A 4-methylumbelliferone-based fluorescent probe for sodium new houttuyfonate

Article abstract of DOI:10.1002/cjoc.201090251

A new fluorogenic probe for sodium new houttuyfonate (SNH) was proposed. 4-Methylumbelliferyl-2,4-dinitrobenzenesulfonate (4-MUDNBS) was a nonfluorescent compound and was synthesized via the one-step reaction of 4-methylumbelliferone (4-MU) with 2,4-dinitrobenzenesulfonyl chloride. In basic media, SNH was decomposed to produce sodium sulfite, which then reacted with 4-MUDNBS to yield highly fluorescent 4-MU, hence leading to the fluorescence increase of the reaction solution. A linear correlation existed between the emission intensity and the concentration of SNH within the range from 0.5 to 15 μg ^?mL ¹ with a detection limit of 0.15 μg ^? mL ¹ (3σ). The effect of substituents on the benzenesulfonyl moiety of the probe is discussed, and the presence of electronegative groups is favorable for the proposed cleavage reaction.

Full text of DOI:10.1002/cjoc.201090251

FULL PAPER
A 4-Methylumbelliferone-based Fluorescent Probe for Sodium
New Houttuyfonate
Yang, Xiaofeng*(杨小峰)
Wang, Liping(王丽萍)
Zhao, Minglei(赵明雷)
Qi, Haiping(齐海萍)
Wu, Yao(毋尧)
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Institute of
Analytical Science, Department of Chemistry, Northwest University, Xi'an, Shaanxi 710069, China
A new fluorogenic probe for sodium new houttuyfonate (SNH) was proposed. 4-Methylumbelliferyl-2,4-
dinitrobenzenesulfonate (4-MUDNBS) was a nonfluorescent compound and was synthesized via the one-step reac-
tion of 4-methylumbelliferone (4-MU) with 2,4-dinitrobenzenesulfonyl chloride. In basic media, SNH was decom-
posed to produce sodium sulfite, which then reacted with 4-MUDNBS to yield highly fluorescent 4-MU, hence
leading to the fluorescence increase of the reaction solution. A linear correlation existed between the emission in-
tensity and the concentration of SNH within the range from 0.5 to 15 µg•mL－ with a detection limit of 0.15 µg•
1
mL－ (3σ). The effect of substituents on the benzenesulfonyl moiety of the probe is discussed, and the presence of
1
electronegative groups is favorable for the proposed cleavage reaction.
Keywords sodium new houttuyfonate, 4-methylumbelliferone, fluorescent probes, fluorescence spectroscopy
Introduction
mechanism, several fluorescent probes for thiol-
containing compounds have been developed.11-14 In our
recent studies, we observed that other nucleophiles, such
as sulfide anion and bisulfite can also lead to the cleav-
age reaction. Encouraged by this observation, we envi-
sioned that this cleavage reaction might be utilized for
the determination of SNH, and such an idea has been
attempted in this work by designing a new fluorogenic
probe, 4-methylumbelliferyl-2,4-dinitrobenzenesulfonate
(4-MUDNBS), for the selective determination of SNH
in basic solution.
Sodium new houttuyfonate (SNH, sodium lauroyl-α-
hydroxyethyl sulfonate) is a novel antimicrobial medi-
cine for clinical practice in recent years (Scheme 1).
Previous studies showed that houttuyfonate homologues
(HOU-Cn) had immunoregulatory activities and typical
adjuvanticity, and could improve the immune ability of
mice and inhibit the growth of Staphylococcus aureus,
Bacillus subtilis and pneumococci.1-3 And the immu-
noregulation and antibacterial activity of SNH were
higher and stronger than that of sodium houttuyfonate.4,5
HPLC method has been successfully applied to the de-
termination of SNH in injections.6,7 Though HPCL
method is accurate, specific, it suffered from the disad-
vantages of low sensitivity, narrow linear range and
time-consuming.
Fluorimetric analysis is widely used for its high sen-
sitivity, but no work has been reported for the fluorimet-
ric determination of SNH. Herein, we developed a sim-
ple and sensitive fluorogenic probe for determining
SNH with a new interaction mechanism.
