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Spectrochimica Acta Part A 72 (2009) 572–576

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

tudy of fluorescence quenching mechanism between quercetin andyrosine-H2O2-enzyme catalyzed product

iao Zhang, Qingluan Lv, Ningning Yue, Huaiyou Wang ∗

ollege of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, PR China

r t i c l e i n f o

rticle history:eceived 3 July 2008eceived in revised form 29 October 2008ccepted 30 October 2008

a b s t r a c t

Because of catalysis of horseradish peroxidase, the tyrosine reacted with H2O2 to form the product S whichwas a strong fluorescence substance. To the product S, the quercetin was acted as a quencher. The fluores-cence quenching mechanism was studied by the measurement of fluorescence lifetime and based on the

eywords:orseradish peroxidase (HRP)luorescence quenchinguercetinhermodynamic parameters

Stern–Volmer plot. The reaction mechanism, which was the static quenching process between quercetinand product S, was studied. The binding constant, K = 4.03 × 105 L mol−1 and the number of binding sitesn = 1.09, were obtained against this reaction. The thermodynamic parameters were estimated. The data,�H = −75.68 kJ mol−1, �S = −147.9 J K−1 mol−1 and �G = −29.17 kJ mol−1 showed that the reaction wasspontaneous and exothermic. What is more, both �H and �S were negative values indicated that van derWaals interaction and hydrogen bonding were the predominant intermolecular forces between quercetin

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ntermolecular force and product S.

. Introduction

In recent years, enzymatic assays have been widely used innalytic biochemistry because of their rapidity and high selectiv-ty. Horseradish peroxidase (HRP) is one of the most importantnzymes obtained from a plant source [1]. Now it is widely used inarious fields such as biotechnological applications [2,3], biosen-ors [4,5] and synthesis [6]. That the tyrosine reacted with H2O2o form the product S which was a strong fluorescence substancey the catalysis of horseradish peroxidase was reported [7,8], theeaction equation is shown in Fig. 1.

Quercetin is a family of compound called the flavonoid amongsthich hesperidin, eriodictyl and rutin. Quercetin, a natural sub-

tance found in plants, fruits and vegetables, possesses theroperties of vitamins, anti-oxidant effect [9,10], protective effectn human cells [11] and anti-inflammation [12]. It is important thatuercetin has preventive activity against various cancers [13,14]. Inecent years, researches showed that quercetin has many benefitsn human health by promoting a healthy immune and cardiovas-ular system as well as a very active antioxidant. The effect of

uercetin on liver disease has also been concerned [15]. The struc-ure of quercetin is shown in Fig. 2.

As all know, there are tyrosine and tryptophan residues in pro-ein [16]. Many researches showed that the tyrosine fluorescence

∗ Corresponding author. Tel.: +86 531 89880458; fax: +86 531 82615258.E-mail address: wanghuaiyou@sdnu.edu.cn (H. Wang).

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386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2008.10.045

© 2008 Elsevier B.V. All rights reserved.

as used to study the lipid-binding properties of protein. Further-ore, tyrosine is one of the hydrolysates of protein. Obviously,

he method we proposed in this paper has potential applicationn the interaction between pharmaceutica molecule and protein

olecule. Because of catalysis of horseradish peroxidase, the tyro-ine reacted with H2O2 to form the product S which was a stronguorescence substance. It was found that quercetin was a quencherf product S fluorescence. The quenching mechanism, a staticuenching process, was confirmed by the measurement of fluo-escence lifetime and based on the Stern–Volmer plot.

. Materials and methods

.1. Apparatus

The measurements of fluorescence lifetime were carried outn FLS-920 (Edinburgh, UK) spectrofluorimeter, equipped with aydrogen discharge lamp and 1.0 cm quartz cell. Other fluorescenteasurements were carried out on a LS-50 (Perkin-Elmer, Amer-

ca) spectrofluorimeter, equipped with a xenon discharge lamp and.0 cm quartz cell. A pH meter pHS-3C (Shanghai Leici Instrumentsactory, China) was used for pH adjustment. The deionized wateras made from Direct-Q 3 (Millipore, America) super water system.

