A novel pH-controlled transfer process of 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin in CTAB micelles

Journal of Colloid and Interface Science 302 (2006) 620–624 www.elsevier.com/locate/jcis

A novel pH-controlled transfer process of 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin in CTAB micelles Lin Guo Key Laboratory of Mesoscopic Chemistry of MOE and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China Received 23 April 2006; accepted 27 June 2006 Available online 28 July 2006

Abstract By analysis of the UV–visible and fluorescence spectra of 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl)porphyrin (P) in different microenvironments of micelle and solvent solutions, a novel pH-controlled transfer process of P in CTAB micelle was reported. In neutral CTAB micelles, porphyrins may locate at the inner layers of micelles. With pH increases to 11.19, the porphyrin can be completely deprotonated and transfers to the outer surface of CTAB micelle. The investigation of kinetics of porphyrin complexing with Cu(II) indicates that the metallation rate of porphyrins in CTAB micelles could also be controlled by changing pH. © 2006 Published by Elsevier Inc. Keywords: Porphyrin; CTAB; Micelle; Location; Metallation

1. Introduction Binding of porphyrin and metalloporphyrin guests to models for membrane hosts has attracted much interest due to the possibility of understanding many biological and photochemical processes, such as photosynthesis, oxygen transport, oxidation–reduction, and electron transport. It has been shown that porphyrins anchored to lipid bilayers and micelles could be applied successfully for reversible binding of dioxygen or nitric oxide in aqueous solutions [1,2]. Incorporation of porphyrins into micelles dramatically influences the aggregation mode and location of these molecules and alters their metallation rate and nitric oxide transfer rate [2–4]. Generally, it is suggested that hydrophobic porphyrins could penetrate the lipid regions of the membranes and distribute into protein-rich membrane domains [5], while highly polar species were assumed to partition mainly into the aqueous compartments [6]. Recently, the interaction of water-soluble synthetic porphyrins with ionic micelles and reversed micelles has been studied inE-mail address: [email protected]. 0021-9797/$ – see front matter © 2006 Published by Elsevier Inc. doi:10.1016/j.jcis.2006.06.048

tensively [4,7–9]. In the present of ionic surfactants below their critical micelle concentration (CMC), both water-soluble ionic porphyrins [10–13] and some meso-tetraaryl-substituted picket fence porphyrins [14] have been shown to form aggregates. Above the CMC, micelles are usually considered only as a means to solubilize the aggregates of porphyrin derivatives into monomers [7–10,15,16]. So far, however, a structural understanding is still unclear, especially for the solubilization site of porphyrinic molecules, because of their versatile substituents. Although some efforts have been made to study porphyrin aggregation and location in micelle surfactant solutions as well as aqueous solutions, most of them have concentrated on water-soluble porphyrinic molecules, and there are still few reports about controlling amphiphilic porphyrin transfer processes in CTAB micelles. On the other hand, metallation in microheterogeneous media has been less investigated despite its biological occurrence [17,18]. We have reported previously the results of studies of the location, metallation, and aggregation of series of amphiphilic porphyrins in nonionic and anionic micelles [19–22]. In this work, cationic cetyltrimethylammonium bro-

L. Guo / Journal of Colloid and Interface Science 302 (2006) 620–624

Fig. 1. Structure of the amphiphilic porphyrins P.

