Article pubs.acs.org/Langmuir
Interaction of FeIII-Tetra-(4-sulfonatophenyl)-porphyrin with Copolymers and Aggregation in Complex Micelles Lizhi Zhao,† Ang Li,‡ Rui Xiang,‡ Liangliang Shen,‡ and Linqi Shi*,‡ †
State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, People’s Republic of China ‡ Key Laboratory of Functional Polymer Materials, Ministry of Education, and Institute of Polymer Chemistry, Nankai University, Tianjin 300071, People’s Republic of China ABSTRACT: Aggregation of FeIII-tetra-(4-sulfonatophenyl)-porphyrin (FeIIITPPS) was studied in the presence of block copolymers, poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP), poly(ethylene glycol)block-poly(2-(dimethylamino)ethyl methylacrylate) (PEG-b-PDMAEMA), and poly(ethylene glycol)-block-poly(β-cyclodextrin) (PEG-b-PCD). The interaction between the iron porphyrin and the blocks, P4VP, PDMAEMA, and PCD, led to the formation of copolymers/FeIIITPPS complex micelles with a PEG shell and determined the species of FeIIITPPS. The electrostatic interaction of protonated P4VP and PDMAEMA with FeIIITPPS remarkably decreased the apparent pKd of FeIIITPPS and led to a micellar μ-oxo dimer of the iron porphyrin. At pH above the pKa of P4VP, FeIIITPPS was kept inside the hydrophobic P4VP core and formed an encapsulated μ-oxo dimer. However, when above the pKa of PDMAEMA, FeIIITPPS escaped from the hydrophobic PDMAEMA core, existing as a free μ-oxo dimer. PCD caused the monomer of the porphyrin because of the inclusion complexation between the β-cyclodextrin residues and FeIIITPPS. The two micellar monomer species FeIIITPPS(H2O)2 and FeIIITPPS(OH) were obtained with an equilibrium pKa ∼ 6.4.
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INTRODUCTION Porphyrins have been well-investigated in recent years because of their special structural, optical, and electrical properties.1 Water-soluble tetra(4-sulfonatophenyl)-porphyrin (TPPS) has attracted more attention for a wide variety of applications in medicine and materials chemistry.2 Metal derivatives of TPPS, in particular Fe III -tetra(4-sulfonatophenyl) porphyrin (FeIIITPPS), are known to be considered as prototypes for tumor-specific contrast agents in radiological and magnetic resonance imaging3 and raw materials to develop artificial O2 carriers (AOCs) by simulating natural hemoglobin (Hb) or myoglobin (Mb).4,5 As model catalysts of cytochrome P-450, FeIIITPPS has also been used in many chemical reactions,6,7 while the iron porphyrin aggregation may greatly influence the activity as sensitizers,8 contrast agents,9 O2 carriers,10 and catalysts.11 Therefore, the examination of the aggregation properties of FeIIITPPS is quite important in view of their biomimetic, medical, and catalysis applications. In aqueous solutions, the aggregation of FeIIITPPS has been proven to be μ-oxo dimerization upon alkalization, with a pKd around 7.8.12,13 Contaminants may influence the aggregation. Yushmanov et al. investigated the solution properties and aggregation of FeIIITPPS on proteins in charge and non-ionic surfactant micelles. Moreover, they detected the presence of different species.13−15 It has been reported in many studies that β-cyclodextrins (β-CDs) with a hydrophobic cavity can inhibit μ-oxo dimerization of FeIIITPPS by forming an inclusion © 2013 American Chemical Society