.2. Reagents and solutions

l-Tyrosine (l-Tyr, BR) solution (10.0 �g mL−1). 3.0 × 10−3% (v/v)f H2O2 solution was stored in amber bottle. A 100 �g mL−1 solu-

M. Zhang et al. / Spectrochimica Acta Part A 72 (2009) 572–576 573

Fig. 1. Reaction equation.

Fig. 2. Structure of quercetin.

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Fig. 5. The fluorescence lifetime of product S.

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0.50 mL HRP (100 �g mL−1), 1.00 mL l-Tyr (10.0 �g mL−1),

ig. 3. Excitation and emission spectra of product S. Excitation spectra (a) and emis-ion spectra (b) of product S; excitation spectra (c) and emission spectra (d) ofroduct S in the presence of quercetin; concentration of quercetin: 5.00 �g mL−1.

ion of HRP (enzymatic activity: 250–300 U mg−1, Rz: 2.5–3.0).tock solution of quercetin (100 �g mL−1). The quercetin solu-

ion was diluted to 50.0 �g mL−1 before being used. The aboveolutions were all stored at 4 ◦C. A pH 8.85 Tris (hydroxymethylminomethane)–HCl buffer solution was adopted. Other reagentsere of analytical reagent grade.

Fig. 4. Stern–Volmer plot.

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Fig. 6. The fluorescence lifetime of product S in the presence of quercetin.

.3. Procedure

.050 mL H2O2 (3.0 × 10−3%), 2.00 mL Tris–HCl buffer solutionere transferred sequentially to a 10.0 mL volumetric flask, the

Fig. 7. lg[(F0 − F)/F] vs. lg[Q].

574 M. Zhang et al. / Spectrochimica Acta Part A 72 (2009) 572–576

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olution was diluted to the mark with water and kept stand-ng for 30 min, which was identified as solution A. Next, 1.00 mLuercetin (50.0 �g mL−1) was added to another 10.0 mL volumet-ic flask, other components were the same as in solution A, thisolution was identified as solution B. Then the excitation andmission spectra of A and B solutions were recorded and theelative fluorescence intensities were measured at 400 nm withhe excitation at 307 nm. The slit width (10 nm) and scan rate240 nm min−1) were constantly maintained for all the experi-

ents.

. Results and discussion

.1. The study of fluorescence quenching mechanism

.1.1. Excitation and emission spectraAccording to Section 2.3, the excitation and emission spectra

f product S in the absence and the presence of quercetin wereecorded. As shown in Fig. 3, the excitation and emission wave-ength were 307 and 400 nm, respectively. The relative fluorescencentensity of product S decreased in the presence of quercetin, whichhowed that the quenching of fluorescence occurred in the process.

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mechanism.

.1.2. Fluorescence quenching mechanismGenerally, several mechanisms can describe the nature of fluo-

escence quenching, such as dynamic quenching, static quenchingnd the combined static and dynamic quenching. In the case of theombined static and dynamic quenching, the Stern–Volmer plot isharacterized by a non-linear behavior with an upward curvature.he polynomial equation is as follows [17]:

0/F = 1 + (KD + KS) [Q ] + KDKS [Q ]2 (1)

here KD and KS are the dynamic and static quenching con-tants, respectively. F0 and F are the fluorescence intensities inhe absence and the presence of quencher. [Q] is the molar con-entration of quencher. The quercetin was used as quencher inhis experiment. The linear regression equation of the calibrationraph was F0/F = 0.9086 + 0.1979C (F0 and F are the fluorescencentensities in the absence and the presence of quercetin and C

as the concentration of quercetin) with the coefficient 0.9992.

he result in Fig. 4 indicated that the quenching mechanismetween quercetin and product S was not the combined staticnd dynamic quenching because the Stern–Volmer plot wasinear.

M. Zhang et al. / Spectrochimica Acta Part A 72 (2009) 572–576 575

Table 1The thermodynamic parameters.

