Berberine azoles as antimicrobial agents: Synthesis, biological evaluation and their interactions with human serum albumin

Article abstract of DOI:10.1039/c3md00032j

A series of berberine azoles was synthesized and characterized by NMR, IR, MS and HRMS spectroscopy. All the newly prepared compounds were screened for their antimicrobial activities. Bioactivity assays manifested that most of the berberine azoles exhibited good antimicrobial activities. Especially compound 7a displayed remarkable anti-Proteus vulgaris and anti-Candida mycoderma efficacies, which were comparable to or even better than for the reference drugs. The binding behavior of compound 7a to human serum albumin (HSA) revealed that hydrophobic interactions and hydrogen bonds play important roles in the association of compound 7a with HSA. Molecular docking experiments showed that compound 7a has moderate affinity to HSA, and the theoretical calculations were in accordance with the experimental results. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3md00032j

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CONCISE ARTICLE
Berberine azoles as antimicrobial agents: synthesis,
biological evaluation and their interactions with human
serum albumin†
Cite this: DOI: 10.1039/c3md00032j
*
Shao-Lin Zhang, Juan-Juan Chang, Guri L. V. Damu,‡ Rong-Xia Geng
and Cheng-He Zhou
*
A series of berberine azoles was synthesized and characterized by NMR, IR, MS and HRMS spectroscopy. All
the newly prepared compounds were screened for their antimicrobial activities. Bioactivity assays
manifested that most of the berberine azoles exhibited good antimicrobial activities. Especially
compound 7a displayed remarkable anti-Proteus vulgaris and anti-Candida mycoderma efficacies, which
were comparable to or even better than for the reference drugs. The binding behavior of compound 7a
to human serum albumin (HSA) revealed that hydrophobic interactions and hydrogen bonds play
important roles in the association of compound 7a with HSA. Molecular docking experiments showed
that compound 7a has moderate affinity to HSA, and the theoretical calculations were in accordance
with the experimental results.
Received 29th January 2013
Accepted 10th March 2013
DOI: 10.1039/c3md00032j
www.rsc.org/medchemcomm
excellent safety proles and favorable pharmacokinetic char-
acteristics. However, with increasing misuse and the abuse of
1 Introduction
Azole compounds are an important class of nitrogen-containing
heterocycles with desirable electron-rich properties. This type of
special structure endows azole derivatives with the ability to
readily bind to enzymes and receptors in biological systems via
noncovalent interactions such as hydrogen bonds, coordination
bonds, ion–dipole, cation–p, p–p stacking, hydrophobic effects
and van der Waals forces. Thereby they exhibit various bioac-
tivities, such as antifungal,1–5 antibacterial,6–10 anticancer,11,12
antitubercular,13 antiviral,14 antiproliferative,15 anticonvul-
sant,16 antiparasitic,17 antihistaminic,18 antihypertensive19 and
other effects. Especially in antimicrobial applications,20 azole
compounds have been extensively investigated as antibacterial
and antifungal drugs and some of them, such as clotrimazole,
miconazole, tioconazole, terconazole, itraconazole and uco-
nazole have been employed in the clinic to treat various types of
microorganism infections. One of the most signicant
achievements is the successful exploitation of uconazole,
which has established an exceptional therapeutic record for
candida infections, and has become the rst choice for the
treatment of antifungal infections due to its potent activities,
azole-based drugs, more and more bacterial and fungal strains
have become resistant to these agents. Thus, these trends have
facilitated the urgent need to develop more effective and safe
antimicrobial agents. Among the attractive approaches to ach-
ieve this goal, structural modication of uconazole in order to
broaden its antimicrobial spectrum and increase therapeutic
indexes has provoked special interest in the realm of medicinal
chemistry. In our previously reported work,6 we developed new
structural analogues of uconazole, in which the tertiary
alcohol was replaced by a tertiary amino group, and the meth-
ylene bridge between the tertiary alcohol and the triazolyl
moieties was substituted by an ethylene chain. Bioactive eval-
uation manifested that these types of uconazole analogues
exhibited good antibacterial and antifungal activities which
were comparable to or even better than for the reference drugs
chloromycin, noroxacin and uconazole, showing their great
potential and development value. Therefore, we have interest in
further investigating this type of new frame.