complex, where sulfonatophenyl groups of the porphyrin are included inside β-CD cavities.16,17 Block copolymers with controlled architectures and narrow molecular weight distributions are apt to self-assemble into micelles under certain conditions. Such polymeric micelles, which have the potential applications as nanoreactors, drug and gene delivery devices, and structure directive templates,18−20 may serve well as catalyst or photosensitizer carriers and confined reactors for FeIIITPPS. However, interactions of FeIIITPPS with block copolymers and aggregation of the metalloporphyrin in polymeric micelles have not been investigated thus far. In our previous studies, we reported that the interaction between TPPS and poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP) led to micellization of the copolymer and an enhanced aggregation of TPPS.21,22 In the present work, we discussed the aggregation of FeIIITPPS in complex micelles. Three kinds of double hydrophilic block copolymers (DHBCs) are used in this work: PEG-b-P4VP and poly(ethylene glycol)block-poly(2-(dimethylamino)ethyl methylacrylate) (PEG-bPDMAEMA) with pH-sensitive blocks and poly(ethylene glycol)-block-poly(β-cyclodextrin) (PEG-b-PCD) with a block rich in β-CD units. The resultant complex micelles, interactions Received: May 19, 2013 Revised: June 14, 2013 Published: June 15, 2013 8936
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Scheme 1. Structures of the Block Copolymers and FeIIITPPS
Preparation of Copolymer/FeIIITPPS Complex Micelles. The FeIIITPPS solution was prepared by dissolving in the neutral Milli-Q water (18 MΩ), stored in the dark and used within a day after preparation. The concentration of the FeIIITPPS solution (50 μmol/L) was determined spectrophotometrically using ε393 = 1.52 × 105 L mol−1 cm−1 at the Soret maximum from an aliquot of the stock solution diluted 1/5 by volume with 0.1 mmol/L phosphate buffer.29 The block copolymer PEG-b-PCD was dissolved in the neutral Milli-Q water. PEG-b-P4VP and PEG-b-PDMAEMA were dissolved in water at pH 3. All of the copolymer solutions were prepared 1 day in advance. Micellization was carried out by adding a certain volume of the FeIIITPPS stock solution into the copolymer solutions, which were diluted with 50 mmol/L acetate−phosphate buffer. The complex micelle solutions of different pH values were obtained by adjusting the initial micellar solutions at pH 3 with 0.2 mol/L HCl or NaOH solution. Characterizations. The micelle solutions for DLS analysis were prepared by filtering samples (about 1 mL) through a 0.45 μm Millipore filter into a clean scintillation vial. The DLS measurements were performed on a laser light scattering spectrometer (BI-200SM) equipped with a digital correlator (BI-10000AT) at 532 nm. The measurements of the 1H NMR spectra were performed on a Varian UNITY plus 400 MHz NMR spectrometer. UV−vis absorption spectra were measured on a TU-1810 UV−vis spectrophotometer (Purkinje General, China). CD spectra were recorded by a Hitachi Jasco-J710 CD spectrophotometer (Japan).
between these blocks and FeIIITPPS, and different species of the iron porphyrin are investigated with the aid of dynamic light scattering (DLS), nuclear magnetic resonance (NMR) spectrometry, circular dichroism (CD), and ultraviolet−visible (UV−vis) spectrophotometry.