mide (CTAB) micelles were used as the simplest model for membranes and potential reaction centers to gain more insight into the nature of amphiphilic porphyrin guest interaction with biological structure hosts. 5,10,15-Tri(4-hydroxyphenyl)20-(4-hexadecyloxyphenyl) porphyrin (P) was solubilized in CTAB micelle solutions (see Fig. 1). The coexistence of the big porphyrin moiety and the long hydrophobic chains in the same molecule suggests that such molecules can be solubilized in organic solvents as well as in the nonpolar regions of micelles; their conjugated π electronic structures relate to the Soret band, and Q bands would not be affected by the substituted chains. Based on this property, we report a novel pH-controlled transfer process of P in CTAB micelles. 2. Experimental The amphiphilic porphyrins P, 5,10,15-tri(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin, were synthesized as reported in the literature [23–25]. CTAB was an analytical reagent and was recrystallized twice from 90% ethanol. Water was doubly distilled after passing through an ion-exchange resin column. All the organic solvents were analytical grade pure and used without further purification. CTAB micelle, aqueous, and organic–water mixture solutions of porphyrins were prepared by injection of a certain amount of 2.5 × 10−3 mol dm−3 dioxane solution of porphyrins into different solvents to obtain 25-ml solutions. The ratio of dioxane and water or other mixture solvents was greater than 1000, so the effects of trace of dioxane on the solution polarity could be neglected. After 20 min of sonication, the fluorescence spectra of solutions were recorded on a Shimudu UV-3100 spectrophotometer and a Perkin Elmer LS50B fluorescence spectrophotometer using a 1-cm quartz cell. The pH value of the solution was measured by a pH-250 pH meter and the pH value of solutions was controlled by 1.5 mol dm−3 NaOH and 1:4 HCl aqueous solutions. The kinetic processes of porphyrins incorporated with Cu(II) were studied at room temperature by adding a certain amount of 0.1 mol dm−3 CuSO4 aqueous solution into different pH porphyrin CTAB micelle solutions and then the UV–visible spectra of the mixture solutions at different reaction times were recorded.

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Fig. 2. UV–visible spectra of P in different pH CTAB micelle aqueous solutions. [P] = 2.2 × 10−6 mol dm−3 ; [CTAB] = 2.5 × 10−3 mol dm−3 .

3. Results and discussion 3.1. Deprotonization of P in CTAB micelles Fig. 2 shows the titration UV–vis spectra of P in CTAB micelle solutions. It can be seen from this figure that at pH 6.20, P shows a strong Soret band at 423 nm and four Q bands at 520, 560, 595, and 655 nm. With pH increasing to 11.19, the Soret band of porphyrin at 423 nm experiences obvious broading and decrease, while the four Q bands of P gradually disappear and two new Q bands appear at 585 and 675 nm. The number of Q bands observed changes from 4 to 2, indicating that P has a higher molecular symmetry (D4h ) similar to that of metal porphyrins [26]. However, the present system contains only one kind of metal ion, Na+ , that is difficult to complex. Hence, this phenomenon indicates that hydrogen atoms bonded to the pyrrole nitrogen atoms of the P moiety may also be deprotonized to form P5− ions (Fig. 3) similar to P in strong basic aqueous solutions when the bulk pH approaches 11.19 (see Table 1). It should be pointed out that the complete deprotonization of P could take place only in strong basic aqueous solutions (such as 1.5 mol dm−3 NaOH aqueous solution); it could not be observed in pH 11.19 aqueous solutions (see Table 1). However, it was found that the P moiety could be deprotonized to form P5− ions in pH 11.19 CTAB micelle solutions. This function is based on the surface potential properties of CTAB micelles. The relationship between surface potential and surface effective concentration of H+ (αH ) of micelle is shown by the equation [27] i w −F Ψ/RT = αH e , αH

(1)

i and α w are the effective concentrations of H+ ions where αH H in micelle surface and bulk aqueous phase respectively, F is Faraday constant, Ψ is the surface potential of micelle, T is temperature in Kelvin, and R is the Boltzmann constant. As we know, that cation surfactant CTAB micelle exhibits a posii  α w according to Eq. (1). tive surface potential (Ψ > 0), αH H This means that the pH value of the CTAB micelle surface is greater than that of the bulk aqueous phase. In fact, Fromherz and Masters [28] have found that the surface potential of CTAB micelles is about 148 mV, which corresponds to an increase

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Fig. 3. Deprotonization of P as the pH of the solution increases. Table 1 The UV–visible spectra data of porphyrins in different solutionsa Porphyrin

Solutions

pH

λmax

ε

P P P P P P ZnP ZnP

CTAB CTAB Methanol Water Water 1.5 M NaOH CTAB CTAB

6.20 11.19 – 11.28 6.20 – 6.20 11.60

423 441 419 435 428 436 430 443

2.61 × 105 1.25 × 105 3.48 × 105 0.97 × 105 0.91 × 105 1.09 × 105 3.09 × 105 1.87 × 105

