Resolving the difference in electric potential within a charged macromolecule

Article abstract of DOI:10.1021/ma302276b

The difference of the electric potential between the middle and end of polystyrenesulfonate (PSS^-) chain is discovered experimentally. Using a pH-responsive fluorophore attached to these two locations on the PSS ^- chain, the local pH value was determined by single molecule fluorescence technique: photon counting histogram (PCH). By the observation of a very high accumulation of proton (2-3 orders of magnitude in concentration) at the vicinity of the PSS^- as a result of the electrostatic attraction between the charged chain and protons, the electric potential of the PSS ^- chain is determined. A higher extent of counterion adsorption is discovered at the middle of the PSS^- chain than the chain end. The entropy effect of the counterion adsorption is also discovered - upon the dilution of protons, previously adsorbed counterions are detached from the chain.

Full text of DOI:10.1021/ma302276b

Article
pubs.acs.org/Macromolecules
Resolving the Difference in Electric Potential within a Charged
Macromolecule
Shuangjiang Luo,†,‡ Xiubo Jiang,†,‡ Lei Zou,†,‡ Fei Wang,†,‡ Jingfa Yang,† Yongming Chen,†
and Jiang Zhao*,†
†Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: The difference of the electric potential between the middle and end of
polystyrenesulfonate (PSS−) chain is discovered experimentally. Using a pH-responsive
fluorophore attached to these two locations on the PSS− chain, the local pH value was
determined by single molecule fluorescence technique: photon counting histogram (PCH). By
the observation of a very high accumulation of proton (2−3 orders of magnitude in
concentration) at the vicinity of the PSS− as a result of the electrostatic attraction between the
charged chain and protons, the electric potential of the PSS− chain is determined. A higher extent
of counterion adsorption is discovered at the middle of the PSS− chain than the chain end. The
entropy effect of the counterion adsorption is also discoveredupon the dilution of protons,
previously adsorbed counterions are detached from the chain.
density or electric potential of the charged chain molecules. In
this study, the difference of electric potential between the
middle and the chain end of a model polyelectrolyte system,
sodium polystyrenesulfonate (NaPSS), has been investigated
successfully. By positioning a pH-sensitive fluorescent molecule
with chemical linkage to the middle and the end of the NaPSS
chain and by adopting a combination of fluorescence
fluctuation spectroscopy−fluorescence correlation spectroscopy
(FCS)26−28 and photon counting histogram (PCH),29,30 the
difference in counterion distribution and the electric potential
at these positions has been investigated at single molecular
level.
INTRODUCTION
■
Polyelectrolyte is an important type of macromolecule carrying
multiple permanent ions or ionizable groups on their main
chain. The study of the physics and chemistry of polyelec-
trolytes has been attracting intensive research attention because
polyelectrolytes have broad applications in numerous fields
such as water treatment, daily life products, smart pharmaceut-
ical products, bio- and medical materials, etc. More importantly,
the physics of polyelectrolytes has been considered to be the
key knowledge for the understanding of biological processes.
Because of the long-range electrostatic interaction and the
existence of multiple counterions, polyelectrolytes exhibit
unique and complicated properties, which in turn have brought
about difficulties to the understanding of their properties.1−10
The most important aspect of polyelectrolytes is the charge
density or the electric potential of the chain as a whole or a
part, which depends not only on the original chemical structure
but also highly on the distribution of counterions.5,6 Because of
its chainlike molecular architecture, the inhomogeneity of the
counterion distribution along the polyelectrolytes molecule has
long been believed to exist. There have been studies by both
theoretical analysis and computer simulation7−9,11−19 that the
charge density at the chain end is different from that of the
middle portion of the molecular chain. Such a chain end effect
of the charged macromolecules is a very important issue,
considering the numerous processes in which the chain ends
may play very important and essential roles, such as self-
assembly of charged linear micelles,20 polyelectrolyte chains
adsorption on charged surfaces,21 polyelectrolyte brushes and
polyelectrolyte hydrogels,22,23 and DNA translocation through
protein channels,24,25 etc.
EXPERIMENTAL SECTION
■
The spatial difference in electric potential within the NaPSS molecule
is resolved by measuring the local pH value at the middle and end of
the chain, based on that protons serve as the counterions of the PSS−
chain partially. By single molecule fluorescence methods, the local pH
value was determined by measuring the fluorescence intensity of single
pH-responsive fluorophore chemically positioned at different
locations. The chemical structure of PSS− polymer and the pH-
responsive fluorescent molecule, Oregon Green 488 (OG488), are
shown in Chart 1.
