Terminal Peptide Directed Assembly of Naphthalene-Bisimides

Article pubs.acs.org/crystal

Terminal Peptide Directed Assembly of Naphthalene-Bisimides Poulami Jana, Santu Bera, Arpita Paikar, and Debasish Haldar* Department of Chemical Sciences, Indian Institute of Science Education and Research−Kolkata, Mohanpur, Nadia, West Bengal 741252, India S Supporting Information *

ABSTRACT: The self-assembly of two naphthalene-bisimide based nonionic bolaamphiphiles containing two terminal tripeptide moieties has been investigated. The bisimide 1 containing a core of adjacent aromatic rings and two termini of folded tripeptide moieties (-Tyr-Aib-LeuOMe) adopts a dumbbell shape conformation, and it self-assembles through noncovalent interactions to fabricate microspheres. In contrast, the bisimide 2 containing two termini of extended tripeptide moieties (-Phe-Phe-Tyr-OMe) adopts a wrist band shape structure, and it self-assembles to produce elongated fibrils. The X-ray crystallography reveals that the bisimide 1 adopts a dumbbell shape with two terminal β-turns, and it self-associates to form a rhombuslike structure in higher order packing. Moreover, the conductivity of the bisimide 2 is 2 orders of magnitude higher than that of the bisimide 1 in room light. The secondary structures of the terminal tripeptides of bisimide systems and the peptide−peptide interactions are the driving forces for the assembly process.

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INTRODUCTION The directed assembly of building blocks by noncovalent interactions including hydrogen bonds, electrostatic interactions, π−π stacking, and hydrophobic interactions is highly important for fabrication of novel materials and devices.1 The n-channel organic semiconductors2 having aromatic cores such as naphthalene3 and perylene4 derivatives are frequently employed as interesting building blocks due to their planarity and relatively low reduction potentials.5,6 The bisimides usually have high melting points and π−π stacking interaction propensity, which enhances the intermolecular π-orbital overlap and facilitates charge transport.7 Various types of aggregation patterns like J-aggregation, H-aggregation, brickwork, ladder, and staircase structure have been reported for the self-assembly of bisimide building blocks.8 However, there are few examples where the self-assembly of bisimides can be controlled by rational design.9 Recently, Würthner and co-workers have reported the design of an organogelator without solubilizing side chains by backbone contortion of a perylene bisimide.9e Previously, we have reported the self-assembly of various bisimides containing chiral amino acids.10 We have also reported the fabrication of self-assembled naphthalene bisimide-based fluorescent microspheres that can be used as turn on fluoride sensor in aqueous medium.11 Intrigued by the previous report, we wanted to investigate whether naphthalene bisimides can be designed with peptide secondary structure at terminal position and to study their intrinsic folding and self-assembling nature. In this report, we have described the design, synthesis, and structural evaluation of bisimides containing a core of adjacent aromatic rings and two termini of diverse tripeptide moieties. We show that out of this series the bisimide 1 containing a core of adjacent aromatic rings and two termini of folded tripeptide moieties (-Tyr-Aib© 2014 American Chemical Society

Leu-OMe) adopts dumbbell shape and self-assembles to produce microspheres morphology. However, the bisimide 2 containing two termini of extended tripeptide moieties (-PhePhe-Tyr-OMe) form wrist brand shape and self-assembles to form elongated fibrils. The X-ray crystallography shed some light on the atomic level structures and self-assembly of the reported bisimide 1.

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RESULTS AND DISCUSSION Background. For naphthalene bisimides 1 and 2 the design principle explored the effect of peptide secondary structures and peptide−peptide interactions in the realm of the central aromatic ring of the naphthalene bisimides. The reported naphthalene bisimides 1 and 2 (Figure 1) were synthesized by dissolving naphthalene dianhydride and respective tripeptide methyl ester (NH2-Tyr-Aib-Leu-OMe/NH2-Phe-Phe-TyrOMe) in glacial acetic acid and refluxed for 6 h. The tripeptides

