Spectrometric Study on the Interaction of Dodecyltrimethylammonium

ARTICLE pubs.acs.org/Langmuir

Spectrometric Study on the Interaction of Dodecyltrimethylammonium Bromide with Curcumin Dan Ke, Xiaoyong Wang,* Qianqian Yang, Yumeng Niu, Shaohu Chai, Zhiyun Chen, Xueqin An, and Weiguo Shen School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: The interaction between dodecyltrimethylammonium bromide (DTAB) and curcumin has been studied in pH 5.0 sodium phosphate buffer using absorption and fluorescence measurements. With increasing DTAB concentration (CDTAB) from 0 to 20 mM, the absorption peak of curcumin at 430 nm, corresponding to the conjugated structure of curcumin, first weakens gradually into a shoulder but increases back into one peak with much higher absorption intensity. On the contrary, as CDTAB increases, the initial small absorption shoulder of curcumin at 355 nm, corresponding to the feruloyl unit of curcumin, first increases gradually into a clear peak but decreases back into one shoulder until almost disappeared finally. By remaining at nearly the same wavelength, the fluorescence of curcumin first decreases at CDTAB lower than 5 mM and then increases gradually up to CDTAB = 10 mM, which is followed by sharp increases of fluorescence intensity with marked blue-shifts at higher CDTAB. The values of anisotropy and microviscosity of curcumin obtained from the fluorescence polarization technique also showed pronounced changes at different surfactant concentrations. The interaction mechanisms of DTAB with curcumin have been presented at low, intermediate, and high surfactant concentrations, which is relating to interaction forces, surfactant aggregations, as well as structural alterations of curcumin.

’ INTRODUCTION Because many synthesized and natural drugs with poor aqueous solubility have possible application limitations in their formulation and delivery developments, surfactants are widely used in the drug industry in order to enhance drug aqueous solubility, maintain drug stability, control drug release and uptake, and improve bioavailability of drugs.1
3 Therefore, the knowledge of the mechanism of interaction between surfactants and drugs has important values for understanding of various surfactant actions. Curcumin is a natural polyphenolic compound that is isolated from the rhizome of turmeric (Curcuma longa). Chemically, curcumin is a diferuloylmethane molecule, containing two ferulic acid residues linked by a methylene bridge, as shown in Figure 1. While the β-diketone moiety undergoes keto
enol tautomerization, curcumin exists in solutions predominantly in the enol form.4,5 Research over the last few decades has shown that curcumin possesses a great variety of beneficial biological and pharmacological activities. Besides its effective antioxidant,6 antitumor,7 anti-inflammatory,8 anticarcinogenic9 and free radical scavenger properties,10 it is believed that curcumin is a potent agent against many diseases such as anorexia, coughs, diabetes, hepatic disorders, rheumatism, and Alzheimer disease.11
13 However, the major problem with curcumin is its reduced bioavailability.14 One reason for the low bioavailability of curcumin is that curcumin is poorly soluble in water at acidic and neutral pHs, which makes curcumin hard to absorb.15 Another cause of reduced bioavailability of curcumin is due to its limited stability in aqueous environments. Curcumin is stable at acidic pH but unstable at neutral and basic pHs, under which conditions r 2011 American Chemical Society

curcumin is degraded to trans-6-(40 -hydroxy-30 -methoxyphenyl)2,4-dioxo-5-hexanal, ferulic acid, feruloylmethane, and vanillin.16,17 So far, some possible ways, like surfactant micelles, are being used to improve the water solubility, stability, and bioavailability of curcumin.14 In comparison with the high insolubility in water at acidic and neutral conditions, it was reported that hydrophobic curcumin could be easily solubilized in micellar solutions up to about 40 times.18 At pH 5 and 8, Tønnesen et al. observed that surfactant micelles such as sodium dodecyl sulfate, Triton X-100, and tetradecyl trimethylammonium bromide were highly effective in stabilizing curcumin by nearly 1800 times.19 Kee et al. demonstrated that at pH 13 cationic surfactant micelles can greatly suppress alkaline hydrolysis of curcumin with a yield of suppression close to 90%.20 Moreover, Khopde et al. investigated the free radical scavenging ability of curcumin and its substituted derivative in neutral and cationic micellar solutions.21 Recently, Barry et al.22 and Hung et al.23 found that binding of curcumin with phospholipid micelles can significantly alter the microstructural properties of micelles. Barry et al. further suggested that curcumin molecules were inserted deep into the phospholipid bilayer in a transbilayer orientation by hydrogen bonding and hydrophobic interaction in a manner analogous to cholesterol.22 While curcumin has three pKa values at 8.38, 9.88, and 10.51 in aqueous solution, corresponding to deprotonation of the enol group and Received: September 14, 2011 Revised: October 13, 2011 Published: October 17, 2011 14112