It was reported that strongly electron-withdrawing
nitro group is the most effective in labilizing the car-
bon-sulfur bond in benzenesulfonamide.8 The resulting
benzenesulfonamide can be readily cleaved by thiolate
anion, derived from thiol under basic conditions through
nucleophilic aromatic substitution.9,10 Based on this
In the present study, protection of the hydroxyl
group in 4-MU with 2,4-dinitrobenzenesulfonyl chloride
gives a nonfluorescent 4-MUDNBS. Upon incubation
with SNH in basic solution at room temperature, the
equilibrium reaction between SNH and sodium bisulfite
in aqueous solution moves toward right, and thus sulfite
is formed, which can then react with 4-MUDNBS to
yield the highly fluorescent 4-MU and hence leading to
a dramatic increase in fluorescence intensity (Scheme 1).
Based on this mechanism, a new fluorescent probe for
SNH was developed. The fluorescence increase is line-
arly w－ith SNH concentration in the range 0－.5—15
1
1
µg•mL with a detection limit of 0.15 µg•mL . The
effect of substituents on the benzenesulfonyl unit of the
probe is discussed, and the presence of electronegative
groups is favorable for the cleavage reaction.
*
E-mail: xfyang@nwu.edu.cn; Tel.: 0086-029-88302604; Fax: 0086-029-88303798
Received December 28, 2009; revised February 27, 2010; accepted March 23, 2010.
Projected supported by the Science & Technology Department (No. SJ08B05) and the Education Department (No. 09JK787) of Shaanxi Province of
China, and NWU Graduate Experimental Research Funds (No. 09YSY19).
Chin. J. Chem. 2010, 28, 1469— 1474
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Yang et al.
FULL PAPER
Scheme 1 Proposed mechanism for the detection of SNH with 4-MUDNBS
Experimental
2,4-dinitrobenzenesulfonyl chloride (1.2 equiv.) in 5 mL
dried pyridine was stirred overnight at room temperature
(Scheme 2). The resulting reaction solution was poured
into saturated brine and yellow precipitate was observed.
The precipitate was filtered off and dried to give a light
yellow solid. This solid was subjected to silica gel (200—
300 mesh) chromatography eluted with chloroform-
ethyl acetate (7∶1, V∶V) to afford the desired product
(0.16 g, 69% yield) as a white solid. The product was
shown to consist of only one substance by thin layer
chromatography (TLC), and its Rf value was 0.68
(chloroform/ethyl acetate, V∶V＝4∶1). 1H NMR
(DMSO-d6, 400 MHz) δ: 9.12 (d, J＝2.0 Hz, 1H), 8.60
(dd, J＝2.0, 8.8 Hz, 1H), 8.29 (d, J＝8.4 Hz, 1H), 7.85
(d, J＝8.4 Hz, 1H), 7.35 (d, J＝2.4 Hz, 1H), 7.22 (dd, J＝
2.4, 8.8 Hz, 1H), 6.47 (d, J＝0.8 Hz, 1H), 2.41 (d,－J＝1.2
Apparatus
The fluorescence spectra and relative fluorescence
intensity were measured with a Sanco CRT-970 spec-
trofluorimeter (Shanghai, China) with a 10 mm quartz
cuvette. The excitation and emission wavelength band-
passes were both set at 10 nm. The absorption spectra
were measured with a Shimadzu UV-1700 spectro-
phometer. The pH was measured with a Model pHs-3B
meter (Shanghai, China). IR spectra were taken in KBr
pellets on a Bruker Tensor 27 IR spectrometer. Mass
spectra were obtained with AXIMA-CFR plus MALDI-
1
TOF Mass Spectrometer. H NMR spectra were meas-
ured with Varian unity INOVA-400 spectrometer (400
MHz) with tetramethylsilane (TMS) as internal standard.
Elemental analysis were obtained with a vario EL III
Elemental Analyzer.
1
Hz, 3H); IR (KBr) ν: 1732.4, 155＋7.7, 1537.7 cm ; MS
(MALDI-TOF) m/z: 406.1 (M ). Anal. calcd for
C16H10H2O9S: C 47.30, H 2.48, N 6.89; found C 47.65,
H 2.45, N 6.38.
