Synthesis of three novel anionic gemini surfactants and comparative studies of their assemble behavior in the presence of bovine serum albumin

Article abstract of DOI:10.1021/la204212s

Three novel anionic sulfonate gemini surfactants, sodium 4,4′-(10,19-dioxo-9,11,18,20-tetraazaoctacosane-9,20-diyl) dibenzenesulfonate (Surfactant I), sodium 4,4′-(12,21-dioxo-11,13,20,22- tetraazadotriacontane-11,22-diyl) dibenzenesulfonate (Surfactant II), and sodium 4,4′-(14,23-dioxo-13,15,22,24-tetraazahezatriacontane-13,24-diyl) dibenzenesulfonate (Surfactant III), with different lengths of hydrophobic tail have been synthesized, and their assembly behavior in the presence of bovine serum albumin (BSA) has been studied using spectral methods and molecular modeling methods at physiological pH and 298 K. Critical micelle concentrations (CMCs) of the three surfactants have been determined by surface tension measurements. Despite the obvious decrease of CMC with the increase of tail length, fluorescence spectra have shown much closer CAC in the presence of BSA. Surfactant II shows the highest CAC of 3.19 × 10^-5 mol L ^-1 compared with the other two. The polarity of the microenvironment in BSA-surfactant systems has been investigated using pyrene as the probe. In addition, far-UV CD spectra studied the change of the secondary structure content of BSA caused by the three surfactants. The features of the assembly behavior were discussed by three concentration regions. Surfactant II could unfold the protein much more efficiently than the other two surfactants at low concentration, but at high concentration, the change of the secondary structure and the formation of hydrophobic microenvironment show a direct relationship to the length of the hydrophobic tail with the increase of the surfactant concentration.

Full text of DOI:10.1021/la204212s

Article
pubs.acs.org/Langmuir
Synthesis of Three Novel Anionic Gemini Surfactants and
Comparative Studies of Their Assemble Behavior in the Presence of
Bovine Serum Albumin
Yu-Shu Ge,† Shu-Xin Tai,‡ Zi-Qiang Xu,† Lu Lai,† Fang-Fang Tian,† Dong-Wei Li,† Feng-Lei Jiang,†
Yi Liu,*,† and Zhi-Nong Gao*,‡
†State Key Laboratory of Virology & Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education),
‡Department of Polymer Sciences, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China
S
* Supporting Information
ABSTRACT: Three novel anionic sulfonate gemini surfac-
tants, sodium 4,4′-(10,19-dioxo-9,11,18,20-tetraazaoctacosane-
9,20-diyl) dibenzenesulfonate (Surfactant I), sodium 4,4′-
(12,21-dioxo-11,13,20,22-tetraazadotriacontane-11,22-diyl) di-
benzenesulfonate (Surfactant II), and sodium 4,4′-(14,23-
dioxo-13,15,22,24-tetraazahezatriacontane-13,24-diyl) dibenze-
nesulfonate (Surfactant III), with different lengths of hydro-
phobic tail have been synthesized, and their assembly behavior
in the presence of bovine serum albumin (BSA) has been
studied using spectral methods and molecular modeling
methods at physiological pH and 298 K. Critical micelle concentrations (CMCs) of the three surfactants have been determined
by surface tension measurements. Despite the obvious decrease of CMC with the increase of tail length, fluorescence spectra
have shown much closer CAC in the presence of BSA. Surfactant II shows the highest CAC of 3.19 × 10−5 mol L−1 compared
with the other two. The polarity of the microenvironment in BSA−surfactant systems has been investigated using pyrene as the
probe. In addition, far-UV CD spectra studied the change of the secondary structure content of BSA caused by the three
surfactants. The features of the assembly behavior were discussed by three concentration regions. Surfactant II could unfold the
protein much more efficiently than the other two surfactants at low concentration, but at high concentration, the change of the
secondary structure and the formation of hydrophobic microenvironment show a direct relationship to the length of the
hydrophobic tail with the increase of the surfactant concentration.