As is known to us, berberine is a well-known natural iso-
quinoline alkaloid which has been demonstrated to possess a
variety of pharmacological activities such as antimicrobial,21
antineoplastic,22
antiviral,23
antiinammatory,24
anti-
School of Chemistry and Chemical Engineering, Southwest University, Chongqing
400715, People's Republic of China. E-mail: geng0712@swu.edu.cn; zhouch@swu.
edu.cn; Fax: +86-23-68254967; Tel: +86-23-68254967
protozoal,25 and antileishmanial26 effects. Especially as an
antimicrobial agent, berberine has been playing signicant
roles in the clinic for the treatment of infectious diseases such
as acute gastroenteritis, cholera and bacillary dysentery. This
overwhelmingly compels us with great interest to develop
berberine-based antimicrobial agents.
† Electronic supplementary information (ESI) available: Experimental procedures
for the prepared compounds and bioassay procedures; spectral data of the
prepared compounds. See DOI: 10.1039/c3md00032j
‡ Postdoctoral fellow from Indian Institute of Chemical Technology (IICT),
Hyderabad-500 607, India.
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In view of this, we combined berberine and the tertiary precursors 6a–h, 8, 10a–c and 12. Notably, the berberine triazole
amino frame to afford a series of hybrids as a new type of u- 7a gave the best antimicrobial activities (MIC ¼ 2–8 mg mLꢀ1),
conazole analogues, which were expected to increase their which were comparable to or even better than the reference
bioactivities and extend their antimicrobial spectrum. More- drugs. This result suggests that the incorporation of the
over, various azole rings including 1,2,4-triazole, benzotriazole, berberine moiety into the target compounds was favorable for
imidazole, benzimidazole, 2-mercaptobenzimidazole and 5,6- the enhancement of antimicrobial activities.
dimethyl-benzimidazole were incorporated into our target
For the tested berberine triazoles 7a–h, all compounds
compounds to explore their contributions to the antimicrobial showed good inhibitory efficacy at concentrations of 2–32 mg
activities. Further binding behaviors between the synthesized mLꢀ1 against the tested strains. Particularly, compound 7a was
compound and human serum albumin (HSA) were investigated the most active one and displayed remarkable antimicrobial
to preliminarily evaluate their transportation and pharmacoki- activities, especially for proteus vulgaris (MIC ¼ 2 mg mLꢀ1),
netic properties. It is well accepted that the overall distribution, which was 4-, 16- and 64-fold more potent than the reference
metabolism, and efficacies of drugs can be modied by their drugs noroxacin, chloromycin and berberine. Notably,
affinity to HSA,27,28 and many promising new drugs have been compounds 7a and 7f–h have a low inhibitory concentration
rendered ineffective because of their unusually high affinity to (MIC ¼ 8 mg mLꢀ1) against MRSA, which makes them 2- and 16-
this protein.29 Full investigations of interactions between drugs fold more potent than the reference drugs chloromycin and
or bioactive small molecules and HSA are not only benecial for berberine, respectively, and equipotent to noroxacin. This
providing a deeper understanding of the absorption, trans- indicated that this type of compounds could be employed to
portation, distribution, metabolism and excretion properties of further study as potent novel anti-MRSA agents.
drugs, but are also signicant to the design, modication and
Berberine benzimidazoles 11a–b and 13 showed weaker
screening of drug molecules. In this work, information on activities than berberine triazoles 7a–h, but compound 11b,
interactions between HSA and a target compound with high bearing an electron donating methyl group, displayed good
antimicrobial activities was obtained by uorescence and UV-vis anti-shigella dysenteriae activity with an MIC value of 4 mg mLꢀ1
,
absorption spectroscopy on a molecular level. Finally, molec- which was 8- and 64-fold more potent than that of chloromycin
ular docking experiments were employed to reconrm the and berberine, respectively. Compared to berberine triazoles
interaction behaviors between the prepared compound and imidazole, berberine benzotriazole 11c and its precursor
and HSA.