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EXPERIMENTAL SECTION
Materials. FeIIITPPS was purchased from Frontier Scientific as a chloride of the acid form, FeIII-tetra(4-sulfonatophenyl)porphyrin chloride, and used as received. β-Benzyl-L-aspartate (BLA) was obtained by the esterification reaction of L-aspartic acid and benzyl alcohol with a catalyst of H2SO4 (60%). β-Benzyl-L-aspartate Ncarboxyanhydride (BLA-NCA) was synthesized according to the literature with a yield of 73.8%.23 α-Methoxy-ω-amino-polyethylene glycol (PEG-NH2) was synthesized by the reaction of tosylation PEGOH (PEG-OTs; Mw = 2000) and potassium phthalimide (PPI) and hydrazinolysis afterward. Mono-6-OTs-β-CD used in the research was synthesized according to the method reported in the literature.2 Synthesis of PEG-b-P4VP and PEG-b-PDMAEMA. The block copolymers PEG-b-P4VP24 and PEG-b-PDMAEMA25 were synthesized by atom transfer radical polymerization (ATRP) of 4VP and DMAEMA with PEG-Br (Mw = 2000) as a macroinitiator. The composition of the block copolymers (PEG45-b-P4VP23 and PEG45-bPDMAEMA68) was characterized by 1H NMR spectrum in CDCl3 using the PEG blocks as the inner standard. The polydispersity indexes (Mw/Mn) of the block copolymer were characterized to be 1.15 and 1.27 by gel permeation chromatography (GPC) using N,Ndimethylformamide (DMF) as the eluent and narrowly distributed poly(methyl methacrylate) as the calibration standard. Synthesis of PEG-b-PCD. Copolymer poly(ethylene glycol)-blockpoly(β-benzyl-L-aspartate) (PEG-b-PBLA) was synthesized by ringopening polymerization (ROP).26 Briefly, BLA-NCA was polymerized under nitrogen in CH2Cl2 at 30 °C by the initiation from the terminal primary amino group of PEG-NH2 to obtain PEG-b-PBLA. PEG-bPAspEDA was prepared through the quantitative aminolysis reaction of PEG-b-PBLA in dry DMF at 40 °C in the presence of 50-fold molar excess of ethylenediamine (EDA).27 After 10 h, the reaction mixture was dialyzed against deionized water [molecular weight cut-off (MWCO) = 3500], and the final aqueous solution was lyophilized to a white powder. PEG-b-PCD was synthesized by the reaction of PEG-b-PEDA and 20-fold excess of mono-6-OTs-β-CD in anhydrous dimethyl sulfoxide (DMSO).28 After 5 days, the reaction product was purified by dialysis, and the resultant aqueous solution was lyophilized to a yellow to brown powder. The composition PEG45-b-PCD17 was characterized by 1H NMR spectrum in D2O using the PEG blocks as the inner standard.
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RESULTS AND DISCUSSION Fabrication of Copolymer/FeIIITPPS Complex Micelles. In acidic aqueous solution, PEG-b-P4VP and PEG-bPDMAEMA transform into typical neutral−cationic DHBCs because of protonation of the P4VP (pKa = 4.5−4.7)30 and PDMAEMA (pKa = 7.0−8.0) blocks.25,31 Thus, at pH 3, micellization of the block copolymers in the presence of FeIIITPPS takes place easily because of the electrostatic attraction between the positive charged units of copolymers and the −SO3− groups of the iron porphyrin. Scheme 1 shows the molecular structures of these block copolymers and FeIIITPPS. PEG-b-PCD is another DHBC with a PCD block carrying βCD units on the side chain. β-CD is one of the naturally oligosaccharides constructed from α-1,4-linked glucose units with hydrophilic rim and hydrophobic cavity. The host−guest interaction between β-CD and FeIIITPPS takes place with the peripheral groups of the metalloporphyrin including inside the 8937
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Figure 1. (A) Transition moment directions for the iron porphyrin Soret transition and the angle between the Soret transition moment and the symmetry axis of β-CD. The directions labeled Bx and By are directed along the N−N axes, respectively. (B) ICD signal of the PEG-b-PCD/ FeIIITPPS complex micelles at pH 3 and 9. The concentration of PEG-b-PCD is 0.8 g/L, and the concentration of FeIIITPPS is 0.01 mmol/L.