Soret band

Q bands

W1/2

Q1

Q2

Q3

Q4

18 60 15 60 50 65 16 40

520

560

518 525 525

556 568 566

595 585 595 596 595 585 565 570

655 675 649 655 655 675 609 628

a Aqueous solutions, [P] = 2.2 × 10−6 mol dm−3 ; [CTAB] = 2.5 × 10−3 mol dm−3 ; [ZnP] = 1.2 × 10−6 mol dm−3 .

of 2.5 units in the intrinsic pH value at the surface of CTAB micelles compared to the bulk pH value. Therefore, CTAB micelles could provide a strong basic surface microenvironment for P although the bulk pH is not too high, and make P completely deprotonized to form hydrophilic P5− ions. 3.2. pH-controlled transfer processes of P in CTAB micelles 3.2.1. UV–visible spectra It is well known that absorption Soret bands of porphyrins are often sensitive to the nature of the local environment in micelle, solvent microenvironment, and aggregation forms [14,29]. From the analysis of the spectral parameters, such as peak position, intensity, and full width at half height of the Soret band of P in different solvent environments and in the micelle CTAB solutions, we can further conclude as to what is happening concerning the site solubilization of P in CTAB micelles in this pH titration process. It is clear from Fig. 2 and Table 1 that the absorption spectral characteristics of P in micelle CTAB solutions in neutral conditions are very similar to those of monomeric P in organic solvents rather than those of P in aqueous solutions. It can be suggested that the porphyrin moiety of P should be located in the inner layers of CTAB micelles with relative week polarity microenvironments. Such a solubilizing location is thermodynamically stable for the hydrophobic chain and porphyrin moiety, even though there are three hy-

droxyphenyl groups having a weak hydrophilic ability. However, the hydroxyphenyl groups of P can be deprotonized to form P− , P2− , and P3− ions and hydrogen atoms bonded to the pyrrole nitrogen atoms of P moiety may also be deprotonized to form P5− in basic solutions and the hydrophilic ability of P increased accordingly (Fig. 3). Therefore, the hydrophilic ability of P can be adjusted by changing the pH values. There is a force for the porphyrin macrocycle transferring out of the micelles. Furthermore, with the electronegativity of P increasing, there still exists strong electrostatic attractive action between P5− and the cation polar head of CTAB. In fact, with bulk pH increasing, decrease of Soret band intensity as well as increase of the full width at half height was observed, and the overall absorption characteristics of P in the pH 11.05 CTAB micelle are quite like those of P in basic aqueous solutions (Table 1). This phenomena implies that P may transfer to a strongly polar microenvironment in the outer surface of CTAB micelles when pH increases. Fig. 4 shows the scheme of this transfer process; it is clear from this figure that the porphyrin moiety may transfer to the outer surface of the CTAB micelle with increasing pH, while the hydrophobic chain part of P is still located at the inner layer of the micelle under basic conditions. 3.2.2. Fluorescence spectra It is well known that the inner cores of micelles show low polarity, and the effective dielectric constant of the Stern layer

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Fig. 4. Scheme of pH-controlled transfer process of P in CTAB micelles.

Fig. 5. (a) Fluorescence emission spectra of P in different pH CTAB micelle aqueous solutions. λex = 423 nm, [P] = 4.1 × 10−6 mol dm−3 ; [CTAB] = 2.5 × 10−3 mol dm−3 . (b) Plot of dielectric constants against the pH value of CTAB micelle solutions of P.

is around 36 [30], while the outer surfaces of micelles, as well as nearby bulk aqueous phases, exhibit strong polarity. In other words, the greater the distance from the hydrophobic center of the micelle is, the greater the microenvironment polarity is. The polarity-dependent fluorescence emission spectral characteristics are likely to be useful sensors of the nature of microenvironments of molecules in micelle systems, and the polar microenvironment of P in different solvents could be presented by the fluorescence intensity ratio of 610 and 660 nm R (I610 /I660 ) (see supporting materials). Fig. 5 shows the fluorescence emission spectra of P in CTAB micelle solutions with different bulk pH. It is clear from this figure that in bulk pH 6.20 solution, P shows a strong emission band at 660 nm and a shoulder band at 610 nm, which are similar to those of P in organic solvents with low polarity. With pH increasing to 11.26, the fluorescence emission band at 660 nm decreases rapidly and a weak broad band appears gradually at around 710 nm. Nonetheless, the shoulder band at 610 nm shows little influence with the change of the bulk pH. The overall fluorescence spectral characteristics of P are similar to those of P in high-polarity aqueous solutions. In this