T (K) 298 308 318 328 338

K 5 5 7.22 × 104 3.52 × 104 1.36 × 104

� −29.38 −27.90 −26.12� −75.68� −147.9

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5.60 × 10 2.25 × 10G (kJ mol−1) −31.60 −30.86H (kJ mol−1)S (J K−1 mol−1)

The dynamic quenching can be expected by the classicaltern–Volmer relationship [18]:

0/F = 1+kq�0 [Q ] = 1 + ksv [Q ] (2)

here kq is the bimolecular quenching rate constant in M−1 S−1, �0s the lifetime of the fluorophore in the absence of quencher, ksv ishe Stern–Volmer quenching constant in M−1. In this case, a linearlot of F0/F vs. [Q] will be obtained.

In the case of static quenching, the Stern–Volmer equation isbserved [18], giving a decrease of fluorescence intensity due tohe formation of nonfluorescence complex.

0/F = 1 + K [Q ] (3)

here K is the formation constant, the Stern–Volmer plot is linearoo.

The measurement of fluorescence lifetime can confirm aynamic or static quenching process. The lifetime (�0) of fluores-ence molecule on excited state has no change in the presence ofuencher if static quenching takes place. Reversely, �0 has to behorter if dynamic quenching occurs. That is, �0/�1 = 1 (�0 and �1 arehe fluorescence lifetimes of fluorescence molecule in the absencend the presence of quencher) for static quenching; �0/�1 = F0/F forynamic quenching [18]. In our research, the fluorescence lifetimesf product S in the absence and the presence of quercetin, �0 and1 were 4.98 and 4.97 ns, respectively. As shown in Figs. 5 and 6,0/�1 ∼= 1, therefore, we suggested that a static quenching processas occurring between quercetin and product S.

.2. The formation constant of static quenching

For the static quenching interaction, if there are some similarnd independent binding numbers in the fluorescence molecule,he following formula can be concluded between the fluorescence

olecule and quencher [19]:

Q + B → Qn − B (4)

here B is the fluorescence molecule, Q is the quenchable pharma-eutica molecule, Qn − B is the nonfluorescence molecule, Ka is theormation constant of the reaction:

a = [Qn − B]/([Q ]n [B]) (5)

B0] is the total concentration of fluorescence moleculeunbound and bound with the quenchable molecule), there-ore [B0] = [Qn − B] + [B], here [B0] is the concentration of unbounduorescent molecule. The relationship between fluorescence

ntensity and the concentration of the quenchable medicamentolecule is [B]/[B0] = F/F0, so there is the following equation:

g [(F0 − F)/F] = lg K + n lg [Q ] (6)

here K is the formation constant. From Eq. (6), n was the slopend lg K was the intercept. In our research, as shown in Fig. 7, theormation constant, K = 4.03 × 105 L mol−1 and the number of bind-ng sites n = 1.09, were obtained. The correlation coefficient of Eq.6) was 0.9994.

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Fig. 9. ln K vs. T−1.

.3. Study of reaction mechanism

The experiment result showed that the number of binding sitesere n = 1.09. We presumed that the amidocyanogen of product Sound with the carboxyl of quercetin [20,21]. The reaction mecha-ism was shown in Fig. 8.

.4. Thermodynamic parameters

The thermodynamic parameters, Gibbs free energy change�G), enthalpy change (�H) and entropy change (�S) of the reac-ion were obtained. The �H and �S were calculated from the slopend intercept of the van’t Hoff equation ln K = −�H/RT + �S/R. �Gas obtained according to the equation �G = �H − T�S. The resultsere shown in Fig. 9 and Table 1.

�G < 0, �H < 0 showed that the reaction was spontaneous andxothermic. What is more, both �H and �S were negative valuesndicated that van der Waals interaction and hydrogen bonding

ere the predominant intermolecular forces between quercetinnd product S [22].

. Conclusion

The fluorescence quenching mechanism between quercetinnd product S was studied. The reaction, which was the staticuenching process, was spontaneous and exothermic. Both �Hnd �S were negative values indicated that van der Waals inter-ction and hydrogen bonding played a major role in the bindingf quercetin to product S. In addition, the binding constant,he number of binding sites and the reaction mechanism were

btained.

cknowledgement

The work was financed by Natural Science Found of Shandongrovince Government (no. Y2006B31).

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