hydrophobic β-CD cavities, and leads to a trans-type 1:2 (iron porphyrin/β-CD) complex.32 Therefore, such a host−guest interaction is an inclusion interaction, and the obtained complex is an inclusion complex. The induced circular dichroism (ICD) spectra (Figure 1B) were used to investigate the inclusion interaction. The ICD signals centered at 395 nm at pH 3 and 414 nm at pH 9, corresponding to the Soret bands of different FeIIITPPS species, which will be discussed in the absorption spectra, indicate that the inclusion interaction occurs. Furthermore, the sign of the ICD signals to some extent indicates the spatial relationship between the iron porphyrin and β-CD. As shown in Figure 1A, the Soret transition moments of FeIIITPPS, which are responsible for the Soret band, are directed along the N−N axes.33 When the symmetry axis of β-CD is parallel to that of the sulfonatophenyl group, the angle between the Soret transition moment and the symmetry axis of β-CD is 45°. Such an angle, which is less than 54.7°, causes a negative ICD signal.34 Therefore, the negative signals in Figure 1B are probably caused by the interacting type of the sulfonatophenyl group inserting the cavity along the symmetry axis of β-CD. The inclusion interaction also enables PEG-b-PCD and FeIIITPPS to construct the micelles. The interaction between copolymers and FeIIITPPS and the micellization can be confirmed by the 1H NMR and DLS analyses. As shown in Figure 2, the almost unchanged signal of PEG blocks (a) indicates that FeIIITPPS has little influence on the solution behavior of the PEG blocks. The peaks contributed by the P4VP (b, c, and d), PDMAEMA (e, f, g, h, and k) and PCD (H1−H6) blocks in complex micelles (curves D, H, K and L of Figure 2) are reduced in terms of intensity compared to those in FeIIITPPS-free solution (curves C, G and J of Figure 2). That means that the mobility of these blocks is partially restricted. In addition, for PEG-b-PCD, a broadening of the cyclodextrin peaks at δ = 5.0 ppm (H1), δ = 3.5−3.6 ppm (H2 and H4), and δ = 3.8−3.9 ppm (H3, H5 and H6) is observed in curves K and L of Figure 2, which is due to the strong T1 and T2 relaxation effects induced by the paramagnetic metal.16 This further indicates the spatial proximity and the interaction between FeIIITPPS and PCD. Thus, the electrostatic or inclusion interaction enables copolymers and the iron porphyrin to construct a core−shell structure with a complex P4VP/FeIIITPPS, PDMAEMA/FeIIITPPS, or PCD/FeIIITPPS core and a PEG shell.21,35 The hydrodynamic diameter distributions of the three kinds of micelles at the scattering angle of 90° are shown in Figure 3.
Figure 2. 1H NMR spectra of the block copolymers, FeIIITPPS, and the complex micelles recorded in D2O. FeIIITPPS at pH 3 (A) and pH 9 (B), PEG-b-P4VP at pH 3 (C), PEG-b-PDMAEMA at pH 3 (G), PEG-b-PCD at pH 7 (J), PEG-b-P4VP/FeIIITPPS at pH 3 (D), pH 4.7 (E), and pH 11 (F), PEG-b-PDMAEMA/FeIIITPPS at pH 3 (H) and pH 11 (I), and PEG-b-PCD/FeIIITPPS at pH 3 (K) and pH 11 (L).
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Figure 3. Hydrodynamic diameter distributions of the copolymers/FeIIITPPS complex micelles. All DLS measurements were performed at the scattering angle of 90° at room temperature.
the antiferromagnetic coupling between the metal ions and the resulting loss of the effective magnetic moment. Aggregation of FeIIITPPS in Complex Micelles: PEG-bP4VP/FeIIITPPS and PEG-b-PDMAEMA/FeIIITPPS. The considerable spectral changes of FeIIITPPS can be observed in the complex micelles. Figure 5 shows the changes in
The diameters are centered at 55, 77, and 168 nm for PEG-bP4VP/FeIIITPPS, PEG-b-PDMAEMA/FeIIITPPS, and PEG-bPCD/FeIIITPPS, respectively. It is evident that PEG-b-PCD/ Fe III TPPS shows a bigger size and broader diameter distribution than the others. This may be caused by the large size of β-CD and low complexation ratio to the iron porphyrin, which acts as a cross-linker during the micellization. Aggregation of FeIIITPPS in the Absence of Copolymers. In aqueous solutions, FeIIITPPS molecules exist as a monomer at lower pH. The μ-oxo dimer forms upon alkalization with a pKd around 7.8.12 The dimerization equilibrium can be expressed as follows: pKD = 7.8
2FeTPPS(H 2O)n HooooooooI O(FeTPPS)2 2 − + 2H+ + (H 2O)2n − 1
Figure 4 shows the absorption spectra of FeIIITPPS. At pH 3, the absorption spectra show bands at 393 nm (Soret band) and
Figure 5. UV−vis absorption spectra of the complex PEG-b-P4VP/ FeIIITPPS at pH 3 with a fixed concentration (0.01 mmol/L) for FeIIITPPS and a variable concentration for PEG-b-P4VP. The arrow describes the spectral changes induced by increasing the concentration of the copolymer (Cp = 0, 2.0 × 10−3, 4.0 × 10−3, 1.0 × 10−2, 2.0 × 10−2, 5.0 × 10−2, 0.1, and 0.2 g/L). (Inset) Changes in the ratio of absorption from 406 to 393 nm as a function of Cp.