Figure 1. Schematic presentation of bisimides 1 and 2. Received: April 11, 2014 Revised: June 11, 2014 Published: June 18, 2014 3918

dx.doi.org/10.1021/cg500501r | Cryst. Growth Des. 2014, 14, 3918−3922

Crystal Growth & Design

Article

were obtained by deprotection of Boc-Tyr-Aib-Leu-OMe12 and Boc-Phe-Phe-Tyr-OMe13 using a TFA−anisole (19:1) mixture. For bisimide 1, we used the helicogenic Aib (α-aminoisobutyric acid)14 and Leu residues to introduce additional hydrophobicity and steric hindrance. For bisimide 2, we used the Phe residues to introduce additional π−π interactions. The synthesized compounds were purified and characterized by 1H NMR, 13C NMR, FTIR, and mass spectrometry (MS) analysis. Previously, we have reported that the tripeptide Boc-Tyr-Aib-Leu-OMe adopts a very compact β-turn structure12 and tripeptide BocPhe-Phe-Tyr-OMe13 adopts the extended structure. We assume that the intrinsic folding propensity of these peptides will develop a dumbbell shape for bisimide bolaamphiphilie 1 and wrist band shape for the extended structure of bisimide bolaamphiphilie 2. The aggregation behaviors of the reported bisimides were observed by UV/vis spectroscopy, CD spectroscopy, polar optical microscopy, and atomic force microscope (AFM). Solution Analysis. Initially, the self-assembly of the reported naphthalene bisimides bolaamphiphiles 1 and 2 containing chiral amino acids have been studied using a wide variety of different spectroscopic techniques. The typical UV/ vis absorption spectra of bisimide bolaamphiphiles 1 and 2 in methanol show absorption bands at 380 and 360 nm due to characteristic π−π* transition (ESI Figure 1a and 1c, respectively, Supporting Information). With increasing the concentration, bisimides 1 and 2 exhibit no change in their respective absorption band. For emission spectra, we have started with a lower concentration and then gradually increased the concentration of the corresponding bisimides. For bisimide bolaamphiphile 1, with increasing concentration, the fluorescence intensity at 433 and 466 nm gradually decreases upon excitation at 375 nm (ESI Figure 1b, Supporting Information). For bisimide bolaamphiphile 2, with increasing concentration, the fluorescence intensity at 410 and 425 nm gradually decreases upon excitation at 375 nm (ESI Figure 1d, Supporting Information). The aggregation of these bisimide bolaamphiphiles due to hydrophobic effects may enhance static quenching. CD is an excellent method to determine the aggregation propensity of the reported bisimide bolaamphiphiles. The CD spectra of bisimides also exhibit general information on secondary structure of the terminal peptides. Circular dichroism spectroscopy of bisimide bolaamphiphilie 1 in dilute methanol solution indeed shows a strong negative Cotton effect centered around 198 and 222 nm. Bisimide bolaamphiphilie 2 in dilute methanol solutions shows strong positive Cotton effect around 200 and 217 nm (Figure 2). The results indicate that the selfassembly pattern of bisimide 2 is significantly different from that of bisimide 1. Morphology. Further, polar optical microscope has been used to study the morphology of the reported bisimides bolaamphiphiles 1 and 2 in solid state. The solution of the corresponding bisimides in methanol (0.2 mM) was aged for 12 h and drop casted on a microscopic coverslip and allowed to dry under vacuum at 30 °C for 2 days. Figure 3a shows representative polar optical micrograph (POM) of bisimide bolaamphiphile 1 under cross polarizer. The micrograph with higher magnification and black background (Figure 3a inset) clearly shows the birefringence color. For bisimide bolaamphiphile 2, birefringence febrile appeared under cross polarizer (Figure 3b).

Figure 2. CD spectra of bisimide bolaamphiphiles 1 (black) and 2 (red) in methanol.

Figure 3. (a) Optical micrograph of bisimide bolaamphiphilie 1 under polarizer and high magnification (100×) with black background (inset). (b) Polar optical micrograph (10×) of bisimide bolaamphiphilie 2 fibers.