yethanol (Triton X-100).11,12 The binding behavior between
surfactants and proteins usually depends on the features of
1. INTRODUCTION
Surfactants have been widely used in industrial, biological,
pharmaceutical, and cosmetic systems.1,2 Thus, great effort has
surfactant. Recently, several reports discussed the unique
behaviors of the interactions between protein and gemini
been paid to the synthesis of new surfactants during the past
few decades.3,4 Gemini surfactant, which consists of two
surfactants. Y. S. Wang et al. comparatively studied the
interactions of hemoglobin with single-chain and gemini
hydrophobic chains, two polar head groups, and spacer linker at
surfactants.13 Y. J. Li et al. reported the investigation on the
or near the head groups, has been proven to be greatly superior
interactions of bovine serum albumin with cationic gemini and
single-chain surfactants.14 D. Wu et al. reported the interaction
of bovine serum albumin with dissymmetric and symmetric
gemini surfactant by spectral method.15 These studies
contribute toward an understanding of the behavior of gemini
surfactants as denaturants and as solubilizing agents for proteins
and the influence on the assembly behavior caused by proteins.
Anionic surfactants interact strongly with proteins and cause
their denaturation. It is well-known that different chain length
usually leads to great difference in the surfactants’ proper-
ties.16,17 So, it is interesting and important to study the
to the conventional single-chain surfactant in many properties
such as lower critical micelle concentration (CMC) and better
wetting, foaming, and dispersing properties.5 Anionic gemini
surfactant (mainly sulfate and sulfonate gemini surfactants) has
played an important part in the new round of development and
application of gemini surfactants.6 Various new anionic gemini
surfactants with different hydrophobic chain length and spacer
linker have been designed and synthesized.7,8
With the development of surfactants, protein−surfactant
systems have always been studied as a more and more
important role in the application in drug delivery, cosmetics,
foods, and so on.9,10 Up to now, much investigation has been
focused on the interaction between protein and single-chain
surfactants such as cetyltrimethylammonium bromide (CTAB),
sodium dodecyl sulfate (SDS), and tert-octylphenoxypolyethox-
Received: October 27, 2011
Revised: March 7, 2012
Published: March 16, 2012
© 2012 American Chemical Society
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behaviors of gemini surfactants with different chain lengths in
the presence of serum albumin.
In this work, three novel anionic gemini surfactants with
different chain lengths were synthesized and characterized.
Their structures were given in Figure 1. We focus on the
Figure 1. Chemical structures of the synthesized anionic sulfonate
surfactants.
assembly behavior of the surfactants in the presence of BSA
using fluorescence spectroscopy, circular dichroism, and
molecular modeling methods. BSA is one of the most
extensively studied serum albumins and shares a very high
level of homology (80%) with human serum albumin, which is
an important transfer protein.18 Study on the behavior of the
surfactants in the presence of BSA can contribute to
pharmacokinetics and pharmacodynamics. For comparison,
parallel measurements have been made on the three BSA−
surfactant systems.
Figure 2. Schemes of reaction routes for the synthesis of the anionic
sulfonate gemini surfactants.
cm path-length, a step interval of 0.5 nm, and an integration time of
0.5 s. Each spectrum was corrected by PBS buffer solution (pH 7.4).
The concentration of BSA was kept at 1.0 × 10−5 mol L−1. Far-UV CD
spectra were recorded from 200 to 250 nm in 5 mM PBS buffer
solution at 298 K.
2.5. Molecular Modeling Study. The initial structure of the
surfactant molecules were generated by molecular modeling software
Sybyl 8.1 package.21 The molecules were charged using the Gasteiger
and Marsili method, and the formal charges of two O atoms of the
sulfonic groups were changed to −1 before computing the charges.
The molecules of surfactants I, II, and III were optimized for use in the
following docking study. And they had their minimized total energy of
9.77, 8.84, and 8.60 kcals mol−1, respectively.