The preparation of NaPSS was conducted through sulfonation of
polystyrene (PS). Two original PS samples were used: end-
functionalized and middle-functionalized PS. Amino group-terminated
PS (Mn = 120 × 103 g mol−1, Mw/Mn = 1.05) was purchased from
Polymer Source (Canada). Middle-functionalized PS (Mn = 117 × 103
g mol−1, Mw/Mn = 1.16) was home-prepared by atom transfer radical
polymerization.
Received: November 2, 2012
Revised: March 3, 2013
Published: April 4, 2013
However, up to now, there has been no direct experimental
evidence reported about the chain end effect of the charge
© 2013 American Chemical Society
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Sulfonation and Labeling. The sulfonation of both polystyrene
samples was conducted according to a published protocol,31 and
careful measurements have shown that the degree of sulfonation is
more than 96% without any breakage of backbone under optimized
experimental condition. The NaPSS polymers were later labeled with a
pH-responsive fluorescent molecule, OG488 succinimidyl ester
(chemical structure provided in Chart 1a). The labeled polymers
were purified by both ultrafiltration and dialysis, and the sample purity
was verified by careful control experiments. Control experiments also
demonstrated no change in the fluorescence properties of OG488 due
to the chemical linkage to the polymer chain (details in the Supporting
Information).32 It was also verified that OG488 itself showed no
response to the presence of NaCl in the concentration range studied.
Measuring Local pH Value in the Vicinity of a Single PSS−
Chain. Single molecule fluorescence techniquesfluorescence
correlation spectroscopy (FCS) and photon counting histogram
(PCH)were adopted to investigate the fluorescence emission
intensity of single OG488 fluorophore chemically attached to PSS−
chain. These two techniques with single molecule sensitivity were
combined together, monitoring the fluctuation of the fluorescence
intensity inside the confocal excitation-detection volume. FCS
analyzed the time-lag autocorrelation function of the fluctuation of
the photon count and determined the diffusion coefficient (therefore
the hydrodynamic radius), while PCH analyzed the deviation of the
fluorescence photon counts from the standard Poisson distribution
due to the fluctuation (the resulted super-Poisson distribution) and
determined the fluorescence emission from single fluorophore within
unit time, named the “brightness”. The principles of these two well-
established methods are detailed in a number of publications.26−30 The
experimental setup in this study was a home-built one based on the
platform of an inverted microscopy (IX-71, Olympus, Japan) with a
confocal geometry.33−37 The 488 nm output of a solid state laser
served as the excitation light source and was introduced into the
sample solution through a water-immersion objective lens (Plan
Apochromat 60×, numerical aperture = 1.2). The excited fluorescence
was collected by the same objective lens. After passing a confocal
pinhole with a diameter of 50 μm, the fluorescence was split into two
parts with identical intensity and detected separately by two
photomultiplier tube-based single photon counting module (H-7421,
Hamamatsu, Japan). The single photon counting signal was recorded
by a FCS-PCH data acquisition board (ISS, USA), and the data
analysis was conducted by its software.
Chart 1. Chemical Structure of the pH-Responsive Oregon
Green 488 (OG488), Succinimidyl Ester (a), Amino-
Terminated Polystyrene Sulfonate (b), and Middle-
Functionalized Polystyrene Sulfonate (c)
Synthesis of Polystyrene Middle-Functionalized by an
Amino Group. The synthesis of the polystyrene functionalized at
the middle by an amino group was conducted by atom transfer radical
polymerization (ATRP), as shown in Scheme 1. The initiator (1) is
first synthesized by the following protocol: 1,3,5-tris(bromomethyl)-
2,4,6-trimethylbenzene (2.0 g, 5.0 mmol) and sodium azide (0.26 g,
4.0 mmol) were dissolved in anhydrous dimethylformamide (30 mL).
After stirring at 50 °C for 24 h, the solvent was evaporated and the
residue was partitioned between dichloromethane (200 mL) and water
(50 mL). The organic phase was washed with brine three times and
dried over MgSO4. The product was purified by flash column
chromatography (silica gel, petroleum ether/dichloromethane 3:1).