10c showed lower antimicrobial efficacies.
The above results clearly pointed out that the antimicrobial
efficacies should be closely related to azolyl moieties and hal-
obenzyl groups to some extent. For this series of compounds,
2 Chemistry
The synthetic routes to the target berberine azoles were outlined azolyl moieties contributed to the antimicrobial activities in the
in Schemes 1 and 2. The target berberine azole compounds were order of triazolyl > imidazolyl > 5,6-dimethyl-benzimidazolyl >
prepared starting from halobenzyl halides 3a–h. The N-alkyl- benzimidazolyl > benzotriazolyl. For berberine triazoles, diha-
ation of diethanolamine with a series of halobenzyl halides 3a– lobenzyl groups are more helpful for increasing antimicrobial
h produced the intermediates 4a–h in excellent yields of 92.3– efficacies in comparison to the monohalobenzyl ones, and a
98.6%, and then the latter were brominated in chloroform with diuoro-substituted derivative showed the highest antimicro-
phosphorus tribromide to afford the dibromides 5a–h accord- bial potency.
ing to procedures similar to our previous work.6 Subsequently,
compounds 5a–h were reacted with 1,2,4-triazole, benzo-
triazole, imidazole, benzimidazole, 2-mercaptobenzimidazole
3.2 Binding discussion
and 5,6-dimethyl-benzimidazole through N-alkylation to
3.2.1 UV-vis absorption spectral study. UV-vis absorption
conveniently afford the corresponding mono-triazoles 6a–h, measurement is a very easy operational method and applicable
mono-imidazole 8, mono-benzimidazoles 10a–c and mono- to explore the structural change of protein and to identify
benzotriazole 12. Finally, these mono-azoles were coupled with complex formation. In our binding experiment, a UV-vis
the berberrubine 2 to afford the desired target berberine- absorption spectroscopic method was employed to evaluate the
derived triazoles 7a–h, imidazole compound 9, benzimidazoles binding interactions between compound 7a and HSA. As shown
11a–c and benzotriazole derivative 13. All the newly synthesized in Fig. 1, two absorption peaks were observed at 278 and
compounds were characterized by NMR, IR, MS and HRMS 338 nm, respectively, and the peak intensity increased with the
spectra (ESI 1†).
addition of compound 7a. The obvious enhancement of
absorbance intensity revealed the interactions between HSA
and compound 7a. This result can be explained by the change of
environment of the Tryptophan (Trp-214) residue, and the
slight increase in hydrophobicity of the microenvironment of
3 Results and discussion
3.1 Antimicrobial activities
The tested antimicrobial results (ESI 2†) in Table 1 revealed that the Trp-214 residue.30
most of the prepared berberine azoles 7a–h, 9, 11a–c and 13
3.2.2 Fluorescence quenching mechanism. Fluorescence
inhibit the growth of Gram-positive bacteria, Gram-negative spectroscopy is an effective method to study the interactions of
bacteria and fungi, to a superior amount compared to their small molecules with HSA. The uorescence intensity of
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Scheme 1 Synthetic routes to berberine triazoles and imidazole. Reagents and conditions: (a) berberine, 20 mm Hg, 190 ꢂC, 15 min; (b) CH3CN, diethanolamine, 50 ꢂC,
10–12 h, 92.3–98.6% yield; (c) CHCl3, PBr3, reflux, 2 h, 88.1–98.5% yield; (d) 1,2,4-triazole, K2CO3, CH3CN, 30 ꢂC, 12 h, 21.9–64.1% yield; (e) imidazole, NaH, THF, 60 ꢂC,
57.3% yield ; (f) DMF, 110 ꢂC, 20 h, 30.8% yield; (g) DMF, 110 ꢂC, 20 h, 21.0–38.6% yield.
