Integrated Nanostructures Based on Self-Assembled Amphiphilic

Chapter 2

Downloaded by UNIV OF FLORIDA on October 26, 2017 | http://pubs.acs.org Publication Date (Web): October 25, 2017 | doi: 10.1021/bk-2017-1252.ch002

Integrated Nanostructures Based on Self-Assembled Amphiphilic Polypeptides Motoki Ueda,1,2,* Stefan Müller,2 Siyoong Seo,1,2 Md. Mofizur Rahman,2 and Yoshihiro Ito1,2 1Nano

Medical Engineering Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan 2Emergent Bioengineering Materials Research Team, RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan *E-mail: [email protected]

Nano-ordered materials are requiring on demand shape control in various fields; biomaterials, medicals, pharmaceuticals and so on. Self-assembly of molecules is a typical bottom-up phenomenon observed in nature to form nano-materials. Self-assembly leads to the formation of various structures that impart functions. We found that amphiphilic block polypeptides having helical structure as a hydrophobic block self-assemble to form different structures. First, the hydrophobic segments form helical structures that define specific structures of self-assemblies such as fibers, tubes, vesicles and sheets depending on the diameter and length of helix. Small diameter of 310-helix induced fibers and micelles reasonably due to small critical packing parameter. On the other hand, sheet-like assembly were obtained by peptide having large sectional area of α-helix. Depending on the length of hydrophobic helix, these sheets transformed into various morphology by heat treatment. Length of helix changes the thickness and size of sheet and thus, changes the flexibility of the sheet, resulting that the difference of just two residues induced drastically different shapes of assembly, tube, vesicle and sheet. Next, hydrophilic polymers, polyethylene glycol (PEG) and poly(sarcosine), didn’t affect drastically the shape but did the size of assembly owing to the difference of hydrophilicity and flexibility. Poly(sarcosine) made even small assembly of ~200 nm length and 80 nm © 2017 American Chemical Society Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


diameter more stable and PEG enhanced the elongation of tubular assembly (more than 1 μm length) with the same diameter of 80 nm. Furthermore, we demonstrate some integration of the structures by mixing with other components such as lipids, cholesterol, or amphiphilic polypeptides having a different hydrophilic chain of polyethylene glycol or a different-length hydrophobic helix. This work shows one of the methods to prepare complex morphology of molecular assembly.

Introduction To prepare molecular assemblies with control over the shape formed, numerous different amphiphiles have been developed. These amphiphiles can be divided into particular groups: lipid-like surfactants (1–3), peptide amphiphiles (4–6) and amphiphilic polymers (7, 8), and other amphiphiles (9). Like nature, some research groups reported phase-separated assemblies by optimization of lipid components and preparation conditions. For example, using the phase-separation property of lipid rafts, a large lateral separation into two domains of cholesterol/sphingomyelin/saturated phospholipids and unsaturated phospholipids in giant unilamellar vesicles was demonstrated to generate Janus vesicles of a conjugate morphology of two vesicles (1–3). This Janus assembly is artificial but is the closest mimic of a natural plasma membrane because this vesicle is composed of a combination of natural components and controlled by only optimizing the mixing ratio and the preparation conditions. Thus, this vesicle is an excellent mimic and model of plasma membranes but is difficult to use in applications owing to instability issues and the inability to control size and shape. However, most of the lipid and lipid-like assemblies are not stable, which limits their applications. In contrast, amphiphilic polymers produce stable molecular assemblies (10) and form various shapes besides the commonly formed sphere, such as tubular structures, mesoporous spheres and sheets. Another instance of a conjugate morphology of self-assemblies was reported where one end of a nanotube was capped with a vesicle to generate a round-bottom flask-type morphology, which was prepared from the mixture of phospholipids and amphiphilic molecules with two hydrophobic legs (11, 12). Here, we focus on polypeptide amphiphiles, which assembly to form stable complexes owing to numerous inter- and intra-interactions, including van der Waals forces, H-bonding, π-π stacking, dipole moments, disulfide bonding, electrostatic interactions, concavo-convexo and steric interactions. The polypeptide assemblies show lipid-like membrane fluidity, morphological diversity and biocompatibility. In addition, it has recently been shown that polypeptide assemblies can create a Janus soft matter consisting of a phase-separated membrane by integration. Using these unique properties, attempts were made to use these polypeptide assemblies for biomedical applications, including bio-matrices, carriers of drug delivery systems and in vivo imaging. 20 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