2. MATERIALS AND METHODS
2.1. Materials. Surfactant I, surfactant II, and surfactant III were
synthesized and purified. Lauric acid (≥99%, AR), chlorosulfuric acid
(≥97%, AR), thionyl chloride (≥99%, AR), aniline (≥98%, AR), and
LiAlH4 (≥95%) were purchased from Sinopharm Chemical Reagent
Co. Ltd. 1,6-Diisocyanatohexane (≥99%) was obtained from Sigma
Chemical Company. All the other reagents were of analytical grade
and were used without further purification.
The crystal structure of BSA was modeled using Modeller_9v7.22
The sequence of the protein was from the ncbi database (http://www.
ncbi.nlm.nih.gov/Database/;ID:CAA76847). The structure was ana-
lyzed with Sybyl 8.1 software. All the side chains were repaired, the
bumps were fixed, and H atoms were added. The biopolymer was
charged using the AMBER7 FF99 method. This structure was used for
the Surflex_Dock program, which was used to compute the docking
mode of our complex with BSA.
Bovine serum albumin (BSA, fatty acid free) and Probe pyrene (for
fluorescence, ≥99.0%, GC) were purchased from Sigma Chemical
Company and were used without further purification. BSA was
prepared in 5 mmol L−1 sodium phosphate buffer of ionic strength
0.01 mol L−1 at pH 7.4. Buffer salts (AR) were purchased from
Sinopharm Chemical Reagent Co. Ltd. Water used was triple-distilled.
2.2. Synthesis and Structure Characterization. The schemes of
the synthetic routes of the anionic sulfonate surfactants are shown in
Figure 2. Both detail information about synthesis and corresponding
The AutoDock program was applied to calculate the possible
binding mode of the surfactants with the protein.23
1
spectral data including FTIR, H NMR, 13C NMR, and electrospray
ionization/mass spectrometry (ESI-MS) are given in the Supporting
Information. The structures and the purity of the surfactants were
confirmed. Plots of surface tension were shown in Figure S1. CMC (T
= 298 K) of surfactant I, surfactant II and surfactant III were
calculated, respectively, as 1.16 × 10−3 mol L−1, 1.89 × 10−4 mol L−1,
and 5.78 × 10−6 mol L−1 by measuring surface tensions using Wihelmy
plate method19 and are given in Table S1.
2.3. Measurement of Fluorescence Spectra. All fluorescence
spectra of BSA−surfactant systems were recorded on a Perkin LS-55
fluorescence spectrophotometer. The excitation wavelength is fixed at
280 nm for steady-state emission spectra of BSA. The excitation and
emission slits were set at 15.0 and 3.5 nm. The emission wavelength of
BSA is 353 nm.
Pyrene spectra were recorded with fixed excitation wavelength at
335 nm. Intensities of peaks labled as I1 and I3 were taken from the
emission intensities at 373 and 384 nm, respectively.20 The excitation
and emission slits were set at 3.0 and 3.0 nm. All measurements were
taken at 298 K.
2.4. Measurement of Circular Dichroism. Circular dichroism
spectra were recorded on a Chirascan circular dichroism spectrometer
(Applied PhotoPhysics, Britain), using a cylindrical cuvette with 0.1
3. RESULTS AND DISCUSSION
3.1. BSA-Surfactant Systems Studied by Fluorescence
Spectra. BSA has strong fluorescence due to tryptophan and
tyrosine residues. This intrinsic fluorescence shows apparent
sensitivity for the change to the microenvironment. Thus,
fluorescence analysis has been employed as an effective method
to study the interaction between small molecules and BSA.24
Figure 3 shows the effect of different concentrations of three
surfactants on the fluorescence spectra of BSA−surfactant
solutions excited at 280 nm (cBSA = 1.0 × 10−5 mol L−1). As
concentration of the surfactants increases, the fluorescence
spectra of BSA show different changes for three surfactants. For
surfactant I, the fluorescence intensity decreases rapidly at first
and then its rate slows down, while the fluorescence intensity of
BSA with the other surfactants shows a sharp decrease and then
a slight increase, all with a blue shift from 353 to 333 nm at the
maximal emission wavelength.