Reagents
－1
A stock solution of 0.2 mg•mL SNH was prepared
Synthesis of 4-methylumbelliferyl-p-toluenesulfonate
(4-MUTS)
by dissolving appropriate amount of SNH in 50 mL wa-
ter. This stock solution was stored at 4 ℃ in the refrig-
erator. Standard test solutions were prepared daily by
appropriate dilution of－the stock solution. 4-MUDNBS
A mixture of 4-MU (0.2 g, 1.14 mmol) and
p-toluenesulfonyl chloride (1.5 equiv.) in 5 mL dried
pyridine was stirred overnight at room temperature
(Scheme 2). The resulting reaction solution was poured
into saturated brine and white precipitate was observed.
The precipitate was filtered off and dried to give a white
solid. This solid was dissolved by 60 mL of CH2Cl2,
1
solution (1.0 mmol•L ) was prepared by dissolving
20.3 mg of the probe in 50 mL ethyl acetate. 4-MUTS
solution (1.0 mmol•L ) was prepared by dissolving
8.25 mg of 4-MUTS in 25 mL ethyl acetate. 4-MUNBS
solution (2.0 mmol•L ) was prepared by dissolving
－1
－1
－1
18.1 mg of 4-MUNBS in 25 mL ethyl acetate. A 0.1 mol•
washed with 0.1 mol•L Na2CO3 (10 mL×2) and wa-
－1
L
NH3NH4Cl buffer solution (pH 8.9) was employed.
ter (10 mL×2), and dried over anhydrous Na2SO4. The
solvent was removed by evaporation to dryness, giving
the desired product as white solid (0.23 g, 61.3% yield).
The product was shown to consist of only one substance
by thin layer chromatography (TLC), and its Rf value
was 0.72 (ethyl acetate/petroleum ether, V∶V＝1∶1).
1H NMR (CDCl3, 400 MHz) δ: 7.73 (dd, J＝1.6, 6.8 Hz,
2H), 7.57 (d, J＝8.4 Hz, 1H), 7.34 (dd, J＝0.8, 8.0 Hz,
2H), 7.11 (dd, J＝2.4, 8.8 Hz, 1H), 6.83 (d, J＝1.2 Hz,
1H), 6.28 (d, J＝1.2 Hz, 1H), 2.48 (s, 3H), 2.42 (d, J＝
1.6 Hz, 3H); MS (MALDI-TOF) m/z: 330.0 (M＋). Anal.
calcd for C17H14O5S: C 61.81, H 4.27; found C 61.47, H
4.29.
The source of reagents were as follows: SNH was ob-
tained from Qinghai Xiadu Pharmaceutical Co., LTD;
4-MU was obtained from Beijing Hengye Zhongyuan
Chemical Reagent Co., 2-nitrobenzenesulfonyl chloride
and 2,4-dinitrobenzenesulfonyl chloride were obtained
from Alfa Aesar. p-Toluenesulfonyl chloride was ob-
tained from Tianjin Chemical Reagent Plant.
All the reagents were of analytical-reagent grade,
and doubly distilled water was used throughout.
Synthesis of 4-MUDNBS
A mixture of 4-MU (0.1 g, 0.57 mmol) and 2,4-di-
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Chin. J. Chem. 2010, 28, 1469— 1474

A 4-Methylumbelliferone-based Fluorescent Probe for Sodium New Houttuyfonate
Scheme 2 Synthesis of 4-MUDNBS, 4-MUTS and 4-MUNBS
Synthesis of 4-methylumbelliferyl-2-nitrobenzene-
sulfonate (4-MUNBS)
enhancement of fluorescence with an emission maxi-
mum at 446 nm was observed when SNH was intro-
duced. This may result from the fact that sodium sulfite
is generated for the decomposition of SNH in basic me-
dia, which then reacted with 4-MUDNBS to generate
the strongly fluorescent 4-MU through nucleophilic re-
action, and thus leading to the fluorescence increase of
the reaction mixture. Furthermore, the fluorescence
spectra of 4-MUDNBS solution containing different
concentrations of SNH were measured, and a character-
istic fluorescence emission maximum centered at about
446 nm was recorded (Figure 1), which were consistent
with the fluorescence emission maximum of 4-MU at
the present conditions. In addition, it can be observed
that the fluorescence emission increases dramatically
upon increasing the SNH concentration, which forms
the basis of SNH determination.