absorption spectra of FeIIITPPS at pH 3 with the addition of PEG-b-P4VP. Obviously, the absorption band at 393 nm red shifts to 406 nm gradually, and Q bands evolve into two bands at 581 and 624 nm, upon increasing the amount of the copolymer. These changes indicate a transformation from monomer to aggregates because of the interaction between P4VP and FeIIITPPS. When the concentration of PEG-b-P4VP is above 0.02 g/L, the aggregation is completed, indicated by the almost unchanged ratio of absorption at 406 nm to that at 393 nm. Thus, a high enough concentration of block copolymers was adopted during the preparation of complex micelles to ensure the adequate interaction between the copolymers and FeIIITPPS. In PEG-b-PDMAEMA/FeIIITPPS at pH 3, the iron porphyrin shows similar absorption features, Soret band at 407 nm and Q bands at 571 and 611 nm (Figure 8). Correspondingly, the 1H NMR signals of FeIIITPPS are centered at 13.9 ppm for the pyrrole β-protons and 8.0 ppm for the phenyl protons (curve H of Figure 2). Judging from the
Figure 4. UV−vis absorption spectra of Fe III TPPS with a concentration of 0.01 mmol/L.
530 and 682 nm (Q bands). The 1H NMR signals (curve A of Figure 2) at 51.6 ppm arising from the pyrrole β-protons and 14.2 and 10.2 ppm from the ortho- and meta-phenyl protons correspond to a dihexacoordinated monomer FeIIITPPS(H2O)2 in an admixed intermediate spin state.12,14 At pH above the pKd, the Soret band centered at 407 nm and the Q bands around 567 and 608 nm, together with a small decrease in the maximum absorption intensity, suggest the specific spectral features of the μ-oxo dimer.36 The 1H NMR spectrum in curve B of Figure 2 for the μ-oxo dimer shows signals at 13.4 ppm for the pyrrole β-protons and 8.2 and 7.3 ppm for the meta- and ortho-phenyl protons. The upfield-shifted signals are caused by 8939
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spectra features in comparison to those of the μ-oxo dimer in pure buffer (Figure 4), the aggregates in complex micelles are also in the form of the non-paramagnetic μ-oxo dimer. The broadening of the FeIIITPPS 1H NMR bands further supports the binding with copolymers. However, in PEG-b-P4VP/ FeIIITPPS, the 1H NMR signals for the iron porphyrin disappear, as shown in curve D of Figure 2. It is supposed that the mobility of FeIIITPPS may be further restricted by steric hindrance of the aromatic units on P4VP blocks, besides the electrostatic interaction. Therefore, when pH is below the pKa of pH-sensitive blocks, the electrostatic interaction induces not only the formation of complex micelles but also the dimerization of FeIIITPPS, even at pH far below its pKd. The dimer formed in the micellar core is called the micellar μ-oxo dimer here. A decrease of pH leads to a dissociation of the micellar μ-oxo dimer into the dihexacoordinated monomer FeIIITPPS(H2O)2 indicated by the emerging absorption shape of the monomer bands at 403, 537, and 685 nm (Figure 6). The red shift with
Figure 7. UV−vis absorption spectra of the complex PEG-b-P4VP/ FeIIITPPS at different pH. The concentration of PEG-b-P4VP is 0.1 g/ L, and the concentration of FeIIITPPS is 0.01 mmol/L.