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Figure 3. Fluorescence spectra of BSA (10 μmol L−1) with presence of different concentration of three surfactants. Concentration of surfactant I
(10−5 mol L−1): 0; 0.2; 0.67; 1.33; 2.00; 2.67; 3.34; 4.00; 4.67; 5.34; 6.00; 6.67; 7.34; 8.67; 10.00; 11.34. Concentration of surfactant II (10−5 mol
L−1): 0; 0.63; 1.26; 1.89; 2.52; 3.15; 3.78; 4.41; 5.04; 5.67; 6.30; 7.56; 8.19. Concentration of surfactant III (10−5 mol L−1): 0; 0.24; 0.64; 1.04; 1.44;
2.04; 2.44; 2.84; 3.24; 3.64; 4.04; 4.64; 5.24; 5.84.
Figure 4. Stern−Volmer curves of the systems of surfactant/BSA.
To obtain more information about this change, the data of
the fluorescence spectra are analyzed using the well-known
Stern−Volmer eq 1:25
It is known that the surfactants can interact with amino acid
chains of the protein and cause the unfolding of the protein.26
These binding surfactants will form micelle-like structures
above the CAC.27 At first, the Trp and Tyr residues were
exposed to a more and more polar environment; thus, the
fluorescence intensity drops rapidly. With the increase of the
surfactant concentration, the residues locate in the forming
compact structures as a more nonpolar environment, and the
fluorescence intensity rises slightly. From the data shown in
Table 1, it is easy to observe the similar unfolding process of
the protein caused by the three surfactants. Figure 5 has
provided a better vision of the similarity from the perspective of
relative fluorescence intensity I/I0. Here, I0 is the fluorescence
intensity of the BSA solution without any surfactants, and I is
the intensity of the mixed solution of surfactants and BSA.
The key point is the different changes caused by the different
length of hydrophobic tails. It is well-known that, the longer the
hydrophobic tail is, the stronger the ability it has in micelle
formation.28 Thus, surfactant III with the longest tail shows the
lowest CAC of 2.70 × 10−5 mol L−1, and surfactant I with the
shortest tail gives the smallest Ksv; meanwhile, it also cannot
affect the amino acid chains effectively enough to change the
polar environment around the intrinsic fluorescence residues
while the other two can achieve this absolutely. However, it is
interesting to observe that surfactant II instead of surfactant I
(with the shortest tail) shows the highest CAC and surfactant
F
F
0
= 1 + K [Q]
SV
(1)
where F0 and F are the fluorescence intensities in the absence
and presence of the quencher, Ksv is the Stern−Volmer
quenching constant, and [Q] is the concentration of the
quencher. Figure 4 gives the Stern−Volmer curves of the three
BSA−surfactant systems. In order to clarify the difference
among the three systems, the quenching constant Ksv and the
critical concentration of aggregation (CAC) which can be
obtained by calculating the intersection point of two curves
with different slope are shown in Table 1.
Table 1. Ksv and CAC of the Systems of the Gemini
Surfactants and BSA
10−4 KSV
10−4 KSV
(2)/(L·mol
gemini
surfactants
(1)/
CAC/(μ
−1
(L·mol−1
)
R1
)
R2
mol L−1
)
Surfactant I
Surfactant II
Surfactant III
4.48
7.67
5.93
0.98
0.99
0.99
6.04
−1.34
−0.49
0.99
0.99
0.94
29.7
31.9
27.0
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Figure 6. Binding isotherm of the surfactants onto BSA (cBSA = 10
μmol L−1).
Figure 5. Relative fluorescence intensity I/I0 as a function of surfactant
concentration (cBSA = 10 μmol L−1).
where α is the fraction of BSA bound to surfactant, Cs is total
surfactant concentration, and Cp is total protein concentration.