Polystyrene (2) was synthesized via ATRP of styrene with 1. A
mixture of styrene (13.83 g, 0.133 mol), N,N,N′,N″,N″-pentam-
ethyldiethylenetriamine (57.6 mg, 0.33 mmol), and 1 (20 mg, 0.055
mmol) was introduced into a polymerization tube with a magnetic
stirrer and degassed by three freeze−pump−thaw cycles. The tube was
then filled with nitrogen, after which CuCl (21.9 mg, 0.22 mmol) was
quickly added into the frozen mixture. With the flask sealed, the
mixture was evacuated and backfilled with nitrogen three times. The
tube was heated to 80 °C in an oil bath. After stirring for 80 h, the
polymerization was terminated by exposing the mixture to air and
diluted with tetrahydrofuran (THF). The solution was passed through
a column filled with basic alumina, and the polymer was precipitated in
a large amount of methanol for three times and dried in a vacuum at
50 °C, producing the solid of 2.
OG488-labeled PSS− samples were dissolved in deionized water
(18.2 MΩ·cm−1) at a concentration of 5 × 10−9 M. The pH value of
the solution (monitored by a pH meter) was controlled by adjusting
the concentration of HCl or NaOH, instead of using a buffer solution
in order to keep the level of added salt as low as possible. The resulting
concentration of added sodium ions (Na+) in the PSS− solution is
from 10−5 to 10−4 M. In order to suppress the possible contamination
by dust and, more importantly, to avoid the fast uptake of carbon
dioxide in the ambient condition, the sample solution was kept inside a
sealed sample cell. The experiments were conducted at 25 °C.
After three vacuum/H2 cycles to remove air from the reaction tube,
a mixture of 2 (200 mg) and 10% Pd/C (20 mg, 10% of the weight of
the substrate) in THF (5 mL) was stirred under a hydrogen
atmosphere at room temperature for 48 h. The reaction mixture was
filtered using a sintered filter funnel. After the filtrate was
concentrated, the polymer was precipitated in a large amount of
methanol and dried in a vacuum at 50 °C to give 3 (Figure S2 in the
Supporting Information).
Scheme 1. Chemical Protocol of the Synthesis of Polystyrene Functionalized at the Middle of the Chain by an Amino Group
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pH value in bulk solution, the local pH value at the vicinity of
PSS− chain is considerably lower by 2−3 pH units,
corresponding to a difference in H+ concentration by 2−3
orders of magnitude. This fact indicates the existence of the
highly concentrated counterions near the charged chain
because of the fact that H+ constitutes partially the counterions
due to its exchanging process with original counterion (Na+).
The dynamic exchange between counterions has been
discovered by a previous study37 and was also proved by a
control experiment here, which, by spectral characterization,
showed that Na+ can be replaced by H+ at prolonged dialysis of
NaPSS in pure water. Therefore, the above result clearly shows
a much denser distribution of counterions surrounding the
PSS− chain as a result of the electrostatic attraction between the
charged chain and the counterions. This observation agrees well
with the model of counterion adsorption6 and also with a
previous experimental study with a different system,35 in which
case the counterions form a cloudlike distribution around the
charged chain. (2) The local pH value at the middle of PSS−
chain is always lower than that at the chain endabout 1.7
times (at pH 6.8, for example) difference in concentration
(Figure 1b), exhibiting a higher counterion concentration in the
middle of the PSS− chain than at the end. Such a difference
decreases continuously when the pH value is further raised, and
the ratio dropped to 1.2 at pH 8.5.
RESULTS AND DISCUSSION
■
Because of the electrostatic attraction between the PSS− chain
and protons (H+), the local concentration of H+ serves as the
probe for the counterion distribution of PSS− chain. The local
proton concentration was determined by measuring the local
pH value through the fluorescence brightness of the pH-
responsive OG488 attached to the PSS− chain. By taking the
response curve of the brightness of free OG488 to the pH value
of the solution as the master curve (inset of Figure 1), the local
pH value of the PSS− chain was measured.
Figure 1a shows the local pH value at the end and the middle
of PSS− chain as a function of pH value of the solution, in
which the local pH value at the vicinity of the PSS− chain (at
the end and the middle) increases with the pH in the solution.