In this chapter, as shown in Figure 1, we show the assemblies of amphiphilic polypeptides and derivatives with polyethylene glycol (PEG), and composite formation by integration with other components such as lipids or cholesterol.

Figure 1. Amphiphilic polypeptide assemblies with other components.

Amphiphilic Polypeptide Assemblies Ueda and coworkers have reported some unique morphologies prepared from polypeptide amphiphiles (13–18). These amphiphiles have an α-helical forming peptide as a hydrophobic block. An α-helical hydrophobic block is rigid and uniform shape due to the intramolecular hydrogen bonding, resulting that it can enable to form stable assemblies, have lateral fluidity without entangling between hydrophobic blocks and make it easy to control the shape of assembly. Hydrophilic block is composed of poly(sarcosine). Poly(sarcosine) forms polymer brush on the surface of assembly and shows the similar hydrophilicity and stealth ability to polyethylene glycol. In addition, poly(sarcosine) is biodegradable and not toxic in a body, resulting that poly(sarcosine) is very useful tool as a biomaterial for medical application. Furthermore, in previous reports, he also reported that a mixture of two enantiomeric amphiphilic polypeptides with a right- and a lefthanded helical block self-assemble to form large planar and curved sheets. The 21 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

sheets could be converted to a round-bottom flask-type molecular assembly upon heating in buffer, where the neck part was composed of a single component and the sphere part composed of the stereocomplex (14, 15).