Trp-214 may change when HSA interacts with other small suggests an increased hydrophobicity of the region surrounding
molecules, which can be seen in the uorescence spectra of HSA Trp-214.28
in the UV region. The effect of compound 7a on the uorescence
The uorescence quenching data can be analyzed by the
intensity of HSA at 298 K is shown in Fig. S1 (ESI 3†). HSA has a Stern–Volmer equation:31
strong uorescence emission with a peak at 348 nm, owing to
F0
¼ 1 þ KSV½Qꢁ
(1)
the single Try-214 residue. The intensity of this characteristic
broad emission band decreased steadily with the increase in
concentration of compound 7a, accompanied with a blue shi
wavelength (from 348 to 342 nm) in the albumin spectrum. This
F
where F0 and F represent uorescence intensities in the absence
and presence of compound 7a, respectively. KSV is the Stern–Volmer
Scheme 2 Synthetic routes to berberine benzimidazoles and benzotriazole. Reagents and conditions: (h) CH3CN, 60 ꢂC, 10–12 h, 32.9–40.2% yield; (i) DMF, 110 ꢂC,
20 h; (j) CH3CN, 60 ꢂC, 10–12 h, 60.0% yield; (k) DMF, 110 ꢂC, 20 h, 33.4% yield.
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Table 1 In vitro antimicrobial activities for the prepared compounds expressed as MIC (mg mLꢀ1 abc
Gram-positive bacteria Gram-negative bacteria
)
Fungi
CA
Compds
MRSA
SA
BS
ML
EC
SD
PA
PV
CM
CU
BF
6a
6b
6c
6d
6e
6f
6g
6h
7a
7b
7c
7d
7e
7f
128
256
256
256
256
128
128
256
8
16
16
32
16
64
64
128
128
128
64
64
64
4
8
8
8
16
4
256
256
256
256
256
256
256
256
8
8
8
16
8
32
32
64
64
512
64
64
32
8
16
32
32
16
8
32
32
64
128
128
32
64
32
4
8
8
8
16
4
64
128
256
256
256
64
64
128
4
16
32
32
8
128
128
128
256
256
128
128
128
8
16
32
32
16
128
256
512
>512
512
256
256
128
2
8
16
32
32
32
32
64
64
128
32
32
32
4
256
256
256
256
256
256
256
256
2
4
8
4
32
256
64
128
128
256
128
64
128
256
128
128
256
256
8
32
32
8
16
64
128
256
32
32
16
32
32
16
8
16
16
32
4
8
8
4
8
4
2
16
7g
7h
8
8
8
64
16
256
128
128
32
32
64
>512
64
16
8
128
—
8
8
64
8
8
8
128
16
256
32
256
32
16
128
>512
64
16
16
64
32
256
64
256
16
16
128
>512
128
8
8
4
4
8
8
16
32
16
128
64
128
16
8
64
>512
64
32
16
256
—
4
8
4
4
4
4
32
32
64
64
>512
32
16
>512
>512
128
—
—
128
4
8
16
64
32
128
128
256
16
16
32
512
64
—
—
256
8
16
16
128
32
512
128
128
32
32
128
>512
32
32
16
>512
—
128
16
64
16
64
8
256
32
512
128
256
32
64
32
128
128
>512
32
32
>512
>512
64
—
128
32
128
256
256
32
16
16
512
64
—
9
10a
10b
10c
11a
11b
11c
12
13
A
128
64
256
16
32
64
>512
32
16
0.5
512
—
4
8
32
>512
128
32
4
128
>512
64
32
8
32
1
>512
—
B
C
D
2
—
>512
1
—
128
16
>512
—
256
—
128
—
a
Minimum inhibitory concentrations were determined by a micro broth dilution method for microdilution plates. b MRSA, Methicillin resistant
Staphylococcus aureus N315; SA, Staphylococcus aureus ATCC25923; BS, Bacillus subtilis ATCC6633; ML, Micrococcus luteus ATCC 4698; EC, Escherichia
coli DH52; SD, Shigella dysenteriae ATCC51252; PA, Pseudomonas aeruginosa ATCC27853; PV, Proteus vulgaris ATCC8427; CA, Candida albicans
c
ATCC90029; CM, Candida mycoderma; CU, Candida utilis ATCC9950; BF, Beer yeast ATCC9763. A ¼ chloromycin; B ¼ noroxacin; C ¼
berberine; D ¼ uconazole.