paper a high surfactant concentration/porphyrin ratio was used to decrease any disturbance owing to porphyrin aggregation. Consequently, the variation with bulk pH of the fluorescence parameters of porphyrin will now be primarily owing to the change of the local microenvironment polarity of P in CTAB micelle. According to the R (I610 /I660 ) of P under different pH conditions, the corresponding microenvironment polarity of porphyrin in CTAB micelle could be obtained. Fig. 5b shows the plot of pH against the corresponding microenvironment dielectric constants of porphyrins in CTAB micelles. The detailed transfer process of P in CTAB micelles is clear from this plot. At pH 6.21, P may locate in inner layer of CTAB micelle with microenvironment dielectric constant (ε) around 36. With the bulk pH of the solution increasing to 11.26, the porphyrin hydrophilic moiety transfers to the outer aqueous surface of the CTAB micelle with strong polar microenvironment (ε ≈ 73). These results coincide with the transfer process illustrated in Fig. 4. The appearance of the new band at 710 nm indicates that the electric state of P has been changed as the deprotonization of hydrogen phenyl groups and pyrrole nitrogen atoms of porphyrin moiety has occurred under basic conditions. 3.3. Metallation of P in CTAB micelle solution Fig. 6 shows the regional UV–visible spectra of P incorporate with Cu(II) at different reaction times in bulk pH 6.20 CTAB micelle solutions. It is clear from this figure that with the reaction time increasing, the four Q bands of P decrease and finally disappear, while a new band appears at 544 nm and increases gradually. Furthermore, there appear two isosbestic points at 523 and 553 nm, respectively. The formation of isosbestic points indicates that only the relative concentration of reaction material and reaction product changes during the reaction process. This implies that the solubilizing location and polar microenvironment of P in CTAB micelles are not changed during metallation in neutral CTAB solution. Fig. 6b shows a plot of pseudo-first-order reaction rate constants of P complex with Cu(II) (kΨ ) against the pH value of CTAB micelle solutions. It can be seen from this plot that with the bulk pH increasing, the metallation rate constant of porphyrins increases accordingly. It is well accepted that Cu(II) is involved in bonding to porphyrin nitrogen atoms and promoted to a plane structure [31];

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tion with Cu(II) shows that the metallation rate of porphyrin increases with increasing pH, indicating that metallation rate could be controlled by changing pH. Acknowledgment This work was financially supported by the Natural Science Foundation of Jiangsu Province (No. BK2006556) Supporting materials The online version of this article contains additional supporting materials. Please visit DOI: 10.1016/j.jcis.2006.06.048. Fig. 6. (a) Regional UV–visible spectra of P incorporate with Cu(II) at different reaction times in pH 6.20 CTAB micelle aqueous solutions. (b) Plot of pseudo-first-order reaction rate constants of P complex with Cu(II) (kΨ ) against pH value of CTAB micelle solutions. [P] = 2.2 × 10−6 mol dm−3 ; [Cu2+ ] = 2.5 × 10−4 mol dm−3 .

at the same time, the Cu(II) iron diffuses into the inner nonpolar core of the micelle with difficulty [5,32]. Hence, the solubilized location of porphyrin in CTAB influences the metallation rate of porphyrin intensively. If the porphyrin moiety is located at the outer surface of the micelle, the probability of the porphyrin moiety meeting a Cu(II) ion becomes greater, so the metallation rate of porphyrin is greater. When the porphyrin moiety stays at the inner layer of the micelle, although the porphyrin could still form a complex with Cu(II), the metallation rate is relatively low. In this paper, the metallation rates of P were measured only in the mildly basic region (pH 6.20–9.50), and a low porphyrin concentration and high concentration of CTAB were used to prevent porphyrin aggregation. Hence, the variation of the metallation rate of P is mainly due to the changes of the solubilized location of P in CTAB micelles. In the case of neutral CTAB solution, porphyrin moiety stays at the inner layer of the micelle and the metallation rate is relatively low. As the pH of the solution increases, the porphyrin moiety transfers to outer layer of the micelle, and the probability of the porphyrin meeting Cu(II) ions becomes higher; hence, the metallation rate of porphyrin increases accordingly. 4. Conclusion By analyzing the fluorescence spectra of synthesized amphiphilic porphyrin (P) in different solvent environments and the relationship between the solubilizing location of the porphyrin in CTAB micelle and the microenvironment polarity, the aggregation, location, and metallation of porphyrin in CTAB micelles were studied. The well-designed porphyrin P is shown to involve a transfer process for the porphyrin moiety from the inner part of the CTAB micelle to the outer surface layer as the pH is increased. The kinetic study of porphyrin incorpora-

