Article pubs.acs.org/Langmuir
Novel Self-Assembling Amino Acid-Derived Block Copolymer with Changeable Polymer Backbone Structure Tomoyuki Koga,* Eri Aso, and Nobuyuki Higashi* Department of Molecular Chemistry and Biochemistry, Faculty of Science & Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan ABSTRACT: Block copolymers have attracted much attention as potentially interesting building blocks for the development of novel nanostructured materials in recent years. Herein, we report a new type of self-assembling block copolymer with changeable polymer backbone structure, poly(Fmoc-Ser)ester-b-PSt, which was synthesized by combining the polycondensation of 9-fluorenylmethoxycarbonylserine (Fmoc-Ser) with the reversible addition−fragmentation chain transfer (RAFT) polymerization of styrene (St). This block copolymer showed the direct conversion of the backbone structure from polyester to polypeptide through a multi O,N-acyl migration triggered by base-induced deprotection of Fmoc groups in organic solvent. Such polymer-to-polymer conversion was found to occur quantitatively without decrease in degree of polymerization and to cause a drastic change in self-assembling property of the block copolymer. On the basis of several morphological analyses using FTIR spectroscopy, atomic force, and transmission and scanning electron microscopies, the resulting peptide block copolymer was found to self-assemble into a vesicle-like hollow nanosphere with relatively uniform diameter of ca. 300 nm in toluene. In this case, the peptide block generated from polyester formed β-sheet structure, indicating the self-assembly via peptide-guided route. We believe the findings presented in this study offer a new concept for the development of self-assembling block copolymer system.
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INTRODUCTION Self-assembly of block copolymer is attractive as a promising tool for fabricating nanostructured materials both in industrial and biomedical fields, e.g., as nanoreactors, as nano/microtemplates, or as nanocarriers. To date, a number of block copolymers have been extensively investigated and found to form nanoscaled structures spontaneously such as micelles,1,2 vesicles,3,4 nanofibers,5,6 toroids,7,8 mushroom-shape, etc.,9,10 depending on the chemical composition of the block chains, the balance of block length, polymer concentration, and environmental conditions including the nature of solvent. Much of current interest in self-assembling block copolymer system is directed to the design of more complicated, accurate and smart nanoassembly. Recent advances in polymer synthesis offer opportunities to design precisely various block copolymers with tailored character and well-defined structures. A hybridization of structurally regulated biobased polymers and conventional synthetic polymers would be one of the good strategies for this purpose. Especially, artificial peptides are fascinating building blocks due to their exceptional ability to hierarchically selforganize into precisely defined three-dimensional (3D)nanostructures,11−20 as is observed in natural protein folding/ self-assembly system. For example, α-helix and β-sheet, which can be designed intrinsically by the primary sequence of amino acids, are the major secondary structure motifs with wellregulated shape and size. In the α-helix, each peptide bond forms intramolecular hydrogen bonds, and side chains are on the outside of the spiral. Diameter and length of the helix rod © XXXX American Chemical Society
can therefore be controlled by manipulating the kind of amino acids and the degree of polymerization, respectively. β-Sheet peptide is further useful as a structuring block because multiple hydrogen bonds among the peptide strands occur intermolecularly, which induce the self-assembly and stabilize the resultant nanoassembly.14−20 Thus, employment of the artificial peptides as one of the block segments is a very attractive strategy for constructing functional self-assembling block copolymers. Many examples of self-assembling peptide−polymer hybrids have been reported including the α-helix,21−25 β-sheet,26−31 and coiled-coil-type 32,33 block copolymers. Such block copolymer was initially developed by Gallot and co-workers, who employed α-helical poly(γ-benzyl L-glutamate) (PBLG) and flexible polybutadiene, and found the formation of lamellar structures in solid state.21 We have also reported the selfassembly behavior of AB-type diblock hybrid PBLG-b-PSt25 and ABC-type triblock hybrid with central-β-sheet block PSt-b(Leu)4-b-poly(ethylene glycol)31 in aqueous and nonaqueous media. The obtained nano-objects include lamella, cylindrical micelle, spherical micelle, and vesicle. Special Issue: Tribute to Toyoki Kunitake, Pioneer in Molecular Assembly Received: April 28, 2016 Revised: May 31, 2016
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DOI: 10.1021/acs.langmuir.6b01617 Langmuir XXXX, XXX, XXX−XXX