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Figure 1. Chemical structures of the keto (a) and enol (b) forms of curcumin.

’ EXPERIMENTAL SECTION Materials. Dodecyltrimethylammonium bromide (DTAB), curcumin, 2,2-diphenyl-1-picrylhydrazyl (DPPH), and pyrene were purchased from Sigma
Aldrich Chemical Co. All samples were prepared in 10 mM sodium phosphate buffer at pH 5.0. The curcumin concentration was kept at 10 μM in all measurements. All other chemical reagents used were of analytical grade and water was double distilled. Electrical Conductivity Measurement. Electrical conductivity was used to determine the critical micelle concentration (cmc) of DTAB using a plot of electrical conductivity against surfactant concentration. The conductivity of DTAB solutions was measured as a function of surfactant concentration in pH 5.0 sodium phosphate buffer using a CLEAN CON500 Conductivity Meter. During the conductivity run the temperature of the solution was kept at 25
C. Absorption Spectra Measurement. While curcumin concentration was fixed at 10 μM, the absorption spectra of curcumin in 300
600 nm were recorded as a function of DTAB concentration in pH 5.0 sodium phosphate buffer using a Shimadzu UV-2450 spectrophotometer. All the measurements were conducted at 25
C by circulating water through the thermostated cuvette holder. Steady-State Fluorescence Measurement. Steady-state fluorescence measurements were performed with an Edinburgh FLS900 spectrofluorophotometer at 25
C by circulating water through the thermostated cuvette holder. The cmc of DTAB was first determined from

Figure 2. Variations of the electrical conductivity K (a) and the pyrene polarity ratio I1/I3 (b) with DTAB concentration CDTAB in pH 5.0 sodium phosphate buffer at 25
C. measurement of pyrene polarity index (I1/I3) as a function of DTAB concentration. I1/I3 is the ratio of the intensities of first and third vibronic peaks in the fluorescence emission spectrum due to pyrene. Pyrene was excited at 337 nm and the emission spectra were scanned from 350 to 500 nm. On the other hand, the emission spectra of curcumin were investigated as a function of DTAB concentration from 450 to 750 nm with the excitation wavelength at 420 nm. Fluorescence Polarization Technique. An Edinburgh FLS900 spectrofluorophotometer with parallel and perpendicular polarizers was used to determine the fluorescence anisotropy (r) of curcumin as a function of DTAB concentration. Curcumin was excited at 420 nm, and the emission spectra were scanned from 450 to 550 nm. The fluorescence intensities were obtained at 0

0
, 0

90
, 90

0
, and 90

90
angle settings at 25
C. The value of r was calculated according to25 r ¼ ðI||
GI^ Þ=ðI|| þ 2GI^ Þ

ð1Þ

where I and I^ are the fluorescence intensities of the emitted light polarized parallel and perpendicular to the exciting light, respectively, and G is the grating correction factor which is the ratio of sensitivities of the instrument for vertically and horizontally polarized light. Microviscosity (η) was calculated from r following the equation25 )

the phenol groups, English et al. showed that curcumin gives lower values of pKa in cationic surfactant micelles.24 Although this work has provided some insight into the molecular interactions through the investigation of physicochemical properties and bioactivities of curcumin in surfactant micelles, the mechanism of interaction between surfactant and curcumin especially at the surfactant concentrations prior to the micelle formation is still unclear. In the present work, the interaction of cationic surfactant dodecyltrimethylammonium bromide (DTAB) with curcumin has been spectroscopically studied. At a fixed curcumin concentration of 10 μM at pH 5.0, the addition of DTAB into curcumin with different surfactant concentrations was monitored by UV
vis absorption and steady-state fluorescence measurements. The anisotropy and microviscosity of curcumin were determined using the fluorescence polarization technique. The variations of intensity and wavelength of characteristic peaks in the absorption and the fluorescence spectra of curcumin together with the changes of microproperties of curcumin indicated significantly different interaction behaviors of DTAB with curcumin when bound DTAB molecules changed from the monomeric state to premicellar and micellar aggregates. Further testing on the free radical scavenging of curcumin in the presence of DTAB was also carried out.