References [1] E. Tsuchida, H. Nishide, Top. Curr. Chem. 132 (1986) 63. [2] F.S. Vilhena, S.R.W. Louro, J. Inorg. Biochem. 98 (2004) 459. [3] S.C.M. Gandini, V.E. Yushmanov, I.E. Borissevitch, M. Tabak, Langmuir 15 (1999) 6233. [4] D.C. Barber, D.G. Whiten, J. Am. Chem. Soc. 109 (1987) 6842. [5] F. Ricchelli, S. Gobbo, G. Jori, G. Moreno, F. Vinzens, C. Salet, Photochem. Photobiol. 58 (1993) 53. [6] F. Ricchelli, G. Jori, Photochem. Photobiol. 44 (1986) 151. [7] K.M. Kadish, G.B. Maiya, C. Araullo, R. Guilard, Inorg. Chem. 28 (1989) 2725. [8] K.M. Kadish, G.B. Maiya, C. Araullo, J. Phys. Chem. 95 (1991) 427. [9] D.M. Togashi, S.M.B. Costa, A.J.F.N. Sobral, M.R. Gonsalves, J. Phys. Chem. B 108 (2004) 11344. [10] N.C. Maiti, S. Mazumdar, N. Periasamy, J. Phys. Chem. B 102 (1998) 1528. [11] T. Tominaga, S. Endoh, H. Ishimaru, Bull. Chem. Soc. Jpn. 64 (1991) 942. [12] R.H. Schmehl, D.G. Whitten, J. Phys. Chem. 85 (1981) 3473. [13] P.P. Mishra, J. Bhatnagar, A. Datta, J. Phys. Chem. B 109 (2005) 24225. [14] D.C. Barber, R.A. Freitag-Beeston, D.G. Whitten, J. Phys. Chem. 95 (1991) 4074. [15] S. Mazumdar, O.K. Medhi, S. Mitra, Inorg. Chem. 27 (1988) 2541. [16] J. Simplicio, Biochemistry 11 (1972) 2525. [17] D.K. Lavallee, Coord. Chem. Rev. 61 (1985) 55. [18] V.H. Rao, V. Krishhan, Inorg. Chem. 24 (1985) 3538. [19] L. Guo, Y.Q. Liang, Supramol. Chem. 16 (2004) 31. [20] L. Guo, Y.Q. Liang, Colloids Surf. A Physicochem. Eng. Aspects 216 (2003) 129. [21] L. Guo, Y.Q. Liang, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 59 (2003) 219. [22] L. Guo, Y.Q. Liang, Can. J. Chem. 80 (2002) 1655. [23] R.G. Little, J. Heterocycl. Chem. 18 (1981) 129. [24] Y.C. Tian, Y.Q. Wang, T. Huang, Y.Q. Liang, Bull. Inst. Chem. Rev. Kyoto Univ. 71 (1993) 86. [25] Y.Q. Wang, Ph.D. dissertation, Jilin University, Changchun, 1993. [26] M. Gouterman, J. Mol. Spectrosc. 6 (1961) 138. [27] P. Savitski, E.V. Vorobyova, I.V. Berezin, N.N. Ugarova, J. Colloid Interface Sci. 84 (1981) 175. [28] P. Fromherz, B. Masters, Biochem. Biophys. Acta 356 (1974) 270. [29] M. Vermathen, E.A. Louie, A.B. Chodosh, C. Sandra Reid, U. Simonis, Langmuir 16 (2000) 210. [30] J.H. Fendler, Membrane Mimetic Chemistry, Wiley, New York, 1982. [31] J.E. Falk, Porphyrins and Metal Porphyrins, Elsevier, New York, 1975. [32] G.M. Williams, R.F.X. Willams, A.J. Lewis, Inorg. Nucl. Chem. 41 (1979) 441.