Length of the Hydrophobic Chain

The morphologies of assemblies can be controlled by the design of amphiphilic polypeptides. The design refers to the character of the hydrophilic block and the hydrophobic block and the valence of each block type. In particular, the hydrophobic block is one of the most important blocks to control the shape because the molecular orientation of packed amphiphiles depends on the character of the hydrophobic block. Here, we synthesized some amphiphilic polypeptides with the hydrophilic polypeptide polysarcosine (Sar)n and various lengths of a hydrophobic helix block composed of L-leucine (L-Leu) and α-aminoisobutyryl (Aib); (Sar)16-b-(L-Leu-Aib)4 (L8), (Sar)25-b-(L-Leu-Aib)6 (L12), (Sar)26-b-(LLeu-Aib)7 (L14), (Sar)32-b-(L-Leu-Aib)8 (L16), (Sar)40-b-(L-Leu-Aib)9 (L18) and (Sar)48-b-(L-Leu-Aib)10 (L20). According to the Griffin hydrophilic-lipophilic balance (HLB) (19–21), these polypeptides have similar values of 12.0–13.0. There is no significant difference among these hydrophilic-lipophilic balances. However, these polypeptides assembly to form different shapes in aqueous solvent. These assemblies were categorized into two groups. One group is the fiber-shaped assembly of L8 (Figure 2a) and the other group is the sheet-shaped assembly of L12–L20. Circular dichroism spectra demonstrated that the hydrophobic block of L8 formed a 310-helix and others forming an α-helix (Figure 2e). The packing of α-helices is known to form planer sheets because the α-helix has a thicker diameter than the 310-helix. Thus, the critical packing parameter (22) of L8 was different from the other amphiphiles. The reason why L8 formed a fiber-shaped assembly is due to the difference of the helix structure. We also found another difference among the assemblies of L12–L20. When these sheet assemblies were heated at 90 °C, L12 and L14 sheets rolled up to form a tubular assembly (Figure 2b) (18, 23), the L16 sheet transformed into a vesicular assembly (Figure 2c) (14), and L18 and L20 sheets maintained their shapes (Figure 2d). The edge of sheets is considered to be hydrophobic, and sheets tend to roll up to decrease the area of the edge. L12–L16 species were considered to not have sufficient lengths of hydrophilic chains to maintain the planar sheet shape during heating. On the other hand, L18 and L20 have sufficient lengths of the hydrophilic chain to cover the hydrophobic edge of the sheet. As a result, L18 and L20 sheets were stable and did not change shape during heating. The formations of tubes of L12 and L14, and the sphere formed by L16 were most likely due to the difference of concavo-convexo interactions between neighboring helices. The anisotropic nature of L12 and L14 membranes facilitated tube formation. However, L16 was too long to be packed with a tilt angle; thus, L16 packed in a parallel manner to form an isotropic sphere. 22 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 2. Schematic illustration and negative staining images by a transmission electron microscope (TEM). The assemblies were composed of a single component of an amphiphilic polypeptide with various lengths of the hydrophobic block: (Sar)16–48-b-(L-Leu-Aib)n (Ln), (n = 8, 12, ,14, 16, 18, 20). Amphiphilic polypeptides were dissolved in ethanol and the solutions were injected into water and heated at 90 °C for 10 min. The assemblies were negatively stained by 2% samarium acetate for TEM observation. Scale bar: 500 nm (a–c) and 200 nm (d). CD spectra of assemblies from Ln in milliQ water after heating at 90 °C for 1h (e) Helicity of the Hydrophobic Chain The shape of the assembly depends on both the length and the helicity of the hydrophobic block. Here, an amphiphilic polypeptide incorporated with D-leucine, (Sar)25-b-(D-Leu-Aib)6 (dL12), was synthesized and formed a left-handed helix (Figure 1). As expected, dL12 also formed a nanotube assembly. However, an equimolar mixture of enantiomeric peptides, L12 and dL12, demonstrated another shape, a vesicle of 180 nm diameter after heating at 90 °C (Figure 3a). The enantiomeric helix is known to form a stereo complex that is thermodynamically more stable than a single component. The isotropic packing induces the spherical structure; although, the single amphiphilic components form tube structures. 23 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 3. Schematic illustration and TEM images of peptide assemblies prepared from an enantiomeric mixture of L12 and dL12 (a) and a single component of PEG750-b-(L-Leu-Aib)6 (PEG750-L12) (b). Each mixture was dissolved in ethanol and the solutions were injected into water and heated at 90 °C for 60 min. The assemblies were negatively stained by 2% samarium acetate for TEM observation. Scale bar: 500 nm.

Conjugation with Polyethylene Glycol (PEG) The hydrophilic block of the amphiphile also affects the shape of the assembly. Since PEG is a typical hydrophilic polymer, it was used as a hydrophilic block of the amphiphile for peptide assembly. A PEG (Mw: 750 and 2000) chain was modified at the N-terminus of the hydrophobic polypeptide block, (L-Leu-Aib)6, which is completely the same as the hydrophobic block of L12. The PEG-incorporated polypeptides, both PEG750-L12 and PEG2000-L12 formed nanotubular assemblies in saline (Figure 3b). The molecular weight of PEG did not affect the nanotube diameter and the diameter is almost 80 nm which is the same as that of L12 nanotubes. As mentioned above, the orientation between hydrophobic helices determines the curvature of the nanotube. On the other hand, although the length of the L12 nanotube was uniformly ~200 nm (Figure 2b), PEG-incorporated nanotubes were longer and the distributions were broader. Considering the length was similar between polysarcosine and PEG amphiphiles, the chemical differences of the hydrophilic chains is important for covering the hydrophobic edge, which dominates the stability of the assembled structure.