α was evaluated from the fluorescence intensity by eq 3:31
III (with the longest tail) shows a largest Ksv either below or
above the CAC point. It reveals that surfactant II can affect the
environment around the intrinsic fluorescence residues more
effectively than the other two, while the greatest difficulty in
forming a micelle-like structure on the unfolded BSA chains
occurs. The reason for this “abnormality” may be the
complicated factors involved in the process of surfactant
assembly in the presence of BSA. The formation of the micelle-
like structures among the compact amino acid chains is not the
simple aggregation that the surfactants usually show in a pure
water environment.29
In the presence of the serum albumin, the anionic head of the
sulfonate gemini surfactants mainly contributes similar
relevance to the interaction and the assembly. However, the
length of the hydrophobic tails can have a different influence on
the process of binding and aggregation of the surfactants. At the
initial concentration below CAC, the surfactants prefer specific
binding to the protein and change the structure of the amide
acid chains with their hydrophobic tails at the same time. As the
intrinsic fluorescence residues located inside the hydrophobic
cavity rather than the surface of the protein,30 the shorter the
tail is, the easier for the surfactant molecular to approach the
residues inside the cavity and influence the intrinsic
fluorescence residues more easily. However, the longer the
tail is, the more powerful the surfactant change to the protein
structure. It is just like two contradictive characters expressed
on one molecule: as a molecule with a long tail, the power of
changing the chain structure increases the quenching constant
while the powerlessness of entering the cavity and binding the
residues directly is adverse to the constant; and the distribution
of these two characters for the molecule with a short tail is just
opposite. Thus, the balance of two competing powers leads to
surfactant II showing the highest Ksv and the highest CAC.
To get a better insight into this process, the binding isotherm
of the surfactant on BSA is given in Figure 6. The average
number ν of surfactant molecules bound to a BSA molecule was
estimated by eq 2:31
α = (I − I )/(I − I )
min
(3)
0
0
Three characteristic regions are shown in Figure 6: (i)
specific binding, (ii) noncooperative binding, and (iii)
cooperative binding.31 When Cs ≤ 4.4 × 10−6 mol L−1, the
average number ν of surfactant molecules bound to a BSA
molecule seldom shows any increase. When 4.4 × 10−6 mol L−1
≤ Cs ≤ 2.0 × 10−5 mol L−1, ν shows a slight but clear increase
with the specific binding to the protein. When Cs ≥ 2.0 × 10−5
mol L−1, large numbers of surfactant molecules bind to the
protein due to the formation of the micelle-like structures.
3.2. Micropolarity Study. The microenvironmental polar-
ity of the solution can be detected by the fluorescence probe
pyrene. The fluorescence spectra of pyrene were measured, and
the intensity ratio of the first and third major vibrational peaks
(I1/I3) was calculated.32 A lower I1/I3 value stands for higher
hydrophobicity of the microenvironment. The changes of the
I1/I3 values of three surfactant solutions at different
concentrations in the absence and presence of BSA were
recorded and shown in Figure 7.
In the absence of BSA, the curves show a sharp decrease with
the addition of the surfactants, which indicates the formation of
hydrophobic microdomains. At high concentrations, surfactant
III solution showed the lowest I1/I3 value finally, suggesting the
decrease of the polar microenvironment with the increase of
the surfactant tail length. In the presence of BSA, the I1/I3 value
of the solution starts from a much lower point because of the
hydrophobic sites, and gets close to the pure surfactant solution
at high concentration. Also, the polarity of the microenviron-
ment of three surfactant solutions displayed an increasing trend
at the initial point, which is the result of the unfolding of the
protein caused by the surfactant, while at high concentration,
numerous micelle-like structures formed. As a result, the
polarity basically remained unchanging or even showed a slight
decrease. BSA solution with surfactant III shows the lowest I1/
I3 value due to the strong interaction of the long tail with the
protein. It is interesting to observe an obviously low I1/I3 value
of the BSA−surfactant II solution at an intermediate value of
concentration, which is even lower than that of the BSA−
surfactant III solution. The low polarity value indicates a very
ν = α × C /C
s
p
(2)
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Figure 7. Dependence of I1/I3 value on surfactant concentration in different solutions. Cpy = 2 × 10−7 mol L−1, CBSA = 1 × 10−5 mol L−1.