Two features are immediately noticed: (1) Compared with the
The electric potential of the middle and the end of the PSS−
chain is determined by analyzing the distribution of protons
using the universal Boltzmann distribution profile: [H+]local
=
[H+]bulk exp(−eψ/kBT), where [H+]local and [H+]bulk denote
concentration of H+ ions at the local vicinity of PSS− chain and
in the bulk solution, e the element charge, ψ the electric
potential, kB the Boltzmann constant, and T the absolute
temperature. This relation leads to pHlocal = pHbulk
+
0.43(eψ/kBT), where pHlocal and pHbulk denote the local pH
value at the PSS− chain and that in the bulk solution. The data
of the electric potential at the middle and the end of PSS− chain
as a function of pH value are presented in Figure 2, which
provides a clear evidence of the chain-end effect of this charged
macromolecule. The absolute value of the electric potential is
higher for the middle of the PSS− chain (|ψmiddle|) than that at
Figure 1. (a) Local pH value at the middle and the end of the PSS−
chain as a function of the pH value in the bulk solution, as determined
by PCH measurements. The dashed lines denote the fitting by
Boltzmann distribution function. Inset: pH dependence of the single
molecule brightness of OG488 linked to the middle and the end of
PSS− chain. The response curve of free OG488 (black data point) is
displayed as the master curve for local pH determination. (b) Ratio of
proton concentration (β) the middle over that of the end of the PSS−
chain.
Figure 2. Electric potential (ψ) at the middle and the end of a single
PSS− chain as a function of pH value of the solution.
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the chain end (|ψend|). As an example, at pH 6.8, the value of
|ψmiddle| is slightly below 0.16 V and that of |ψend| is 0.14 V.
The inhomogeneity of the counterion distribution of the
PSS− chain is clearly exposedthe counterion concentration is
higher at the middle than that at the end of the chain. This
difference originates from the original topological configuration,
which generates the basic difference in the electric field at the
chaina counterion at the middle experiences the attraction
from both sides of the chain along the dimension of the
backbone while the one at the chain end only experiences
attraction from one half. From another point of view, this is
attributed to the probability for the chain end to be located
with respect to the random coil of the polymer chainthe
chain end has a higher probability to reside at the outer portion
of the random coil of the PSS− chain, where a lower
concentration of counterions is found.38
The absolute values of electric potential of both locations
remain constant below pH 7.0 but increase with the further
elevation of pH value in the solution (Figure 2). Correspond-
ingly, these results show that the charge density of the PSS−
chain is constant below pH 7.0 while the charge density
increases beyond pH 7.0. This is a clear indication of the
entropy effect to the counterion distribution. With the decrease
of proton concentration (for pH < 7.0), the cloud of protons
surrounding the PSS− chain expands at first, resulting in the
decrease of the local proton concentration without affecting the
charge density of the PSS− chain. This is further evidenced by
the good fitting by Boltzmann distribution function (denoted
by the dashed lines in Figure 1a). This observation also agrees
well with a previous observation with a different system.35
However, when the pH value is further increased, the dilution
of protons makes the previously adsorbed (condensed) protons
on the PSS− chain be released and detached from the chain.
The consequence of this process is the increase in the charge
density of the PSS− chain as demonstrated by the elevation of
the amplitude of the electric potential. Meanwhile, with the
further dilution of protons, more counterions are detached
from the chain end than the middle of the chain, making the
charge density or the electric potential increased more than the
middle. This therefore reduces the difference in charge density
between the middle and the end, evidenced by the reduction of
the difference between |ψmiddle| and |ψend|it drops from 0.02 V
at pH 6.8 to 0.004 V at pH 8.5.
The validity of the determination of the local pH values by
PCH method is proved by a number of control experiments.
The pH response of OG488 depends on its protonation
process, and its spectrum changes continuously with the change
of pH values.39,40 As a consequence, the fluorescence intensity
of the fluorophore increase continuously with the elevation of
pH value around it. The advantage of the high sensitivity of the
PCH method allows measurements of fluorescence intensity of
the fluorophore (the brightness) at a single molecular level.
Therefore, it is critical to prove that the chemical linkage to the
polymer chain does not affect the properties of the pH-
responsive fluorophore, for example, the pKa value. This is done
by the following control experiments which are all detailed in
the Supporting Information. (1) The pH response of the
OG488 with the chemical linkage to a monomer (styrenesul-
fonate) is identical to that of the free OG488, as measured by
PCH. This demonstrates that local chemical linkage does not
affect the fluorophore. This can be easily understood by the
existence of the aliphatic spacer between the fluorophore and
the polymer chain. This is further verified by the fact that the
identical fluorescence emission spectra of the OG488 with and
without chemical linkage to the PSS− chain were recorded at
identical pH local values, for both end- and middle-labeling. (2)
The pH response of OG488 with linkage to a neutral polymer,
poly(N-isopropylacrylamide), is identical to that of the free
OG488, indicating the dominant role of the electrostatic
interaction in the effect with PSS− chain.
