to the
C3 protons. The sharp peak at 1.9 ppm is from thiol protons;
1
2
this assignment is supported by the H spectrum of H-Cys in
1
Figure 1. The contribution of the thiol protons to the H
+
−ND3 deuterons. In addition, there is a less intense 2H
spectrum provides strong support for the cysteine bilayer
resonance located where a hydroxyl deuteron would be
expected to appear.
model. The intensity of the contribution made by thiol protons
1
to the H NMR spectrum (assumed to made of the features at
Crystalline cysteine occurs in several forms. When recrystal-
lized from water, cysteine appears in the zwitterion form.
However, hydroxyl protons are present when cysteine is
recrystallized as an HCl salt. Hence, the small nonstoichio-
metric amount of added HCl during recrystallization produced
a sample that contains crystals with zwitterions and other
crystals (a smaller fraction) with cysteine molecules having
hydroxyl groups. Since the deuterated sample was recrystallized
from D2O, it was expected that the thiol groups would be at
least partially deuterated. The expected deuterium resonance of
the thiol deuteron is not clearly identifiable in the 2H spectrum,
but it may overlap with the −CD2 deuterium resonance.
1.9 ppm and lower) is approximately half the intensity
contributed by the two sharp C3 proton resonances. This 1:2
ratio of signal intensities is consistent with two −CH2 protons
contributing to the spectrum for every one -SH proton. Hence,
the set of sharp resonances is from cysteine molecules forming
the outer layer of the bilayer system since only that layer can
have thiol protons. The absence of a 1H resonance in the region
of the spectrum where hydroxyl protons would contribute
(around 19 ppm) indicates that cysteine molecules in the gold
nanoparticle system exist in the zwitterion form, which has rich
hydrogen bonding possibilities.
The sharp peak at 5.5 ppm is assigned to water. The position
+
1
Intense peaks from −CH, −NH3 , and −SH protons
of the peak is consistent with a H NMR signal coming from
1
2
1
appeared in the H spectrum of H-Cys (Figure 1, bottom).
A less intense peak appeared at 19 ppm, consistent with
contributions from hydroxyl protons. The sought after 1H
resonance from the thiol proton became evident because the
−CH2 protons were replaced with deuterons in the starting
material. The thiol proton resonance appears at 1.9 ppm in the
water. Two additional H NMR spectra (Figures S2 and S3
shown in the Supporting Information) were taken about one
year after the original data was taken on 13C-CysAu to support
the assignment of the peak at 5.5 ppm to water. Figure S2
1
shows the H spectrum of the sample after spending one year
in a sealed bottle that was also placed in a zip lock bag. The
sharp 1H resonance at 5.5 ppm is still present. After the
spectrum in Figure S2 was acquired, the sample was removed
from the spectrometer and exposed to deuterated water vapor
1
1H spectrum and makes a contribution to the H spectrum
comparable to that made by C2 protons.
1H MAS NMR Spectroscopy of 13C-Cys and 13C-CysAu.
1
1
The H MAS NMR spectrum of 13C-Cys shown in Figure 2
for two days. A H spectrum was obtained after the exposure
(top) is dominated by three broad resonances. The peak at 8.7
ppm is from the amino protons and the peak at 4.3 ppm is from
C2 protons. The C3 protons contribute to the peak centered at
and is shown in Figure S3. The intense sharp peak at 5.5 ppm
in Figure S2 is no longer present in Figure S3. This is strong
evidence that water molecules are the source of the sharp
resonance at 5.5 ppm.
1
2.0 ppm. A distinct H resonance from thiol protons is not
readily discernible in the spectrum, as addressed earlier, but is
part of the broad peak shared with the C3 protons.
A natural question is why don't the −CH protons have a
1
sharp H resonance feature. Cysteine molecules in the inner
The 1H spectrum of 13C-CysAu, obtained by a simple 90° −
acquire pulse sequence, is shown in Figure 2 (bottom). It is
layer are rigidly held in place, being anchored to the gold
surface at the sulfur end and tied down via hydrogen bonding at
the amino/carboxylate end of the molecule. The environment
of the molecules in the outer layer is not as restrictive. The
amino/carboxylate groups of these molecules are hydrogen
bonded to the inner layer. However, the sulfur end of the
molecule is free to explore a more open space. Hence, large
amplitude motions about the S−C3 and the C3−C2 bonds are
1
evident that two sets of H resonances are present. The broad
resonances are greater in width to those observed in 13C-Cys
1
(top spectrum). Approximate H linewidths for the various
samples are shown in Table 1. The broad feature at 7.6 ppm is
from amino protons and the broad feature at 4.3 ppm is from
C2 protons. The broad feature centered at 2 ppm (under the
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possible, but such motions about the C1−C2 and N−C2 bonds
are prohibited. Consequently, the −CH2 and -SH protons may
undergo large amplitude motional averaging leading to their
sharper resonances whereas motional averaging of the −CH
proton does not occur and results in only a broad resonance
feature. Figure 3 shows a model that is being proposed which
HETCOR spectrum because of the short 1H−13C cross-
polarization contact time and their proximity to the 13C label.