Following the procedure for the preparation of
4-MUTS, 4-MUNBS was prepared as a white solid
(66.7% yield) from 4-Mu and 2-dinitrobenzenesulfonyl
chloride. Rf＝0.57 (chloroform/ethyl acetate, V∶V＝
1
3∶1). H NMR (CDCl3, 400 MHz) δ: 8.02 (d, J＝7.6
Hz, 1H), 7.89—7.87 (m, 2H), 7.75—7.71 (m, 1H), 7.62
(d, J＝9.2 Hz, 1H), 7.26 (dd, J＝2.4, 8.8 Hz, 2H), 6.30
(dd, J＝1.2, 2.4 Hz, 1H), 2.43 (d, J＝1.2 Hz, 3H); MS
(MALDI-TOF) m/z: 362.0 (M ＋ ). Anal. calcd for
C16H11NO7S: C 53.19, H 3.07, N 3.88; found C 53.16, H
3.25, N 3.83.
Procedure
The fluorogenic reaction was performed in a 10 mL
volumetric tube. Typically, to a test tube containing 0.4
－1
mL of 0.1 mmol•L 4-MUDNBS, different concentr－a-
1
tion of SNH was added, and then 2.0 mL of 0.1 mol•L
NH3-NH4Cl buffer solution (pH 8.9) was added and the
solution was diluted to 10 mL with water. The reaction
solution was kept at room temperature for 35 min, and
then the fluorescence intensity of the above solution was
recorded at an emission wavelength of 445 nm with ex-
citation wavelength set at 362 nm. In the meantime, a
blank solution containing no SNH was prepared and
measured under identical conditions for comparison.
Results and discussion
Spectral characteristics
Figure 1 Fluorescence emission spectra (excitation at 350 nm)
of 4-MUDNBS (4.0 µmol•L ) toward different concentrations of
－1
The fluorescence spectra of 4-MUDNBS in the ab-
sence and presence of SNH are shown in Figure 1. It can
be observed that 4-MUDNBS itself was nonfluorescent
prior to its mixing with SNH. However, a significant
SNH (final concentrations: 0, 0.5, 1.0, 3.0, 5.0, 7.0, 9.0, 10, 12, 15
－1
µg•mL ) after incubation at room temperature for 35 min in pH
8.9 NH3-NH4Cl buffer.
Chin. J. Chem. 2010, 28, 1469— 1474
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1471

Yang et al.
FULL PAPER
Effect of substituents on the cleavage reaction
was selected in the following experiment.
To investigate the effect of substituents on the ben-
zenesulfonyl moiety of the probe on the cleavage reac-
tion, the fluorescent responses of 4-MUDNBS,
4-MUNBS and 4-MUTS toward SNH were compared
and the results are shown in Figure 2. It can be observed
that the fluorescence signal of 4-MUDNBS increased
dramatically with the reaction time prolonged, while the
fluorescence signal of 4-MUNBS and 4-MUTS re-
mained almost unchanged. The significant difference
between them lies in the substituents on the benzene-
sulfonyl moiety of the probe. For 4-MUDNBS, the nitro
group on the ortho and para position of benzenesulfonyl
unit can effectively labilize the C—S bond,8 and hence
the cleavage reaction can proceed readily. While for
4-MUTS, because of the lack of electronegative groups,
the cleavage of the sulfonate is generally very unfavor-
able. As for 4-MUNBS, though the presence of the nitro
group in its ortho position, the cleavage reaction is very
slowly compared with that of 4-MUDNBS. Based on
the above results, it can be concluded that the electro-
withdrawing groups on the ortho and para position of
benzenesulfonyl moiety of the probe can markedly fa-
cilitate the nucleophilic reaction.