The self-assembly of the bisimide bolaamphiphiles were further studied by atomic force microscope (AFM). The solution of the corresponding bolaamphiphiles in methanol (0.234 mM) was aged for 12 h and drop casted on a microscopic coverslip and allowed to dry under vacuum at 30 °C for 2 days. Investigation by AFM revealed the formation of microspheres for bolaamphiphiles 1 (Figure 4a). Figure 4b is showing the 3D image of bisimide 1 microspheres. The bisimide 1 microspheres have diameter ca. 120 nm. However, bisimide bolaamphiphilie 2 exhibits entangled nanofiber morphology with diameter ca. 70 nm (Figure 4c,d). The FESEM experiments also exhibit microsphere morphology for bisimide 1 and nanofiber morphology for bisimide 2 (ESI Figure 2). Solid State Analysis. The aggregated mass obtained from the methanol−water solution of the respective bisimides was examined by solid-state FTIR spectroscopy to investigate structures of these peptide bisimides. For bisimide 1, the band at 3387 cm−1 indicates that all NH groups and phenolic OH groups are hydrogen bonded.15 The amide I and amide II bands are appeared at 1664 and 1516 cm−1, respectively (ESI Figure 3, Supporting Information). This suggests that the peptide in bisimide 1 adopts a kink-like backbone conformation, and overall bisimide 1 has a dumbbell-like shape. The bisimide 2 has a band at 3393 cm−1. The amide I and amide II bands appeared at 1670 and 1516 cm−1. This suggest that the peptide in bisimide 2 adopts an extended backbone structure.16 3919



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hydrogen bonds between imide CO and Leu NH resulting in a ten-membered hydrogen bonded β-turn conformation (Figure 5a). Aromatic amides prefer coplanarity of the carbonyl functional group with the aryl system to optimize conjugation. A close look at the central π-scaffold, the β-turn forming tripeptide head groups induce backbone contortion in the central aromatic core of the naphthalene bisimide 1 (Figure 5b). The steric hindrance and competitive intramolecular Hbonding between Leu NH and bisimide core imide CO appear to play a critical role in dictating the overall structural features of bisimide 1. Further, the bisimide 1 molecules are themselves regularly self-assembled through intermolecular π−π interactions between side chain tyrosine ring with electron deficient naphthalene bisimide core and intermolecular hydrogen bond between tyrosine−OH with Aib NH and generate a staircaselike architecture (ESI Figure 4). The centroid to centroid distances between tyrosine side chains and central naphthalene diimide template are 3.64 and 3.55 Å. In higher order packing, the individual staircase-like columns of bisimide 1 are selfassembled through noncovalent interactions to form a rhombus framework stacking motif (Figure 5c). Like the tripeptide BocPhe-Phe-Tyr-OMe,13 the bisimide 2 did not crystallize. Photoelectric Response. A simple conductor device was fabricated by two Au contacts of area 1 mm2 separated by a distance of 3 mm on a pellet made of either bisimide 1 or bisimide 2. The room light (without filtering the UV light) was shined on the conductor area to measure the photoresponse property. The current between the two contacts were measured using a Keithley source meter (model 2401). Under light illumination, the bisimide bolaamphiphilie 1 exhibits very low current conduction, as shown in Figure 6a. However, under

Figure 4. AFM images of (a) bisimide 1 microspheres at 0.234 mM, (b) 3D image of bisimide 1 microspheres, and (c,d) bisimide 2 nanofibers at 0.234 mM.

Crystal Structure Analysis. X-ray crystallography sheds some light on molecular structure and self-assembly pattern of the chiral bisimide 1. From X-ray crystallography, it is evident that the asymmetric unit contains one molecule of bisimide 1 with one molecule of ethyl acetate.17 From Figure 5a, the tripeptide chains are not in complete trans position in bisimide 1 and the C2 symmetry is lost. There are two intramolecular

Figure 6. Plots of I−V curves of (a) bisimide 1 and (b) bisimide 2 under room light. (Inset) Schematic presentation of the conductor device.

light illumination on the bisimide bolaamphiphilie 2 matrix, the current conduction increases (Figure 6b). The results clearly illustrate that the highest current has been obtained for the bisimide 2, which is about 2 orders of magnitude higher compared to that of the bisimide 1. The enhancement is probably due to proper packing of the bisimide 2 core, which

Figure 5. Molecular structure of (a) bisimide 1 in solid state. The intra molecular hydrogen bonds are showing as a dotted line. Aib side chains here appear as pink spheres and Leu side chains as purple spheres. (b) The stick model of bisimide 1 showing contortion in central π-scaffold. (c) The schematic presentation of rhombus packing motif of bisimide 1 in solid state. 3920