quenching constant, and [Q] is the concentration of compound 7a.
The Stern–Volmer plots of HSA in the presence of compound 7a at
different concentrations and temperatures are shown in Fig. S2
(ESI 3†).
Fluorescence quenching can occur via different mecha-
nisms, usually classied as dynamic quenching or static
quenching depending on temperature and viscosity. Because
higher temperatures result in larger diffusion coefficients, the
quenching constants are expected to increase with a gradually
increasing temperature in dynamic quenching. However,
increasing of the temperature is likely to result in a smaller
static quenching constant due to the dissociation of weakly
bound complexes.
Therefore, the KSV values at each temperature were calcu-
lated from Stern–Volmer plots. The result (Table 2) showed
that the Stern–Volmer quenching constants were inversely
correlated with temperature, which indicated that the prob-
Fig. 1 Effect of compound 7a on the UV-vis absorption of HSA, c(HSA) ¼ 1.0 ꢃ
able quenching mechanism of the compound 7a–HSA
binding reaction was initiated by ground-state complex
formation.30
10ꢀ5 mol Lꢀ1; c(compound 7a)/(10ꢀ5 mol Lꢀ1): 0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6,
1.8 (T ¼ 294 K, pH ¼ 7.40). The inset corresponds to the absorbance at 277 and
344 nm at different concentrations of compound 7a.
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Table 2 Stern–Volmer quenching constants for the interactions of compound
7a with HSA at various temperatures
Table 3 Binding constants and sites of the compound 7a–HSA system at
pH ¼ 7.4
pH
7.4
T (K)
10ꢀ4KSV (L molꢀ1
)
Ra
S.D.b
Modied Stern–Volmer
method
Scatchard method
294
302
310
8.73
7.12
6.51
0.9943
0.9940
0.9948
0.1006
0.0799 T (K) 10ꢀ4Ka (L molꢀ1
)
10ꢀ4Kb (L molꢀ1
)
R
S.D.
n
0.0666
294
302
310
4.03
3.74
3.51
4.15
3.79
3.57
0.994 0.009 1.44
0.998 0.004 1.39
0.997 0.005 1.38
a
R is the correlation coefficient. b S.D. is the standard deviation.
In order to reconrm that the probable quenching mecha-
nism of uorescence of HSA by compound 7a was mainly
initiated by ground-state complex formation, the difference in
absorption spectroscopy was employed. The UV-vis absorption
spectrum of HSA and the difference in the absorption spectrum
between the compound 7a–HSA complex and compound 7a at
the same concentration could not be superposed (Fig. S3, ESI
3†), this result suggests that the probable quenching mecha-
nism of uorescence of HSA by compound 7a was mainly a
static quenching procedure.28
When small molecules bind to a set of equivalent sites on a
macromolecule, the equilibrium binding constants and the
numbers of binding sites can also be calculated according to the
Scatchard equation:33
r/Df ¼ nKb ꢀ rKb
(3)
where Df is the molar concentration of free small molecules, r is
the number of moles of small molecules bound per mole of
protein, n is the binding sites multiplicity per class of binding
site, and Kb is the equilibrium binding constant. The Scatchard
plots are shown in Fig. 3 and the Kb and n are listed in Table 3.