24 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Integration of Assembled Polypeptides


Mismatch of a Hydrophobic Block Assembly can be integrated by two kinds of peptide membrane. As shown above, the packing of an amphiphile, especially the hydrophobic block, defines the morphology of the assembly. An enantiomeric peptide tends to form a stereo complex, but peptides with the same helicity do not mix to form one membrane. Two kinds of amphiphilic polypeptides, which having the same helicity but the different lengths of the hydrophobic block, are divided into two membranes of the same length of the hydrophobic helix. In other words, these amphiphiles phase separate in one assembly. Actually, a mismatch in the length of the hydrophobic block induces phase separation. L16 is composed of polysarcosine 32mers and a hydrophobic leucinebased 16 residue block. As shown above, L16 self-assembles into spheres with approximately 70–100 nm diameters in buffer after heating the planar sheets. This diameter is similar to the diameter of L12 nanotubes. Therefore, L16 and L12 were mixed to obtain an AB-type conjugate morphology. L16 planar sheets were incubated with L12 nanotubes at a ratio of 1:1 (w/w), and the dispersion was heated at 90 °C for 1 h. As shown in Figure 4a, a nano-test-tube-shaped self-assembly with round bottoms were predominantly observed. The sizes of the neck and round-bottom parts of the assemblies correspond to those of the L12 nanotube and L16 sphere, respectively. Furthermore, the membrane thickness of the spherical part was 6 nm, which was the same as that of the L16 sphere. These results suggest that the nano-test-tube-shaped self-assembly was composed of a peptide membrane where L12 and L16 phases separated to form the corresponding neck part and spherical part of the nano-test-tube morphology.

Combination with Stereo Complex Assembly A combination of a stereo complex membrane prepared from L12 and dL12, and a single component membrane of L12 also formed successfully an integrated assembly that formed a round-bottom flask shape (Figure 4b). As mentioned above, the stereo complex membrane of L12 and dL12 formed a sphere shape of 180 nm diameter. Thus, when the stereo complex membrane was incubated with the L12 nanotube in saline and the mixture heated at 90 °C for 1 h, the stereo complex membrane attached to the open mouth of the L12 nanotube through hydrophobic interactions and then rolled-up to form a vesicle that capped the open mouth. The size of spherical part is corresponding to that of stereo complex vesicle from L12 and dL12.

25 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 4. Schematic illustration and TEM images of assembly integrations. Assembly integrations were prepared from a combination of some amphiphilic polypeptides: (a) (Sar)25-b-(L-Leu-Aib)6 (L12) and (Sar)32-b-(L-Leu-Aib)8 (L16), (b) L12 and dL12, (c) ((Sar)26)3-b-((L-His)2-(L-Leu-Aib)6) (HL12) and (Sar)25-b-(D-Leu-Aib)6 (dL12), (d) L12 and phospholipid (DPPC), (e) L12, dL12 and DPPC, and (f) L12 and cholesterol. Each mixture was dissolved in ethanol and the solutions were injected into water and heated at 90 °C for 60 min. The assemblies were negatively stained by 2% samarium acetate for TEM observation. Scale bar: 200 nm. Combination with pH-Dependent Assembly Here, we introduce a stimuli-responsive molecule selectively into one of the parts of the self-assemblies. A pH-responsive polypeptide termed HL12, (poly(sarcosine)26)3-b-((L-His)2-(L-Leu-Aib)6), was prepared (12). Since HL12 contains a histidine (His) dipeptide at the junction between the hydrophilic block and the hydrophobic helical block, it was found that disassembly of either the neck or the round-bottom parts of the Janus-type assemblies under acidic conditions with heat treatment would facilitate self-assemble of HL12 into either part. The integration of HL12 enabled preparation of the round-bottom flask-shaped assemblies containing HL12 selectively either in the neck part (12) or the round-bottom part (Figure 4c). The formation of a stereo complex between the right- and the left-handed helices was the basis for creating round-bottom flask-shaped assemblies with dL12 and HL12. The concavo-convex interaction between neighboring α-helical blocks defined precisely the size and shape of the molecular assemblies. Further, the amphiphilic helical peptides were allowed to diffuse laterally in the assemblies at high temperatures (12). The pH-sensitive molecule, HL12, can be confined selectively into one part of the Janus-type assembly of the round-bottom flask-shaped assembly with a suitable combination of other amphiphilic helical peptides. The part containing HL12 can be selectively disrupted by acidification and heat treatment while leaving the other part of the Janus-type assembly intact. By using a combination of some functional amphiphiles, integration of not only structures but also functions also be succeeded. 26 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Combination with Lipid Assembly A lipid assembled liposome also be integrated into a nanostructure assembly. The sealing of a nanotube formed by amphiphilic polypeptides was performed using a liposome. The liposome was generated by mixing dipalmitoyl-phosphatidylcholine (DPPC) and cholesterol at a molar ratio of 55/45. The mixture in ethanol was injected into the L12 nanotube suspension (molar ratio of DPPC/L12, 1/1) in buffer and heated at 90 °C for 1 h. As a result, a round-bottom flask assembly was observed (Figure 4d). The sizes of the diameter and length of the neck part of the round-bottom flask were similar to those of the L12 nanotube. The membrane thickness of the neck part was ca. ~10 nm, which was the same as that of the L12 nanotube, indicating clearly that the neck part was composed of L12. On the other hand, the round bottom part of the round-bottom flask showed a smaller morphology than the liposome, and a membrane thickness of ca. 15 nm, which is obviously larger than that of the liposome, suggesting that the round-bottom shaped membrane is composed of a mixture of DPPC, cholesterol and L12.