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Hz, Tyr-ring proton), 6.86−6.84 (d, 2H, J = 10 Hz, NH), 6.7−6.68 (d, 4H, J = 10 Hz, Tyr-ring proton), 6.45−6.44 (broad, 2H, −OH), 5.95− 5.92 (m, 2H, Cα H Phe), 4.79−4.78 (m, 2H, Cα H Phe), 4.63−4.61 (m, 2H, Cα H Tyr), 3.57 (s, 6H, OMe), 3.52−3.48 (m, 2H, Cβ H Phe), 3.42−3.38(m, 2H, CβH Phe), 3.08−3.06 (m, 2H, Cβ H Tyr), 2.95−2.93 (m, 4H, Cβ H Phe), 2.91−2.82(m, 2H, CβH Tyr). 13C NMR (125 MHz, CDCl3, δ in ppm): 171.41, 170.12, 168.95, 162.54, 155.18, 136, 135.94, 131.04, 130.18, 129.20, 128.58, 127.10, 126.97, 125.94, 115.36, 55.73, 54.36, 53.92, 52.97, 37.58, 36.65, 34.36, 29.67. Mass spectra: m/z 1234[M + Na]+; Mcalcd 1211.27. NMR Experiments. All NMR studies were carried out on a Brüker AVANCE 500 MHz spectrometer at 278 K. Compound concentrations were in the range 1−10 mM in CDCl3 and (CD3)2SO. FTIR Spectroscopy. All reported solid-state FTIR spectra were obtained with a PerkinElmer Spectrum RX1 spectrophotometer with the KBr disk technique. Mass Spectrometry. Mass spectra were recorded on a Q-Tof Micro YA263 high-resolution (Waters Corporation) mass spectrometer by positive-mode electrospray ionization. Atomic Force Microscopy. The morphologies of the reported bisimides were investigated by AFM. A small amount of solution (0.5 mg/mL MeOH) of the corresponding compounds was placed on a microscope cover glass and then dried by slow evaporation. The material was then allowed to dry in a vacuum at 30 °C for 2 days. Images were taken with an NTMDT instrument, model no. AP-0100 in semicontact-mode. Single Crystal X-ray Diffraction Study. Orange crystals of bisimide 2 suitable for X-ray diffraction studies were obtained from their respective ethyl acetate solution by slow evaporation. Singlecrystal X-ray analysis of bisimide 2 was recorded on a Bruker highresolution X-ray diffractometer instrument. UV/Vis Spectroscopy. UV/vis absorption spectra were recorded on a UV/vis spectrophotometer (Hitachi). Photoelectric Response. A conductor device was fabricated by evaporating two Au contacts of area 1 mm2 separated by a distance of 3 mm on a pellet made of bisimides. The room light (without filtering the UV light) was shined on the conductor area to measure the photoresponse property. The current between the two contacts was measured using a Keithley source meter (model 2401).

has a large effect on reducing the number of trap states by defect modification.

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CONCLUSIONS In conclusion, the solution and solid state analysis reveal that naphthalene-bisimide based nonionic bolaamphiphile consisting of two-folded tripeptide moieties at both terminal positions forms a dumbbell shape structure, and this is further selfassembled to form microsphere morphology. However, the bolaamphiphile consisting of two extended tripeptide moieties at terminal positions adopts wrist brand shape, and it selfassembles to form a fibrillar structure. The atomic force microscope (AFM) images showed the approximate size and shape of these microspheres and fibers. In addition, from X-ray crystallographic study, it is evident that the bisimide 1 adopts a dumbbell shape with two terminal β-turns and self-associate to form rhombus framework packing. The photoelectric response of the fibrillar bisimide bolaamphiphile 2 is 2 orders of magnitude higher than that of microspheres bisimide bolaamphiphile 1 in room light. These findings indicate that the modulation of head groups can be used as a crucial tool to control structure and properties of naphthalene bisimides.