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An additional advantage of peptide-based building block becomes prominent with the design of stimuli-responsive polymers. Stimuli-responsive polymers exhibit a drastic change in their character, including conformation, polarity, and phasestructure, in response to physical and chemical signals from their environment such as temperature, pH, ionic strength and guest additives.34−38 The design of such polymer usually involves incorporation of stimuli-sensitive groups in the polymer side chain. Therefore, artificial peptide can be easily converted to stimuli-responsive block by selecting the corresponding monomers appropriately from 20 kinds of natural amino acids. In fact, various stimuli-responsive peptide block copolymers have been previously reported to control the self-assembly behavior.39−42 For example, Klok and co-workers have reported that the block copolymer of poly(L-glutamic acid)-b-polybutadiene forms vesicular assembly in water, and the size and shape of the assembly can be manipulated by changing the pH and ionic strength.39 The same research group has also reported a pH-sensitive schizophrenic micelle from block copolymer composed of PGA and poly(lysine).40 In those cases, self-assembly behaviors of the block copolymers mainly rely on the change in conformation, polarity, and/or hydration state of the peptide block based on a certain functional group located at the side chain of polymer backbone (e.g., ionizable Glu and Lys residues). While such stimuliinduced conformational transition of the peptide has been employed to control a self-assembly of the block copolymer, to the best of our knowledge, there has been no report on a selfassembling block copolymer system triggered by a quantitative conversion of polymer backbone structure. Herein, we report on the synthesis of a new type of block copolymer with changeable polymer backbone structure and its unique self-assembly in toluene. The backbone structure (i.e., the type of linkages in the polymer main chain) is one of the most important factors governing the nature of the polymer chain intrinsically such as conformation, self-assembling character and degradability. However, it is generally difficult to accomplish a polymer-to-polymer conversion, since the main chain linkages are usually created via polymerization of the corresponding monomers. To overcome such difficulties, we focused on the O,N-acyl migration reaction.43−45 We have recently described the synthesis of a unique poly(Ser) ester with photolabile N-6-nitroveratryloxycarbonyl (Nvoc) groups and the direct polymer-to-polymer conversion from polyester to polypeptide through multi O,N-acyl migration in water.46 Moreover, the enzyme-degradability of this polymer could be changed by the structural conversion. This tactic must also be useful for the design of a novel self-assembling building block with switchable character. We newly prepared a block copolymer poly(Fmoc-Ser)ester-b-PSt, in which polyester derived from 9-fluorenylmethoxycarbonyl-serine (Fmoc-Ser) was combined with hydrophobic polystyrene (PSt) as a convertible structural block, via polycondensation and reversible addition−fragmentation chain transfer (RAFT) polymerization. A detailed analysis of structure and self-assembly behavior of the block copolymer was conducted in organic solvent, especially in view of the effect of base-promoted structural conversion from polyester to polypeptide. Exploring a novel block copolymer system and understanding the relationship between polymer structure and the consequent morphology of self-assembled nanoarchitectures are still challenging and valuable toward facilitating a polymer-based bottom-up nanotechnology.
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EXPERIMENTAL SECTION
Materials. Solvents of analytical grade were used unless otherwise stated. Fmoc-L-Ser and 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid N-succineimidyl ester were purchased from Peptide Institute Inc., and from Sigma-Aldrich, respectively. N,N′-Diisopropylcarbodiimide (DIPC), 4-(dimethylamino)pyridine (DMAP), p-toluenesulfonic acid were purchased from Wako Pure Chemical Industries, Ltd. N,NDimethyl-formamide (DMF), piperidine, tetrahydrofuran (THF), diethyl ether, dichloromethane, methanol, acetone, toluene, hexane, DMSO-d6, chloroform-d, acetone-d6, 2,2′-azobisisobutylonitrile (AIBN), styrene, anhydrous sodium sulfate and activated alumina were purchased from Nacalai Tesque, Inc. DMF was used after purification with distillation. Styrene was purified by passing through an activated alumina column. Measurements. 1H NMR spectra were acquired using a JEOL JNM-ECA500 (JEOL Resonance) spectrometer (500 MHz). The number-average molecular weight (Mn) and the polydispersity index (Mw/Mn) of each polymer were determined by gel permiation chromatography (GPC) using a JASCO LC-net II/AD (JASCO Ltd.) equipped with a refractive index (RI) detector (eluent: THF, flow rate: 1 mL/min, temperature: 40 °C, column: GF-710F). Poly(methyl methacrylate)s and poly(styrene)s were purchased from GL Sciences, Inc. and used as the calibration standards for poly(Fmoc-Ser)ester and block copolymer, respectively. Matrix-assisted laser desorption ionization-time-of-flight MS (MALDI-TOFMS) analysis was carried out on an Autoflex speed (Bluker Daltonics) using 2,5-dihydroxybenzoic acid (DHBA) as a matrix. Transmittance- and ATR-FTIR spectra were measured with the Nexus 470 (Thermo Nicolet Co.) using a Mercury−Cadmium-Tellurium (MCT) detector (resolution: 4 cm−1, number of scan: 1024). Transmittance-FTIR was measured by KBr method. For FTIR measurement of the poly(serine) generated from P(Fmoc-Ser)ester, we used the sample obtained by the lyophilization from aqueous solution. For ATR-FTIR, the block copolymer assemblies in toluene were adsorbed onto quartz plate and then dried in incubator for ca. 12 h. CD spectrum was recorded on a J820 spectropolarimeter (JASCO Ltd.) equipped with a Peltier type thermo static cell holder coupled with a controller PTC-423L under a nitrogen atmosphere. Experiment was performed in a quartz cell with a 1 mm path length at 25 °C. The AFM images were collected at ambient temperature on a SPM9700 (Shimazu Co.) operated by tapping using a silicon tip (MPP-11100, tip radius