Fig. 2 and 3 show the Modied Stern–Volmer and Scatchard
plots for the compound 7a–HSA system at different tempera-
ture. The decreasing trend of Ka and Kb with increasing
temperature was in accordance with KSV's dependence on
temperatures. The values of the binding sites n were approxi-
mately 1, which showed that one high affinity binding site was
present in the interactions of compound 7a with HSA. The
results also showed that the binding constants were moderate
and the effects of temperature were not signicant, thus
compound 7a might be stored and carried by this protein.
3.2.4 Binding mode and thermodynamic parameters.
Generally, there are four types of non-covalent interactions
including hydrogen bonds, van der Waals forces, electrostatic
and hydrophobic bonds, which play important roles in small
molecules binding to proteins.34 Changes in the thermody-
namic parameters enthalpy (DH) and entropy (DS) during the
3.2.3 Binding constant and site. For a static quenching
process, the data could be described by the Modied Stern–
Volmer equation:32
F0
1
1
1
¼
þ
(2)
DF faKa ½Qꢁ fa
where DF is the difference in uorescence intensity in the
absence and presence of compound 7a at concentration [Q], fa
is the fraction of accessible uorescence, and Ka is the effec-
tive quenching constant for the accessible uorophores, which
are analogous to associative binding constants for the
quencher–acceptor system. The dependence of F0/DF on the
reciprocal value of quencher concentration [Q]ꢀ1 is linear with
ꢀ1
the slope equal to the value of ( faKa)ꢀ1. The value fa is xed
on the ordinate. The constant Ka is a quotient of the ordinate
ꢀ1
fa and the slope ( faKa)ꢀ1. The Modied Stern–Volmer plots
are shown in Fig. 2, and the calculated results are displayed in
Table 3.
Fig.
2
Modified Stern–Volmer plots of the compound 7a–HSA system at
Fig.
3
Scatchard plots of the compound 7a–HSA system at different
different temperatures.
temperatures.
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binding reaction are the main evidence for the interactions
between small molecules and proteins. If the enthalpy change
(DH) does not vary signicantly over the studied temperature
range, then its value and that of entropy change (DS) can be
evaluated from the van't Hoff equation:
DH DS
ln K ¼ ꢀ
þ
(4)
RT
R
where K is analogous to the associative binding constants at the
corresponding temperature and R is the gas constant. In order to
explain the binding model between compound 7a and HSA, the
thermodynamic parameters were calculated from the van't Hoff
plots. The enthalpy change (DH) was estimated from the slope of
the van't Hoff relationship (Fig. S4, ESI 3†). The free energy
change (DG) was then calculated from the following equation:
Fig. 4 Spectral overlap of compound 7a absorption (curve A) with HSA fluo-
rescence (curve F; T ¼ 294 K). c(HSA) ¼ c(compound 7a) ¼ 1.0 ꢃ 10ꢀ5 mol Lꢀ1
.
DG ¼ DH ꢀ TDS
(5)
intensities of HSA in the presence and absence of compound 7a,
respectively. R0 can be calculated from equation (7):
Table 4 summarizes the values of DH, DG and DS. The
negative values of the free energy DG of the interactions
between compound 7a and HSA suggest that the binding
process is spontaneous, and the negative values of enthalpy
(DH) indicate that the binding was mainly enthalpy-driven and
involved an exothermic reaction, the entropy (DS) was unfa-
vorable for it. A positive (DS) value is frequently taken as typical
evidence for hydrophobic interactions, which was consistent
with the above discussion. The negative (DH) value (ꢀ6.554 kJ
molꢀ1) observed cannot be mainly attributed to electrostatic
interactions since electrostatic interactions DH were very small,
almost zero.35 Therefore, the enthalpy change DH < 0 and the
entropy change DS > 0 obtained in this case indicate that the
hydrophobic interactions and hydrogen bonds play an impor-
tant role in the binding of compound 7a to HSA,30 electrostatic
interactions might also be involved in the binding process.