Figure 8. UV−vis absorption spectra of the iron porphyrin in the complex PEG-b-PDMAEMA/FeIIITPPS with concentrations of 0.1 g/ L for PEG-b-PDMAEMA and 0.01 mmol/L for FeIIITPPS. Figure 6. UV−vis absorption spectral changes of the complex PEG-bP4VP/FeIIITPPS as a function of pH. The concentration of PEG-bP4VP is 0.1 g/L, and the concentration of FeIIITPPS is 0.01 mmol/L. (Inset) Changes in the absorption at 537 nm as a function of pH.
Under alkalinity conditions, as shown in curve I of Figure 2, the deprotonation of PDMAEMA leads to an evident upfield shift and a broadening of the aminomethyl band (peak h), as well as much less intensity of dimethylamino ethyl (f and g) and main chain proton (e and k) signals. Because the PDMAEMA signals do not disappear completely, the block must retain some degree of hydration. More importantly, evident sharp μ-oxo dimer bands, instead of broadening signals, can be observed. Additionally, a large amount of μ-oxo dimer was detected in the dialysate leakage of PEG-b-PDMAEMA/ FeIIITPPS at pH 11 (data were not shown). These results reveal that the deprotonated PDMAEMA turns to be non-interacting with FeIIITPPS and the iron porphyrin escapes from the micelles. Therefore, at pH > pKa (PDMAEMA), FeIIITPPS exists as the free μ-oxo dimer, independent of the polymeric micelles that have a hydrophobic PDMAEMA core instead of a complex PDMAEMA/FeIIITPPS core. In the case of PEG-b-P4VP/FeIIITPPS, the Soret band shifts to 409 nm and Q bands shift to 567 and 608 nm with pH raising above the pKa of P4VP (Figure 7). The pyrrole βprotons signal in the 1H NMR spectra (curve E of Figure 2) appears at 13.5 ppm, indicating that the iron porphyrin still exists as the μ-oxo dimer. In contrast to PEG-b-PDMAEMA/ FeIIITPPS, no FeIIITPPS was detected in the dialysate leakage of the complex micelles at pH > pKa (P4VP). Therefore, such a dimer is different from the free μ-oxo dimer and also differs from the micellar μ-oxo dimer described above. At pH 4.7, P4VP starts turning hydrophobic for the pH close to the pKa of the block. The reduced intensity and upfield shift
respect to the absorption of free FeIIITPPS (Figure 4, pH 3) may arise from the interaction with copolymers. Besides, the conversion of the micellar μ-oxo dimer into the monomer is accompanied by two isosbestic points at 563 and 642 nm and a color change of the micellar solution from green to light red. The apparent pKd of FeIIITPPS can be obtained by the titration of the complex micelles as a function of pH. In PEG-b-P4VP/ FeIIITPPS, the apparent pKd derived from the absorption evolution at 537 nm is about 1.5 (inset of Figure 6). Similar absorption changes were observed in the case of PEG-bPDMAEMA/FeIIITPPS (data not shown), and a lower apparent pKd at about 1.0 was obtained. It is largely demonstrated that the binding of FeIIITPPS to charged polymers changes its apparent pKd considerably.37 The two pH-sensitive block copolymers show different effects on the aggregation of FeIIITPPS when pH is above the pKa of each block. In the case of PEG-b-PDMAEMA/ FeIIITPPS, the absorption bands change little when pH is above the pKa of PDMAEMA (Figure 8), suggesting that FeIIITPPS remains in the form of the μ-oxo dimer. However, this μ-oxo dimer is a different species from the micellar dimer obtained at pH < pKa (PDMAEMA), which can be concluded from the 1H NMR and dialysis results. 8940