η ¼ 2:4r=ð0:362
rÞ

ð2Þ

DPPH-Scavenging Activity Measurement. The radical-scavenging activity of curcumin with and without DTAB were examined according to the DPPH (2,2-diphenyl-1-picrylhydrazyl) method.26 Briefly, curcumin samples as a function of DTAB concentration in pH 5.0 sodium phosphate buffer were mixed with DPPH in the same buffer. The changes of maximum absorption of DPPH at 517 nm were recorded using a Shimadzu UV-2450 spectrometer at 25
C.

’ RESULTS AND DISCUSSION Micellization of DTAB in the Absence of Curcumin. First, we studied the micellization of DTAB without curcumin in pH 5.0 sodium phosphate buffer using electrical conductivity and steady-state fluorescence measurements. It is observed in parts a and b of Figure 2, respectively, that as DTAB concentration (CDTAB) increases, while the electrical conductivity (K) has a higher increasing rate below the cmc than above the cmc, the plot of the pyrene polarity ratio I1/I3 with CDTAB has the sigmoidal shape with a rapid decrease of I1/I3 at CDTAB slightly below the cmc and a leveling off at high surfactant concentrations. As 14113

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Figure 3. Absorption spectra of curcumin at various CDTAB in pH 5.0 sodium phosphate buffer at 25
C.

shown in Figure 2, the cmc of DTAB can be taken as the concentrations corresponding to the interceptions of two furnished straight lines in the curves of K vs CDTAB and I1/I3 vs CDTAB. The values of cmc obtained by electrical conductivity measurement and steady-state fluorescence method are 14.6 and 14.3 mM, respectively, in agreement with the literature value.27 Because surfactant monomers can self-aggregate into surfactant micelles when the surfactant concentration is above cmc, it is expected that the interaction of DTAB with curcumin may possibly exhibit different behaviors at surfactant concentrations below and above the cmc. During the following studies, the concentration of curcumin is kept constant at 10 μM and CDTAB is 0, 0.4, 0.8, 1, 3, 5, 10, 12, 15, 20 mM, respectively. The absorption and the fluorescence measurements are used to disclose the mechanism of interaction between DTAB and curcumin upon increasing surfactant concentrations. UV
Vis Absorption Spectra of Curcumin. The UV
vis absorption spectral properties of curcumin as a function of CDTAB were investigated to examine the interaction of DTAB with curcumin. Figure 3 depicts the absorption spectra of curcumin at various CDTAB in sodium phosphate buffer of pH 5.0. The absorption curve of curcumin alone is characterized by a broad peak at 430 nm with a small shoulder at 355 nm, which is similar to the absorption features of curcumin in aqueous buffers.28,29 However, it is noted that the absorption spectrum of curcumin has striking changes upon adding of DTAB. As CDTAB increases from 0 to 5 mM, the absorption peak at 430 nm loses its intensity gradually into a broad shoulder, while the shoulder at 355 nm increases gradually into a clear peak. It is also observed that the new absorption peak at 355 nm beyond 0.8 mM DTAB has somewhat red-shifts when CDTAB increases from 0.8 to 5 mM. However, when CDTAB = 10 mM, the peak that previously appeared at 355 nm partly loses intensity and turns into a shoulder again, whereas the absorption at 430 nm is enhanced into a peak together with a shoulder at 440 nm. At CDTAB = 12
20 mM, curcumin with higher DTAB concentration presents more pronounced absorption maximum at 430 nm and shoulder peak at 440 nm, whereas the shoulder at 355 nm is almost disappeared. Moreover, the absorption peaks at 430 nm at CDTAB higher than 10 mM move to smaller wavelengths compared with smaller surfactant concentrations of 0
1 mM DTAB. Because the intramolecular exciton coupling occurs between the electric dipole transition moments of two feruloyl chromophores, there allows the low-energy π
π* excitation of the conjugated curcumin structure.5 Therefore, as shown in Figure 3, curcumin without DTAB has an absorption peak at 430 nm and a shoulder at 355 nm, corresponding to the absorptions of