3.2.5 Binding distance. A spectral overlap between the
uorescence spectrum of HSA and the UV-vis spectrum of
compound 7a is shown in Fig. 4. The salient overlap suggests
that there is a direct resonance energy transfer between HSA
and compound 7a.
6
R0 ¼ 8.79 ꢃ 10ꢀ25K2nꢀ44J
(7)
where K2 is the orientation factor related to the geometry of the
donor (HSA) and acceptor (compound 7a) of dipoles; n is the
average refracted index of medium in the wavelength range
where spectral overlap is signicant; 4 is the uorescence
quantum yield of the donor; J is the overlap integral of the
emission spectrum of HSA and the absorption spectrum of
compound 7a, which could be obtained from equation (8):
N
ð
FðlÞ3ðlÞl4dl
0
J ¼
(8)
N
ð
FðlÞdl
0
where F(l) is the uorescence intensity of HSA in the wavelength
range from l to l + Dl, 3(l) is the extinction coefficient of
compound 7a at l.
In the present case, K2 ¼ 2/3, n ¼ 1.36, 4 ¼ 0.074,37 according
¨
According to Forster's theory, the energy transfer efficiency
to equations (6)–(8), we calculated J ¼ 3.42 ꢃ 10ꢀ14 cm3 L molꢀ1
,
(E) is dened by the following equation:36
E ¼ 0.385, R0 ¼ 2.75 nm, and the binding distance r ¼ 2.97 nm.
The values for R0 and r are on the 2–8 nm scale, and 0.5R0 < r <
1.5R0, indicating the existence of interactions between
compound 7a and HSA.38,39
6
F
R0
6
E ¼ 1 ꢀ
¼
(6)
R0 þ r6
F0
where E denotes the efficiency of transfer between the donor
and the acceptor, r is the average distance between donor and
3.2.6 Molecular docking studies. HSA contains three
homologous a-helical domains (I–III), each of which is
composed of subdomains A and B. The principal regions of
ligand binding sites in HSA are located in hydrophobic cavities
in the subdomains IIA and IIIA.
¨
acceptor, and R0 is the Forster critical distance when the effi-
ciency of transfer is 50%, F and F0 are the uorescence
Molecular docking evaluation was performed to understand
the binding mode between compound 7a and HSA. The docking
mode with the lowest binding free energy (ꢀ11.32 kJ molꢀ1) is
shown in Fig. 5 and 6. As clearly evidenced, compound 7a is
surrounded by Arg-218, 257, Lys-195, 199, Leu-198, 238, 260,
Ala-291, 455, Ile-264, 290, Asp-451, Phe-211 and Trp-214.
Moreover, compound 7a partly occupies subdomain IIA,
resulting in uorescence quenching of Trp-214. Hydrophobic
Table 4 Thermodynamic parameters of compound 7a–HSA system at different
temperatures
DH
DG
DS
T (K)
(kJ molꢀ1
)
(kJ molꢀ1
)
(J molꢀ1 Kꢀ1
)
294
302
310
ꢀ6.55
ꢀ25.92
ꢀ26.44
ꢀ26.97
65.86
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7a has moderate affinity for HSA, and the theoretical calcula-
tions were in accordance with the experimental results.
Acknowledgements
This work was partially supported by the National Natural
Science Foundation of China (no. 21172181), the Research
Fellowship for International Young Scientists from Interna-
tional (Regional) Cooperation and Exchange Program (no.
81250110089, 81250110554), the key program from Natural
Science Foundation of Chongqing (CSTC2012jjB10026) and the
Specialized Research Fund for the Doctoral Program of Higher
Education of China (SRFDP 20110182110007).
Fig. 5 Molecular docking of compound 7a with HSA.
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