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of the P4VP peaks in the 1H NMR spectra (curve E of Figure 2) indicate a further restricted mobility of the PVP block. Here, the electrostatic interaction with Fe IIITPPS is partially restrained because of the deprotonation. A π−π interaction between the porphyrin electron-rich benzenosulfonate ring and the P4VP electron-deficient pyridine ring may take place,38 which also makes some contribution to the upfield shift of the P4VP 1H NMR signals. The red shift of the Soret band in the absorption spectra (Figure 7, at 409 nm) compared to that in the copolymer-free dimer spectra (Figure 4, at 407 nm) supports the existence of the interaction that induces the decrease in the relative energy gap between the filled [highest occupied molecular orbital (HOMO)] and empty [lowest unoccupied molecular orbital (LUMO)] orbitals of FeIIITPPS.39 When the pH was further increased, the quite weak signals of phenyl protons at 7.4−7.8 and 8.3 ppm can be observed, with the disappearance of the P4VP pyridine signals caused by almost completely restricted mobility (curve F of Figure 2). It is reasonable to assume that the π−π stacking makes FeIIITPPS encapsulated in the micellar core. Besides, monomer FeIIITPPS(H2O)2 was observed when the porphyrin was kept outside the polymeric micelles in the pH range of pKa (P4VP) < pH < pKd (FeIIITPPS) by adding the iron porphyrin into the PEG-bP4VP micelles self-assembled through a hydrophobic force. That means that the hydrophobic microenvironments inside the micelles lead to the dimerization of the iron porphyrin. The species of FeIIITPPS is named as the encapsulated μ-oxo dimer here. The encapsulated μ-oxo dimer obtained in PEG-b-P4VP/ FeIIITPPS at pH above the pKa of P4VP and that in PEG-bP4VP/FeIIITPPS or PEG-b-PDMAEMA/FeIIITPPS at pH below the pKa are different in the driving force. The former is primarily promoted by the hydrophobility of the polymeric micellar core, while the latter stems from a high local concentration of the iron porphyrin in the micellar core, which is caused by the electrostatic interaction. Aggregation of FeIIITPPS in Complex Micelles: PEG-bPCD/FeIIITPPS. PEG-b-PCD shows the inclusion interaction with FeIIITPPS as discussed in the Fabrication of Copolymer/ FeIIITPPS Complex Micelles section. The formation of the PEG-b-PCD/FeIIITPPS complex is accompanied by the broadening and shift in the pyrrole β-proton 1H NMR signals as well as the changes in the cyclodextrin peaks, as discussed above. Under the acid condition (curve K of Figure 2), the pyrrole β-protons appear at 57.8 ppm, downfield shifting from 51.6 ppm observed in the absence of copolymers. The peaks at 14.0 and 10.2 ppm are ascribed to ortho- and meta-protons. It is suggested that the iron porphyrin exists as the dihexacoordinated monomer FeIIITPPS(H2O)2. The increase in the pH results in marked spectral changes. As the pH is raised to 11, the pyrrole β-protons downfield shift to 82.7 ppm and show a broad band. The corresponding metaproton peak shifts to 11.0 ppm. These results indicate that fivecoordinate high-spin hydroxyl monomer FeIIITPPS(OH) is formed even in a strong alkaline solution. Therefore, the inclusion interaction inhibits the aggregation of the iron porphyrin effectively. The two micellar monomer species, FeIIITPPS(H2O)2 and FeIIITPPS(OH), show absorption bands at 395 and 414 nm in Figure 9a. In the pH range between 4 and 8, the systematic spectral changes with isosbestic points at 410 and 554 nm were observed, indicating a ligand exchange of FeIIITPPS. The
Figure 9. (a) UV−vis absorption spectra of the iron porphyrin in the complex PEG-b-PCD/FeIIITPPS at different pH values. (Inset) Changes in the ratio of absorption from 414 to 395 nm as a function of pH. The concentration is 0.8 g/L for PEG-b-PCD, and the concentration is 0.01 mmol/L for FeIIITPPS. (b) Absorption spectra at pH 8 with a fixed concentration (0.01 mmol/L) for FeIIITPPS and a variable concentration for PEG-b-PCD. The arrow describes the spectral changes induced by increasing the concentration of the copolymer (0, 0.1, 0.2, 0.4, and 0.8 g/L). (Inset) Enlarged details of the Soret and Q bands.