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conjugated diferuloyl structure and feruloyl unit, respectively.30,31 Meanwhile, the very weak electronic dipole forbidden n
π* transition of curcumin is located somewhere above 300 nm, but it cannot be identified due to the strong masking effect of the neighboring absorption bands.5 In addition, the absence of an isosbestic point in the absorption curves of curcumin alone and with DTAB also supports that there are more than two absorption states for curcumin. It is known that the absorptions at 430 and 355 nm in the absorption spectrum of curcumin originated from the π
π* excitations of conjugated curcumin and feruloyl unit, respectively. The opposite changes of the absorption bands at 430 and 355 nm at CDTAB lower than 12 mM therefore suggest that the binding of DTAB with curcumin can disrupt or recover the conjugation characteristics of curcumin. If the binding of DTAB with curcumin occurs at the active groups in the aromatic moiety of curcumin, the added surfactant is expected to always decrease the absorptions of curcumin at both 430 and 355 nm. However, the observation of opposite changes of two absorption bonds at 430 and 355 nm is inconsistent with this speculation. It was calculated by Zsila et al. that the β-diketone group has the maximum electron density in the structure of curcumin.30 Moreover, the β-diketone moiety of curcumin can generally chelate cationic metal ions such as Na+, Cu2+, Mg2+, and Al3+.28,32 Therefore, we can consider that the positively charged headgroup of DTAB may electrostatically interact with the β-diketone group of curcumin to form DTAB/curcumin complexes. Similar to the reduction of the π-orbital overlap between two feruloyl moieties of curcumin in protein/curcumin complexes,4 the binding of DTAB molecule on the central methylene bridge of two ferulic acids may decrease the extended aromatic conjugation of the planar geometry of curcumin. As a consequence, upon increasing CDTAB from 0 to 5 mM, the absorption peak at 430 nm decreases gradually into a shoulder, but the absorption intensity at 355 nm increases gradually, leading the shoulder to become a peak at 355 nm. However, when CDTAB = 10 mM, the inverse changes of absorption curve of curcumin compared with lower surfactant concentrations indicate the departure of the headgroup of DTAB from the β-diketone group of curcumin and the recovery of the conjugation structure of curcumin. Owing to the strong hydrophobicity of aromatic groups of curcumin, hydrophobic interaction is generally observed when curcumin interacts with micelles22,24 and proteins.18 At CDTAB = 10 mM, the alkyl chains of added DTAB may hydrophobically bind with the hydrophobic aryl groups of curcumin, which leads previously β-diketone-bound DTAB to leave the β-diketone group of curcumin gradually. Meanwhile, owing to the hydrophobic interaction, the bound alkyl chains of DTAB molecules may start to aggregate at the aryl groups of curcumin to form small DTAB premicelles, similar to the pure surfactant system without curcumin. At CDTAB = 12
20 mM, the bound DTAB molecules may gradually aggregate into DTAB micelles, and curcumin molecules are solubilized inside DTAB micelles, which is consistent with the reported systems of curcumin with surfactant micelles.19,20,24 The pronounced peak at 430 nm and the almost disappeared characteristics at 355 nm can reveal that curcumin in DTAB micelles recovers the conjugated structure again. Furthermore, the photophysical properties of curcumin were found to be very sensitive to the polarity of the medium.33 As seen in Figure 2b, pyrene polarity index I1/I3 shows big and constant values at CDTAB lower than 5 mM and decreases sharply at CDTAB = 10
15 mM, which is followed by small and constant 14114

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Figure 5. Steady-state fluorescence spectra of curcumin at various CNaBr in pH 5.0 sodium phosphate buffer at 25
C.