The asymmetric dumbbell shaped contour spectrum shows two
different populations of C3 protons are present, which suggests
that there are two different types of cysteine molecules
contributing to the spectrum. The projections of the centers
1
of the large and small lobes onto the H axis are around 2.5
ppm and 2.0 ppm, respectively. The deshielded large lobe at 2.5
ppm is associated with the chemisorbed C3* feature, and the
smaller lobe at 2.0 ppm belongs to the C3 feature. The
deshielded 13C and deshielded 1H NMR resonances are
associated with chemisorbed molecules of cysteine.
The ratio of the intensity of the 13C NMR signals from the
peaks labeled C3* and C3 is approximately 1.8. The lower
intensity of the signal from the C3 carbon (outer layer) may
1
arise from less efficient H−13C cross-polarization because of
the motional averaging of the outer layer mentioned in the
previous section.
CONCLUSIONS
■
Figure 3. Bilayer model of cysteine on gold. Bonds capable of large
amplitude motions are marked with arrows.
The 1H, 2H and {1H}13C (CP) HETCOR MAS NMR
experiments provide evidence that L-cysteine forms two layers
on gold nanoparticles. The {1H}13C (CP) HETCOR and H
1
NMR data show that two types of cysteine are present in 13C-
CysAu, as is evidenced by the double lobed HETCOR
spectrum in Figure 4 and the presence of broad and sharp
resonances in the proton spectrum in Figure 2. Furthermore,
illustrates some of the details just discussed. The arrows around
the S−C3 and C3−C2 bonds of the cysteine molecules in the
outer layer represent large amplitude motions about those
bonds.
{1H}13C HETCOR MAS NMR of 13C-CysAu. The {1H}13C
CP HETCOR MAS NMR20 spectrum of 13C-CysAu in Figure
4 shows the presence of two spectroscopically different C3
carbons, labeled C3* and C3. According to previously published
results, the deshielded feature, C3*, comes from cysteine
molecules chemisorbed to the gold surface, and the feature
labeled C3 is from cysteine molecules that form an outer layer
to the chemisorbed layer. The C3 protons dominate the
1
the H MAS NMR spectrum of 13C-CysAu clearly shows the
presence of a thiol proton and has no contribution from
hydroxyl protons. Together, the data support the picture of
cysteine zwitterions forming a two-layer coverage of the gold
nanoparticles. An inner layer is chemisorbed to the gold surface
via the sulfur atom, and a second layer is hydrogen bonded to
the chemisorbed layer. The sharp set of proton resonances
observed for the −CH2 and −SH protons of 13C-CysAu and
the lack of sharp resonances of the −CH proton in the sample
are indicative of large amplitude motions about the S−C3 and
C3−C2 bonds for cysteine in the outer layer.
Because of the expense associated with using 13C and 15N
labeled cysteine in an earlier study17 and 13C labeled cysteine in
this work, it was not possible to study the structure of cysteine
on gold as a function of pH during sample preparation. This
work shows a signature for the formation of a bilayer of cysteine
is the presence of sharp 1H resonances for the −CH2 and −SH
1
protons. Hence, high speed H MAS NMR may provide an
inexpensive way to determine the number of cysteine layers on
gold nanoparticles as a function of pH since isotopically labeled
samples would not be required.
ASSOCIATED CONTENT
■
S
* Supporting Information
Solution state 13C spectra of of 13C-Cys (Figure S1). H MAS
1
spectrum taken on 13C-CysAu approximately 6 months after
1
the spectrum in Figure 2 was taken (Figure S2). H MAS
spectrum taken on 13C-CysAu after being exposed for 2 days to
a vapor of deuterated water (Figure S3). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 4. {1H}13C HETCOR (CP) MAS NMR spectrum of 13C-
CysAu. For reference, the proton spectrum of 13C-CysAu shown in
Figure 2 is displayed along the 1H axis of the HETCOR spectrum. 128
transients, 64 t1 points and a MAS speed of 53 kHz were used for the
HETCOR experiment.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by Grant CHE-0846583 from the
National Science Foundation. We thank Dr. Novruz Akhmedov
for taking solution spectra during the course of this work.
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