Separate measurements by using another pH-responsive
fluorophore, Oregon Green 514 (OG514), also exhibit very
similar results. (For clarity, all the details of the following
measurements are provided in the Supporting Information.) By
attaching OG514 to the middle and the end of the same PSS−
chain, the difference in electric potential between these two
locations is clearly exposed by PCH measurements. Within
about 10% offset in the electric potential value compared with
what were determined by OG488, the difference between the
end and the middle is obvious at a very similar level.
Meanwhile, by taking the advantage of OG514 being a
ratiometric pH indicator, its steady fluorescence excitation
spectra with and without conjugation with the PSS− chain were
measured, and the results clearly exposed the huge difference in
proton concentration near the PSS− chain and the solution as
well as the difference between the middle and the end of the
PSS− chain. Compared with what observed by the PCH
method, an effect by the increased concentration of PSS− chain
showed up, as discussed later in the text. Besides, another set of
separate measurements of quantum yield of the OG488 with
and without the conjugation to PSS− chain were conducted,
and the results again show very similar results compared with
those by the PCH method. Still, due to the lower sensitivity of
the ordinary spectrometer, a concentration effect showed up
compared with the single molecular PCH measurements. A
discussion on concentration effect is provided later in the text.
Several other issues have been considered: (1) The concern
of why the sodium ions do not fill the vacancies left by the
detachment of the protons with the presence of 10−4−10−5
M
concentration of Na+. This is attributed to the much weaker
binding of Na+ to the PSS− chain than protons.8 The weaker
binding of Na+ with PSS− chain than H+ is evidenced by a
control experiment in which the local pH change of PSS− chain
is monitored under an increase of Na+ concentration. Only a
minor increase of the local pH value (∼0.2) was observed by a
change of NaCl concentration of 10−4 M. This is in a sharp
contrast with the effect by proton dilutiona change of 10−6 M
of protons induced a change of local pH value of 1.0. A detailed
description is provided in the Supporting Information. (2) The
possible effect of polymer concentration. The concentration of
NaPSS is kept constant (5.0 × 10−9 M) during the PCH
measurements, and no concentration effect is supposed to
affect the results. It should be interesting to investigate the
change of counterion distribution within one PSS− chain at
different polymer concentration because this difference may
disappear when the counterion clouds of different molecules
overlap. Our control experiments by addition of unlabeled
NaPSS in the solution failed to provide sufficient information
on this. The results showed that the effect by polymer
concentration appeared at 0.1 mg mL−1, but only the gradual
replacement of H+ by Na+ was observed. However, a slight
reduction of the difference of local pH at the middle and end of
the chain was observed. Details are provided in the Supporting
Information. (3) The difference of the dielectric constant of the
medium in the center and at the outer rim of the random coil of
the PSS− chain.41 With the presence of electrostatic interaction
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CONCLUSIONS
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In summary, the direct experimental evidence on the difference
of counterion distribution and the electric potential between
the middle and the end of PSS− chain is discovered. Because of
electrostatic attraction between the charged chain and counter-
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ASSOCIATED CONTENT
* Supporting Information
■
S
(27) Sukhishvili, S. A.; Chen, Y.; Muller, J. D.; Gratton, E.; Schweizer,
̈
Detailed information on the data analysis of single-molecule
PCH experiments (Figure S1), additional figures about SEC
(Figure S2), FCS (Figure S3), influence of labeling on the
fluorescence properties of OG488 (Figures S4−S6), the
measurements by steady fluorescence spectroscopy as well as
the by using OG514 (Figures S7−S9), the effect of sodium ions
(Figure S10), and effect of polyelectrolyte concentration
(Figure S11). This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jzhao@iccas.ac.cn.
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research is partially supported by the National Natural
Science Foundation of China (NSFC 20874108, 20925416,
51173197) and Chinese Academy of Sciences (KJCX2-YW-
H19, KJCX2-EW-W09). We thank Prof. Pengfei Wang of
Technical Institute of Physics and Chemistry for the help in
quantum yield measurements.
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115−125.
3136
dx.doi.org/10.1021/ma302276b | Macromolecules 2013, 46, 3132−3136