Figure 3 Effect of reaction time on fluorescence intensity of
－1
4-MUDNBS (4.0 µmol•L ) cleavage system by SNH (5.0 µg•
－1
mL ). F and F0 are the fluorescence intensity of 4-MUDNBS
solution in the presence and absence of SNH, respectively. Other
conditions were the same as those described in the experimental
section.
Effect of pH
The pH had a great effect on the sensitivity of the
proposed method and was selected considering the fol-
lowing three aspects. (i) It was reported that SNH can
decompose to produce lauroyl acetaldehyde and sodium
sulfite in basic media,15 and the latter is an essential
compound responsible for the proposed fluorogenic re-
action. Therefore, to ensure the decompose of SNH to
yield sulfite, the reaction solution should be kept at ba-
sic conditions. (ii) As the pKa of the 4-MU phenolic
proton is 7.8,16 it requires relatively high pH medium
(pH ca. 9) to achieve maximum fluorescence intensity.
(iii) Because the nucleophilic attack by hydroxide can
take place and thus leading to the cleavage of the sul-
fonate,8 a high background signal may be recorded un-
der high pH media.
In order to select the optimum conditions for the de-
termination of SNH with 4-MUDNBS, the effect of pH
was studied in the range 8.3—9.8, and the results are
shown in Figure 4. It can be observed that both the
fluorescence signal of reaction system and blank solu-
tion increased with increasing pH. It can also be ob-
served from Figure 4 (insert curve) that F/F0 increased
with increasing pH, obtained the maximum value at pH
8.9, and thereafter decreased. Hence, to obtain a high
F/F0 value, pH 8.9 of NH3-NH4Cl buffer was selected
for the fluorogenic reaction in the subsequent experi-
ments.
Figure 2 Time dependent fluorescence intensity changes ob-
served during reaction of 4-MUDNBS (or 4-MUNBS, 4-MUTS)
－ 1
with SNH (5.0 µg•mL ) in pH 8.9 NH3-NH4Cl buffer. (a)
4-MUDNBS; (b) 4-MUDNBS ＋ SNH; (c) 4-MUNBS; (d)
4-MUNBS＋SNH; (e) 4-MUTS; (f) 4-MUTS＋SNH. The con-
centrations of 4-MUDNBS, 4-MUNBS and 4-MUTS are 4.0
－1
µmol•L .
Effect of reaction time
The effect of reaction time on the fluorescence re-
sponse of 4-MUDNBS with SNH is demonstrated in
Figure 3. It can be seen that the fluorescence intensity of
the reaction system increased rapidly with the reaction
time prolonged. Meanwhile the blank signal also in-
creased slowly at the same conditions. The effect of the
reaction time on the F/F0 value was also investigated
and the results are shown in Figure 3. It can be seen that
F/F0 value of the reaction system increased rapidly at
the beginning, while the F/F0 value increased very
slightly after 30 min. Therefore, a 35 min reaction time
Study of interferences
In order to investigate the possibility of practical ap-
plication in determination of pharmaceutical preparation,
the interference from some metal ions and excipients
often contained in tablets, such as lactose, starch, and
mannitol, were tested in the chosen condition. The
effects of foreign substances were tested by analyzing a
－1
standard solution of SNH (5.0 µg•mL ) to which
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Chin. J. Chem. 2010, 28, 1469— 1474

A 4-Methylumbelliferone-based Fluorescent Probe for Sodium New Houttuyfonate
Figure 4 Effect of pH on the fluorescence intensity of
Figure 5 Linear plots of (F－F0) vs. SNH concentration over
－1
4-MUDNBS (4.0 µmol•L ) in the presence (a) and absence (b)
the entire dynamic concentration range. Other conditions were the
same as those described in the experimental section.
－1
of SNH (5.0 µg•mL ) in pH NH3-NH4Cl buffer solution. Inset:
fluorescence enhancement factor F/F0 as a function pH value.
Application
increasing amounts of interfering substances was added.
The to－lerable concentration ratios with respect to 5.0
Three synthetic samples of SNH with ions and ex-
cipients were determined under the optimal condition.
The recovery of the method was acquired using the
standard addition method by adding a known amount of
standard to the synthetic samples. As presented in Table
2, the determination results were satisfactory. From the
results presented it can be concluded that the proposed
method is a reliable method for the determination of
SNH.