Figure 4. Steady-state fluorescence spectra of curcumin at various CDTAB in pH 5.0 sodium phosphate buffer at 25
C.

values beyond CDTAB = 15 mM. Therefore, the observed blueshifts of absorption maximum at 430 nm above 10 mM DTAB compared with CDTAB = 0
1 mM should have resulted from the lower polarity of DTAB premicelles or micelles. Furthermore, while the absorption peak of π
π* transition has a blue-shift, the absorption of the n
π* transition of curcumin have a red-shift.34 The separation of two absorption bands of curcumin is indicated by the peak at 430 nm and the shoulder at 440 nm at CDTAB = 10
20 mM. In addition, higher absorption values of curcumin at 430 and 440 nm are observed at larger surfactant concentration, which also supports that DTAB micelles with lower polarity can promote the absorption of solubilized curcumin. Steady-State Fluorescence Spectra of Curcumin. Figure 4 shows the fluorescence spectra of curcumin in pH 5.0 sodium phosphate buffer at various DTAB concentrations. It can be seen that curcumin without surfactant emits relatively weak fluorescence with an emission peak at 570 nm. Although the emission of curcumin remains at nearly the same wavelength, the addition of DTAB at CDTAB lower than 5 mM causes the fluorescence intensity of curcumin to decrease. It is also noted that there is a small increase in the fluorescence intensity at CDTAB = 10 mM but with an obvious blue-shift related to curcumin without surfactant. With further increasing CDTAB from 12 to 20 mM, curcumin generally has high fluorescence intensity, that is, the higher CDTAB value corresponds to higher fluorescence intensity. As discussed above, DTAB molecules form premicelles or micelles in the presence of curcumin when CDTAB is higher than 10 mM. Compared with CDTAB lower than 5 mM, the clear blueshifts at CDTAB = 10
20 mM could be ascribed to lower polarity of DTAB premicelles or micelles. Especially at CDTAB = 15
20 mM, the pronouncedly high values of fluorescence intensities obviously indicate that DTAB aggregates can greatly enhance the fluorescence of curcumin owing to the nonpolar-like micellar environment. Upon comparing with curcumin without surfactant, although the fluorescence intensity of curcumin shows markedly enhanced values at CDTAB more than 10 mM, it is interesting to see that the addition of small amounts of DTAB at CDTAB within 5 mM can reduce the fluorescence intensity of curcumin. This change in the fluorescence intensity of curcumin is consistent with the gradual decreasing of absorption intensity of curcumin at 430 nm in Figure 3 when CDTAB increases from 0 to 5 mM. As discussed previously, the headgroup of DTAB may electrostatically bind

Figure 6. Variations of the anisotropy r (a) and the microviscosity η (b) of curcumin with CDTAB in pH 5.0 sodium phosphate buffer at 25
C.

with the β-diketone group of curcumin to form complexes at low surfactant concentrations. Meanwhile, the bromide ion of DTAB may also affect the fluorescence property of curcumin. Some investigators early reported that halogen ions can quench the fluorescence compounds on the basis of a heavy atom effect.35 In the present system, the fluorescence spectra of curcumin at various NaBr concentrations were measured as the control experiment. As shown in Figure 5, when the concentration of NaBr increases from 0 to 5 mM, there is a decreasing change in the fluorescence intensity of curcumin. But the decreasing degree of fluorescence of curcumin with NaBr here is smaller than that of curcumin with DTAB in Figure 4 in the same range of surfactant concentrations. This fact may indicate that the reduced fluorescence intensity of curcumin at CDTAB within 5 mM is mostly due to the formation of DTAB/curcumin complexes and partly because of the quenching effect of the bromide ion of DTAB. Moreover, when CDTAB is higher than 10 mM, the markedly enhanced hydrophobic environment of DTAB premicelles and micelles can result in the significant enhancement in fluorescence intensity with clear blue-shifts. In this case, the bromide ions are left outside DTAB aggregates and do not exhibit quenching on the fluorescence of curcumin anymore. Fluorescence Polarization Technique. Fluorescence polarization measurement can provide the information on the anisotropy (r) and the microviscosity (η) of the environments of curcumin. Figure 6 presents the changes of the values of r and η of curcumin as a function of CDTAB. When CDTAB increases from 0 to 10 mM, r values present a descending trend and η values do 14115

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Figure 7. Simplified mechanism for interaction of DTAB with curcumin.