Figure 8. Circular dichroism (CD) spectra of native BSA and of BSA treated with surfactants.
strong hydrophobic microenvironment. The reason might be
the opposite influence of the tail length on the process of
binding and aggregation of the surfactants as mentioned in the
above paragraph. At about (3−4) × 10−5 mol L−1, surfactant II
could unfold the protein much more efficiently than that of the
other two surfactants, while at lower or higher concentration,
surfactant III shows the largest efficiency on the formation of
the hydrophobic microdomain due to the strong ability of
micelle formation for the surfactants with a long tail.
3.3. Circular Dichroism Spectra Study of the Secon-
dary Structure. Circular dichroism measurement was used to
monitor the changes of the secondary structure of BSA caused
by the surfactants.33 Figure 8 shows the typical far-UV CD
spectra of the native BSA in the absence and in the presence of
three surfactants at different concentrations.
It is obvious that the magnitude of molar ellipticity decreases
after adding the surfactants. The contents of the secondary
structure of BSA were calculated and given in Table 2. For all
three surfactants, the contents of α-helix decrease, while those
of β-sheet, β-turn, and random coil show distinct increases to
varying degrees. It is observed that, at low ratio of BSA and the
surfactant, the unfolding of the protein is slight. At about a ratio
of 1 to 19, surfactant III changes the secondary structure most;
the content of α-helix decreases to 53.5%, the content of β-
sheet increases to 9.4%, and the content of β-turn increases to
16.4%; while surfactant I shows the least effect on the change of
the secondary structure. The content of α-helix changes from
58.3% to 57.0% only, and the contents of β-sheet and β-turn
increase to 7.0% and 15.4%, respectively. With the increase in
ratio, unfolding of the protein became quite obvious. The
Table 2. Content of the Secondary Structure of BSA in the
Absence and Presence of the Surfactant
α-helix
(%)
β-sheet
(%)
β-turn
(%)
random coil
(%)
BSA only
BSA: I
58.3
57.4
57.0
54.7
42.6
41.6
55.5
52.7
48.5
48.0
45.4
56.4
53.5
44.5
39.6
40.2
6.6
6.9
14.8
15.8
15.4
15.9
12.9
18.7
16.1
14.3
17.1
11.7
17.5
15.9
16.4
18.6
16.0
15.8
23.3
22.0
21.6
24.3
31.7
25.6
22.9
24.5
25.2
27.2
26.5
22.2
21.9
28.1
31.0
28.8
1:10
1:20
1:50
1:100
1:140
1:19
1:38
1:57
1:76
1:113
1:6
7.0
9.5
14.7
14.9
8.1
BSA: II
9.3
10.0
12.7
13.3
7.1
BSA: III
1:18
1:36
1:54
1:72
9.4
11.1
13.6
15.1
change of the secondary structure shows a direct relationship to
the length of the hydrophobic tail. The content of α-helix
suddenly decreases to 40% at the ratio of 1 to 54, with no
obvious change at higher concentrations, while for surfactant I
and surfactant II, the content of α-helix could hardly decrease
to 40% even above the ratio of 1 to 100. Especially for
surfactant II, the content of α-helix decreases to 45.4% at the
ratio of 1 to 113, while that of surfactant I has reached 42.6%, a
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Figure 9. Secondary structure of BSA model and the most possible bound surfactant molecules. BSA structure was displayed in the secondary
structure; the surfactant molecules were shown by lines and colored by atom types. The yellow dashed lines point out the most potential binding
sites of BSA.
lower value at lower ratio of 1 to 100. The weakened unfolding
of the protein at high concentration of surfactant II could be
the results of the less binding on the protein, especially deep
inside the protein chains. Although much less surfactant III can
enter deep inside the protein, they could unfold the protein
chains so much more efficiently, so that the protein was
unfolded from side chains outside. Thus, surfactant III could
change the content of the α-helix most, which also indicated the
greatest ability to unfold the protein.