1
µg•mL SNH for interference at 5% level are reported
in Table 1. From the result it was observed that most of
the foreign substances studied did not interfere in the
＋
determination of SNH, except for Cu2 , citrate and
ascorbic acid.
Analytical characteristics of 4-MUDNBS for SNH
The calibration graphs for the determination of SNH
with 4-MUDNBS were constructed under the optimum
experimental conditions. A very good linear relationship
(R＝0.998) was observed up to SNH concentration
Mechanism for the fluorescence enhancement
As is well known, there is a reversible equilibrium
reaction between SNH and sodium bisulfite in aqueous
solution (Scheme 1), SNH, lauroyl acetaldehyde and
sodium bisulfite are all present in the solution, and the
equilibrium lies toward lauroyl acetaldehyde and so-
dium bisulfite in basic media. To investigate whether the
dramatic increase in fluorescence intensity results from
－1
ranging from 0.5 to 15 µg•mL (Figure 5) and the de-
tection limit, calc－ulated following the IUPAC criteria,
1
was 0.15 µg•mL
(3σ). The standard deviation for
seven replicate measurements of a solution containing
－1
5.0 µg•mL SNH was 1.7%.
－1
Table 1 Effects of several cations and excipients on the fluorescent determination of SNH (5.0 µg•mL ) with 4-MUDNBS
－1
－1
Metal ion Coexisting concentration/(µmol•L ) Change of F/% Excipient
Coexisting concentration/(µg•mL ) Change of F/%
K＋
1500
1500
1500
150
3.21
－2.09
－0.56
－0.91
－3.78
－2.40
－0.21
Starch
Mannitol
Glucose
500
500
500
500
5
－0.40
－3.27
－1.12
－4.96
－0.44
1.10
＋
Mg2＋
Ca2
Cd2
＋
Sucrose
＋
Zn2＋
150
Citrate
Al3
Cu2
15
Ascorbic acid
5
＋
0.15
Table 2 Results for the determination of SNH in synthetic samples (n＝3)a
－1
－1
Number
Amount/(µg•mL )
Main in＋terferent
Amount found/(µg•mL )
RSD/%
1
2
3
5.00
5.00
5.00
K＋,＋Al3 ,＋starch, sucrose
4.78±0.03
0.6
3.9
0.7
Ca2 , Zn2 , mannitol, ascorbic acid
5.04±0.19
＋
＋
Cd2 , Mg2 , glucose, citrate
4.97±0.03
－1
a The concentration of all cations was 15 µmol•L . The concentration of starch, sucrose, mannitol, ascorbic acid, glucose and citrate was
－1
5.0 µg•mL .
Chin. J. Chem. 2010, 28, 1469— 1474
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Yang et al.
Surf., A: Physicochem. Eng. Aspects 2006, 279, 218.
FULL PAPER
the deprotection of an benzenesulfonate by sulfite, the
same concentration of SNH, lauroyl acetaldehyde and
sodium bisulfite were added to the 4-MUDNBS solution
and the fluorescence signals were recorded under iden-
tical conditions, respectively. It was found that no fluo-
rescence increase was observed for lauroyl acetaldehyde,
while a 1,2-fold higher fluorescence signal was obtained
by sodium bisulfite than that of SNH. Based on the
above results, we thus believe that the fluorescence in-
crease of the system is ascribed to SNH’s decomposing
product, sodium sulfite. Based on the above experiment,
a possible detection mechanism was given as shown in
Scheme 1. In basic media, SNH was decomposed to
produce sodium sulfite, which can then react with
4-MUDNBS to yield the highly fluorescent 4-MU and
hence leading to a dramatic increase in fluorescence
intensity.
2
3
4
5
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In summary, a 4-MU-based fluorogenic probe for
SNH was prepared and its fluorogenic behavior was
investigated. The method is based on the decomposition
of SNH to produce sodium sulfite, which then reacted
with 4-MUDNBS to yield the strongly fluorescent
4-MU, hence leading to the fluorescence increase of the
reaction solution. The method is proved to be simple,
selective and highly sensitive for SNH assay.
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Chin. J. Chem. 2010, 28, 1469— 1474