not have significant changes except at CDTAB = 0 mM. However, as CDTAB increases from 10 to 20 mM, there are sharp increases both in the values of r and η. The anisotropy of curcumin indicates the extent of restriction of the rotation of curcumin molecule. The higher r value reveals greater restriction of the rotation of curcumin molecule. Considering the changes of I1/I3 values during the aggregation of DTAB shown in Figure 2b, we could attribute the sharp increase of the values of η above CDTAB = 10 mM to the aggregation of DTAB molecules. The formations of DTAB premicelles and micelles will allow curcumin to exhibit higher r values. A similar increase in r values was reported in the case of curcumin bound to proteins.33 As CDTAB is lower than 10 mM, although DTAB molecules may electrostatically bind with curcumin, the nearly constant η values of curcumin reveals that bound DTAB molecules still remain in their monomeric form, which is also indicated by the high and constant I1/I3 values at these surfactant concentrations shown in Figure 2b. Hence, the decreasing r values with increasing CDTAB from 0 to 10 mM should not be caused by the aggregation of DTAB molecules but could be likely attributed to the enhanced fluorescence properties of the feruloyl unit owing to the binding of DTAB with curcumin. Interaction Mechanism of DTAB with Curcumin. While curcumin has two aryl rings with several active groups including β-diketone, methoxy, and hydroxyl groups, DTAB has a cationic headgroup and alkyl chain. There are possibly electrostatic interactions, hydrophobic forces, and hydrogen bonding between DTAB and curcumin. The results of the absorption and the fluorescence spectra suggest that the interaction of DTAB with curcumin is different at surfactant concentrations where DTAB molecules remain in the monomeric form, start to aggregate into DTAB premicelles, and finally form general DTAB micelles. Figure 7 shows an illustration at three surfactant concentration ranges, which are coincident with the micellization of DTAB without curcumin. Although the curcumin structure remains un-ionized under the experimental conditions of pH 5.0, which is lower than the smallest pKa value of curcumin (pKa = 8.38), the β-diketone group was calculated to have maximum electron density in the curcumin molecule.30 At low surfactant concentrations of CDTAB = 0
10 mM, it is thought that the positively charged headgroup of DTAB may electrostatically interact with the β-diketone group of curcumin, forming DTAB/curcumin complexes. The binding of surfactant molecule on the central methylene bridge of curcumin decreases the interactions between two feruloyl moieties. So, as CDTAB increases, the formation of DTAB/curcumin complexes may reduce the absorption of conjugated π
bond characteristics at 430 nm but enhance the absorption of feruloyl units at 355 nm. Because the bound DTAB molecules remain at their monomeric state, there is almost no aggregation among DTAB molecules.

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Figure 8. DPPH absorption at 517 nm as a function of CDTAB in pH 5.0 sodium phosphate buffer at 25
C: (a) DPPH alone, (b) DPPH with curcumin, (c) DPPH with curcumin and 5 mM DTAB, (d) DPPH with curcumin and 10 mM DTAB, (e) DPPH with curcumin and 20 mM DTAB.