ABC-Type Phase-Separated Assemblies As shown some combinations, any amphiphiles form a unique assembly on the edge of L12 nanotube, resulting that complex morphologies are obtained by the assembly integration. By using this assembly integration method, ABC-type dumbbell shaped assemblies were prepared and observed. Two different vesicles and one nanotube were prepared from three types of amphiphilic helical peptides by phase separation. First, round-bottom flask assemblies were prepared from a mixture of L12 and dL12 at a molar ratio of 2:8 and purified using a syringe filter to remove assemblies larger than 450 nm. Subsequently, a DPPC solution was added and the mixture was heated at 90 °C for 1 h. By using this twostep method, ABC-type asymmetric dumbbell-shaped assemblies were obtained. As shown in Figure 4e, the asymmetric dumbbell-shaped morphology had two different spherical parts that were each composed of 180-nm stereoscope vesicles of L12 and dL12 and size-uncontrolled DPPC liposomes, and a middle neck part that was the L12 nanotube with a diameter of 80 nm and a length of 180 nm.

Conjugate with Cholesterol In addition to these shape controls, membrane fluidity of the polypeptide assemblies can be controlled by conjugation with cholesterol. Polypeptide nanotube rigidity was increased and displayed higher stability following insertion of cholesterol into the peptide membrane without changing the shape of the assembly. When the membrane fluidity was evaluated by Laurdan reagent, the fluidity increased linearly as the amount of incorporated cholesterol increased. Controlling fluidity is important for further applications of polypeptide assemblies. 27 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Conclusion


Using amphiphilic polypeptides, it is possible to prepare different nanostructure shapes, such as fibers, sheets, spheres and tubes. In addition, by summation of these nanostructures more complex shapes were prepared. These complex structures are expected to contribute to the fine regulation of various nanosystems. Nanosystem also become to need on demand precise shape- and size-control to improve the function in near future. Development of bottom-up method to prepare various complex morphology is very important challenge. This work in this chapter is one of their advanced methods, show the possibility of molecular assembly integration and give knowledge to establish complex-shaped nanomaterials for researchers in a various fields.