constant pKa for the equilibrium between FeIIITPPS(OH) and FeIIITPPS(H2O)2 is about 6.4 (inset of Figure 9a), much lower than the pK d for the μ-oxo dimer−Fe III TPPS(H 2 O) 2 equilibrium in the absence of PEG-b-PCD, giving more proof that the block PCD markedly prevents the dimerization of FeIIITPPS. Furthermore, the μ-oxo dimer that has formed can be dissociated by the PEG-b-PCD, which can be supported by the spectral changes upon increasing the concentration of the copolymer (Figure 9b). The red shift from 407 to 414 nm and the new Q-band shoulder at 652 nm indicate a dissociation of the μ-oxo dimer into hydroxyl monomer FeIIITPPS(OH). The Fe−O bond length in the crystal structure of dimer O(FeTPPS)2 is 1.76 Å.40 While the cavity diameter of β-CD is 6.0−6.5 Å.41 Thus, the prevented dimerization and dimer dissociation are achieved by the steric hindrance of β-CD, which encloses the tetrasulfonatophenyl groups of FeIIITPPS. The PEG-b-PCD/FeIIITPPS complex micelles that inhibit the formation of the μ-oxo dimer might be regarded as a primary Hb model. Extension of the system might make it possible to prepare the simple Hb model that works in aqueous solution.10 In summary, FeIIITPPS shows different aggregation behavior in the complex micelles from that in the absence of the copolymers. Scheme 2 illustrates the various forms of the iron porphyrin. The μ-oxo dimer was obtained in PEG-b-P4VP/ 8941
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Langmuir Scheme 2. Illustration of the FeIIITPPS Forms in Complex Micelles
ACKNOWLEDGMENTS
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REFERENCES
We thank the National Natural Science Foundation of China (21204064, 51173132, and 91127045) and the National Basic Research Program of China (973 Program, 2011CB932503) for financial support.
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FeIIITPPS and PEG-b-PDMAEMA/FeIIITPPS at pH much lower than the pKd, which can be kept in the former micelles but released from the latter at pH above each pKa of the block, while two monomeric forms, FeIIITPPS(H2O)2 and FeIIITPPS(OH), were achieved in PEG-b-PCD/FeIIITPPS.
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CONCLUSION The present study deals with the aggregation behavior of FeIIITPPS in the copolymer/FeIIITPPS complex micelles. It is established that different iron porphyrin species are closely related to the interaction between copolymers and FeIIITPPS. PEG-b-P4VP and PEG-b-PDMAEMA have the pH-sensitive blocks; therefore, the FeIIITPPS species and the interaction between copolymers and FeIIITPPS are based on the pKa of P4VP or PDMAEMA block. At pH below the pKa, the electrostatic interaction of protonated blocks with FeIIITPPS leads to the micellar μ-oxo dimer, making an especially low pKd for the monomer−dimer equilibrium. At pH above the pKa, the π−π interaction of P4VP with the porphyrin may occur and result in the encapsulated μ-oxo dimer and the deprotonated PDMAEMA makes FeIIITPPS release from the micelle and form the free μ-oxo dimer. In the case of PEG-b-PCD/ FeIIITPPS, the inclusion complexation of PCD with FeIIITPPS prevents the aggregation of the iron porphyrin effectively. The micellar monomer species exist as FeIIITPPS(H2O)2 and FeIIITPPS(OH). The present systems provide valuable insights into the interactions between copolymers and FeIIITPPS and the FeIIITPPS species in polymeric micelles. The complex micelles can give the FeIIITPPS species as needed and may provide a stable microenvironment for the iron porphyrin as a catalyst or sensitizer. This is important in view of the biomimetic, medical, and catalytic applications of metalloporphyrins.
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The authors declare no competing financial interest. 8942
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