Considering that I1/I3 values of DTAB without curcumin at these low surfactant concentrations remain constant and high, we can expect that the binding of DTAB with curcumin does not change the polarity of the environment of curcumin very much. Hence, the fluorescence intensity of curcumin still remains at the low values with almost same wavelengths at these surfactant concentrations. At intermediate surfactant concentrations of CDTAB = 10
15 mM, due to the hydrophobic interaction, the alkyl chains of added DTAB may bind with the aryl groups of curcumin. The hydrophobic interaction among the alkyl chains of DTAB may overcome the electrostatic interaction of headgroup of DTAB with the β-diketone group of curcumin, which may gradually lead the headgroup of DTAB to leave the β-diketone group of curcumin. At the same time, the hydrophobic alkyl chains of different DTAB molecules bound at the aryl group of curcumin may start to aggregate into small DTAB premicelles. The release of β-diketone group of curcumin will help to recover the conjugated structure of curcumin, leading to the enhanced absorption of curcumin at 430 nm but the reduced absorption at 355 nm. The formation of DTAB premicelles may result in the greatly decreased polarity of curcumin, indicated by the changes of I1/I3 values at CDTAB = 10
15 mM shown in Figure 2b. The polarity changes may lead to the marked increasing of fluorescence intensity of curcumin with some blue-shifts at these intermediate surfactant concentrations. Finally at high surfactant concentrations of CDTAB = 15
20 mM, DTAB micelles are formed. Both absorption and fluorescence measurements showed that curcumin is located inside DTAB micelles. In DTAB micelles, curcumin molecule totally recovers the conjugated structure, which will increase the absorption at 430 nm and the fluorescence due to the high hydrophobic micellar environment. Curcumin was reported to be trapped in the palisade layer of the surfactant micelles.36 Similar to curcumin encapsulated in the phosphate bilayer22 and other surfactant micelles,34 curcumin is likely to be trapped inside DTAB micelles in the trans-type orientation, that is, one phenoxy group resides close to the micellar surface, while the other phenoxy group and the β-diketone group are buried near the hydrophobic core of the micelles. In addition to the hydrogen bonding of the phenoxy group with the headgroup of DTAB, there are hydrophobic interactions between surfactant alkyl chains and the aryl groups of curcumin. DPPH-Scavenging Activity of Curcumin. The DPPHscavenging method has been employed to further test the interaction of DTAB with curcumin at some selected surfactant 14116

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Langmuir concentrations. Figure 8 gives the values of maximum absorption of DPPH at 517 nm in the presence of curcumin at CDTAB = 0, 5, 10, and 20 mM compared with DPPH alone. It is observed that the mixture of curcumin with DPPH at CDTAB = 0 mM can reduce the absorption of DPPH about 50%. This strong scavenging ability of curcumin is usually attributed to the donation of H from the β-diketone group of curcumin to DPPH.37 Although the additions of 5 and 10 mM DTAB lead to smaller absorption of DPPH compared to free DPPH, it is seen that the absorbance values of DPPH at 5 and 10 mM DTAB are quite higher than the value at CDTAB = 0 mM. This result reveals that the addition of DTAB plays a minor role in the reduction of DPPH absorption by curcumin at these two surfactant concentrations. As discussed above, the β-diketone group of curcumin is totally or partly bound with the headgroup of DTAB at low or intermediate surfactant concentrations, which may prevent curcumin from donating H to DPPH. On the contrary, when CDTAB = 20 mM, the absorbance of DPPH shows the lowest value among five samples. This observation suggests that the β-diketone group of curcumin, which is loaded near the hydrophobic core of DTAB micelles, has the strongest H-donating ability to reduce radical DPPH.

’ CONCLUSIONS The measurements of absorption and fluorescence spectra of curcumin have revealed that the interaction of DTAB with curcumin is significantly different upon increasing surfactant concentrations. At low surfactant concentrations of CDTAB = 0
10 mM, the headgroup of DTAB may electrostatically bind with the central β-diketone group of curcumin to form DTAB/curcumin complexes. When the addition of DTAB reaches intermediate surfactant concentrations of CDTAB = 10
15 mM, the alkyl chains of DTAB hydrophobically bound with the aryl groups of curcumin start to aggregate into DTAB premicelles, which may lead the previously electrostatically bound headgroup of DTAB to leave the β-diketone group of curcumin. At high surfactant concentrations of CDTAB = 15
20 mM, DTAB micelles are formed, and curcumin is totally solubilized inside. This work reveals that the interaction of DTAB with curcumin is involved with the changes of driving forces, surfactant aggregations, as well as structural alterations of curcumin, which has important influences on the physicochemical properties and the bioactivities of the natural drug curcumin. ’ AUTHOR INFORMATION Corresponding Author

*Tel: 86-21-64252012. E-mail: [email protected].

’ ACKNOWLEDGMENT This project is sponsored by the National Natural Science Foundation of China (Grant 21173081), the Natural Science Foundation of Shanghai (Grant 11ZR1408600), the Fundamental Research Funds for the Central Universities (Grant WK0914037), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China. ’ REFERENCES (1) Zhang, H. X.; Annunziata, O. Langmuir 2008, 24, 10680–10687. (2) Bhat, P. A.; Rather, G. M.; Dar, A. A. J. Phys. Chem. B 2009, 113, 997–1006.

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