Experimental Section Materials All amino acids were purchased from Watanabe Chemical Industries Ltd. (Japan). Lipid of DPPC and cholesterol were obtained commercially from NOF Co. (Japan) and Wako Pure Chemical Industry (Japan). mPEG was purchased from Sigma-Aldrich Co. Preparation of Molecular Assembly The amphiphilic peptide (12 mg) was dissolved in ethanol (240 μL) to make a stock solution. An aliquot (10 μL) of the stock solution was injected into a saline (1 mL) with stirring. After stirring for 30 min, this dispersion was heated at 90 °C for 1 h. Molecular assemblies of different compositions were prepared similarly with keeping the total volume of the mixed ethanol solution to be 10 μL. Transmission Electron Microscopy (TEM) TEM images were taken using a JEOL JEM-1230 at an accelerating voltage of 80 kV. Peptide aqueous solutions were applied on a carbon-coated Cu grid, and the samples were negatively stained with 2% samarium acetate, followed by suction of the excess fluid with a filter paper. Synthesis of Amphiphilic Peptides The amphiphilic peptides (L8, L12, L14, L16, L18 and L20) were synthesized by conventional procedures in solution following essentially protocols reported previously (13–18, 23). The hydrophobic helical segments (L-Leu-Aib)m (m = 4, 6, 7, 8, 9, 10) were prepared by fragment condensation, whereas the polysarcosine extension at the N-terminal of the helical segments was obtained by the NCA (N-carboxy anhydride) polymerization. MALDI-TOF MS analysis and the area ratios of Sar N-CH3 peaks against those of OCH3 of the C-terminal in 1H NMR spectra were used to determine the degrees of polymerization of 16–48, 28 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

respectively. Amphiphilic peptide having hydrophobic right-handed helix, dL12, was synthesized by the same way as previous reports (15–18).


Synthesis of PEG750-L12 Synthetic routes of PEGylated peptide PEG750-L12 was illustrated in Schemes 1. The hydrophobic helical peptide (L-Leu-Aib)6 was synthesized as previously reported (13–18, 23). First, mPEG 1 (Mw = 750) (2.7 g, 3.6 mmol) was dissolved in THF (50 mL) and subsequently, (1.45 g, 7.2 mmol) and trimethylamine (1 mL, 7.2 mmol) were added to the solution. After stirring at r.t. for 4 days, the solution was evaporated and purified through gel filtration column (LH-20) with a mixture of chloroform and methanol (1/1, v/v) as an elution solvent. The solid mPEG-pNP 2 (3.12 g) was obtained. Next, mPEG-pNP 2 (220 mg) and (L-Leu-Aib)6 (100 mg) were dissolved in DMF (4 mL) and then the mixture was stirred at r.t. for 4 hours. After evaporation, the compound was purified through LH-20 column with methanol. The white solid of PEG750-L12 3 was isolated. All intermediates were identified by 1H NMR spectroscopy and were further confirmed by MALDI-TOF MS spectrometry.

Scheme 1. Synthetic scheme of PEG750-L12.

mPEG-pNP 2: 1H NMR (400 MHz, DMSO-d6): δ (ppm) 8.32 (d, 2H, OCCHCHCNO2 of phenyl group), 7.57 (d, 2H, OCCHCHCNO2 of phenyl group), 4.36 (t, 2H, CH3OCH2CH2O), 3.59 (t, 2H, CH3OCH2CH2O), 3.52 (m, 71H, OCH2CH2O, CH3O), 3.24 (t, 4H, OCH2CH2OCOO). MALDI-TOF MS; calcd. for C42H75NO22 [M + Na]+: 968.49, found.: 968.43. PEG750-L12 3: 1H NMR (400 MHz, MeOH-d4): δ (ppm) 8.08–7.72 (m, 12H, amide), 4.86–4.02 (m, 6H, LeuCαH), 3.75–3.61 (m, 68H, OCH2CH2O), 3.53 (s, 3H, OCH3), 1.82–1.50 (m, 54H, LeuCH2, LeuCH, AibCH3), 1.00–0.85 (m, 36H, LeuCH3). MALDI-TOF MS; calcd. for C97H182N12O32 [M + Na]+: 2050.31, found.: 2050.13. 29 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

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Integrated Nanostructures Based on Self-Assembled Amphiphilic

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