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EXPERIMENTAL SECTION

General. All L-amino acids were purchased from Sigma chemicals. 1-Hydroxybenzotriazole (HOBt) and dicyclohexylcarbodiimide (DCC) were purchased from SRL. Bisimides Synthesis. The bisimides were synthesized by solutionphase methodology. The tripeptide C-terminus was protected by a methyl ester. All the bisimides were fully characterized by 500 and 400 MHz 1H NMR spectroscopy, 13C NMR spectroscopy, FTIR spectroscopy, and mass spectrometry. The bisimide 1 was characterized by X-ray crystallography. (a). Bisimide (1). 1,4,5,8-Naphthalene dianhydride (1 g, 3.72 mmol) was dissolved in glacial acetic acid (30 mL) by warming to 40 °C. HTyr-Aib-Leu-OMe (5.86 g, 14.91 mmol) was added to the reaction mixture and refluxed for 4 h. Transparent light yellow solution was concentrated under reduced pressure; 20 mL of DCM was added. The organic layer was washed with 2 M HCl (3 × 50 mL), brine (2 × 50 mL), 1 M sodium carbonate (3 × 50 mL), and brine (2 × 50 mL) and dried over anhydrous sodium sulfate and evaporated in a vacuum to obtain 1 as a white solid. Yield 2 g (52.76%). 1H NMR (400 MHz, CD3OD, δ in ppm): 8.64 (s, 4H, bisimide aromatic ring protons), 8.13 (s, 2H, NH), 7.43−7.41 (d, 2H, J = 8 Hz, NH), 6.84−6.82 (d, 4H, J = 8 Hz, Tyr-ring), 6.37− 6.35 (d, 4H, J = 8 Hz, Tyr-ring), 5.91−5.90 (broad, 2H, −OH), 4.55− 4.52 (m, 2H, Cα H Tyr), 4.20−4.17 (m, 2H, Cα H Leu), 3.71 (s, 6H, OMe), 2.02−2.00 (m, 4H, Cβ H Tyr), 1.70−1.6 (m, 2H, Cβ H Leu), 1.54 (s, 6H, AIB), 1.30 (s, 6H, AIB), 1.28 (m, 2H, CβH Leu), 1.08− 1.06 (d, 6H, J = 8 Hz, CδH Leu), 1.01−1.00 (d, 6H, J = 8 Hz, CδH Leu), 0.90−0.89 (m, 2H, CγH Leu).13C NMR (100 MHz, CD3OD, δ in ppm): 177.01, 174.12, 170.78, 164.67, 156.99, 132.00, 131.32, 128.89, 127.94, 127.79, 115.97, 58.82, 58.55, 57.23, 52.48, 38.63, 34.39, 30.70, 27.12, 26.64, 24.01. Mass spectra: m/z 1042 [M + Na]+; Mcalcd 1019.1. (b). Bisimide (2). 1,4,5,8-Naphthalene dianhydride (1 g, 3.72 mmol) was dissolved in glacial acetic acid (30 mL) by warming to 40 °C. HPhe-Phe-Tyr-OMe (7.28 g, 14.91 mmol) was added to the reaction mixture and refluxed for 4 h. Transparent light yellow solution was concentrated under reduced pressure; 20 mL of DCM was added. The organic layer was washed with 2 M HCl (3 × 50 mL), brine (2 × 50 mL), 1 M sodium carbonate (3 × 50 mL), and brine (2 × 50 mL) and dried over anhydrous sodium sulfate and evaporated in a vacuum to obtain 2 as a white solid. Yield 2.3 g (51.05%). 1H NMR (500 MHz, CDCl3, δ in ppm): 8.39 (s, 4H, bisimide aromatic ring protons), 7.28−7.27 (d, 2H, J = 5 Hz, NH), 7.19−7.08 (20H, aromatic proton), 7.02−7.00 (d, 4H, J = 10

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and spectral characterization of bisimides 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(D.H.) E-mail: [email protected] or deba_h76@iiserkol. ac.in. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the DST, New Delhi, India, for financial assistance Project No. (SR/FT/CS-041/2009). P.J. thanks C.S.I.R., New Delhi, India for fellowship. S.B. and A.P. wish to acknowledge the UGC, India for research fellowship. We gratefully acknowledge Dr. Sudip Malik, I. A. C. S., Kolkata700032, India, for the photoelectric response experiments.

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REFERENCES

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Terminal Peptide Directed Assembly of Naphthalene-Bisimides

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