3.4. Molecular Modeling Study. At region (i) of the
binding process, surfactant molecules show a specific binding to
the protein. Thus, the docking method that aims to study the
specific binding mode between molecule and macromolecule
was used to learn the schematic binding location and preferred
binding mode of each surfactant.34 Through the modeling
study at the molecular level, the different observation of the
macro-experiments may be understood much better.
Figure 10. Lowest-energy binding mode of surfactants I, II, and III.
The secondary structure of BSA was displayed by ribbon and tube.
Small molecules of surfactants I, II, and III were displayed by space
ball, and colored in dark blue, red, and sky blue, respectively.
The top 100 possible docking conformations of the
surfactant molecules were modeled by the AutoDock program
and ranked by the energy. Finally, 92, 67, and 41 conformations
with negative binding energy were considered as reasonable for
surfactants I, II, and III, respectively. All the conformations with
BSA were displayed in Figure 9. By comparison of the six
commonly recognized binding sites of BSA for small molecules
which were outlined in yellow,35 the cleft between domain I
and domain III is obviously the most potential location
preferred by all three surfactants. There are large numbers of
conformations of surfactant I could be fit in site IIIB, while
fewer conformations were suitable on the same site for
surfactant II. For surfactant III, hardly any conformations
could be found. The same trend occurs for site IIIA. The result
ilustrates that the shorter tail contributes to the more direct
approach of the residues inside the cavity and influence on the
intrinsic fluorescence residues. Thus, the analysis on the
experimental observation at low concentration can be
explained.
conformation, and the binding energy is also very close.
Surfactant III shares the same binding site with surfactant I,
while the conformation is totally different. Thus, binding
energy is higher for surfactant III and the binding conformation
is also more unstable. From the detailed analysis about these
lowest-energy conformations for each surfactant, a better
insight into the result of the above modeling experiment is
obtained.
In view of the above, a schematic diagram of the surfactant−
BSA systems was given in Figure 11. The coils stand for the
structure of protein, and green lines represent the location of
intrinsic fluorescence residues. The surfactant concentration
increases from left to right. The surfactant prefers certain
specific binding sites in extremely dilute solution and generally
shows noncooperative binding with the concentration increas-
ing. Finally, a “micelle-like” structure was formed above the
CAC. The hydrophobic chain length increases from the bottom
up. The surfactants bind the protein at different locations and
show different assembly behavior at the same concentration.
Finally, surfactant III reaches the CAC first and surfactant II the
last.
The conformations and locations of the lowest binding
energy for each surfactant were shown in Figure 10. The
calculated binding energies for surfactants I, II, and III are
−4.65, −4.41, and −3.21 kcal mol−1, respectively. Surfactant I
and surfactant II bind in site IB and site IIIB with similar
conformations, which are close to the energy-minimized
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21077081 and 20921062), Program for Changjiang Scholars
and Innovative Research Team in University (IRT1030),
Natural Science Foundation of Hubei Province
(2010CDB01302), and the Fundamental Research Funds for
the Central Universities (201120302020010).
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ASSOCIATED CONTENT
* Supporting Information
Synthesis and characterization. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
■
*Prof. Dr. Yi Liu: E-mail yiliuchem@whu.edu.cn, Tel +86 27
68756667, Fax +86 27 68754067. Prof. Zhinong Gao: E-mail
whugzn@yahoo.cn, Tel +86 27 68752701, Fax +86 27
68754067.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from
National Natural Science Foundation of China (21173026,
■
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