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Dendritic Amphiphiles: Dendrimers Having an Amphiphile Structure in Each Unit Katsuhiko Ariga,* Toshihiro Urakawa, Atsuo Michiue, Yoshihiro Sasaki, and Jun-ichi Kikuchi Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan Received June 27, 2000. In Final Form: September 29, 2000 We have developed a novel approach for forming dendrimer monolayers. A Lys-Lys-Glu tripeptide with a dioctadecylamino tail at the C-terminal and an acetyl head at the N-terminal was newly synthesized as a unit. Three generations of the dendrimers were successfully synthesized by stepwise deprotection and condensation. The molecular area in the condensed phase of all the dendrimers was comparable to the total area of the alkyl chain cross section, but the packing behavior somewhat depends on the generation number. This method will provide a well-defined two-dimensional arrangement of the hydrophobic tails and polar heads with desirable sequences and combinations.
Introduction To create novel properties and/or to accumulate functions, finer and finer control of the molecular arrangement has been challenged for a long time. Although the Langmuir-Blodgett (LB) technique is one of the most successful approaches in this research field, it provides only a defined layer-by-layer structure. Now, methodologies to control the in-plane structures must be developed.1 One of us demonstrated the formation of a regular amphiphile arrangement based on a specific molecular recognition2a and an artificial hexagonal pattern made of amphiphiles with seven alkyl chains.2b However, the size of the unit component using these approaches is limited. The unit molecules capable of a wide extension must be designed. The concept of a dendrimer may satisfy this requirement, because dendrimers have a well-defined size and shape and are freely extended generation by generation. Several attempts3 to form dendrimer monolayers on water showed successful regulation of the occupied area. The design of the monolayer-forming dendrimers can be classified into two types. One of them is to introduce hydrophobic tails at the outermost generation of a hydrophilic dendrimer.3a,3b In another approach, a hydrophilic tail is attached to a hydrophobic dendrimer.3c,3d In both approaches, an amphiphilic nature was added to the conventional dendrimers. (1) Control of in-plane molecular arrangement has been widely discussed for self-assembled monolayers (SAMs). For recent example, see: Yoshimoto, S.; Hirakawa, N.; Nishiyama, K.; Taniguchi, I. Langmuir 2000, 16, 4399. (2) (a) Oishi, Y.; Torii, Y.; Kato, T.; Kuramori, M.; Suehiro, K.; Ariga, K.; Taguchi, K.; Kamino, A.; Koyano, H.; Kunitake, T. Langmuir 1997, 13, 519. (b) Bissel, P.; Onda, M.; Yoshihara, K.; Koyano, H.; Ariga, K.; Kunitake, T.; Oishi, Y.; Suehiro, K. Langmuir 1999, 15, 1791. (3) (a) Schenning, A. P. H. J.; Elissen-Roma´n, C.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (b) Mulders, S. J. E.; Brouwer, A. J.; Kimkes, P.; Sudho¨lter, E. J. R.; Liskamp, R. M. J. J. Chem. Soc., Perkin Trans. 2 1998, 1535. (c) Kampf, J. P.; Frank, C. W.; Malmstro¨m, E. E.; Hawker, C. J. Langmuir 1999, 15, 227. (d) Iyer, J.; Hammond, P. T. Langmuir 1999, 15, 1299. (e) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. V. Langmuir 1998, 14, 7468. (f) Cardullo, F.; Diedrich, F.; Echegoyen, L.; Habicher, T.; Jayaraman, N.; Leblanc, R. M.; Stoddart, J. F.; Wang, S. Langmuir 1998, 14, 1955. (g) Fre´chet, J. M. J. Science 1994, 263, 1710. (h) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1199.
In this Letter, we propose a more straightforward approach where amphiphiles are dendritically connected in a generation-by-generation manner. The concept of our approach is summarized in Figure 1. Unit amphiphiles are based on a Lys-Lys-Glu tripeptide with hydrophobic tails at the C-terminal and a polar head at the N-terminal. The dendrimer syntheses are proceeded by a stepwise deprotection and condensation of the tripeptide part. Therefore, the hydrophobic tails and the polar head can be introduced in each unit. “Our approach converts amphiphiles to dendrimers, while the previous approaches converted dendrimers to amphiphiles.” Our methodology has the possibility of providing a well-defined twodimensional arrangement of hydrophobic tails and polar heads with desirable sequences and combinations. Experimental Section Materials. Water used for the subphase was distilled in an Autostill WG220 (Yamato) and deionized by a Milli-Q Lab (Millipore). Spectroscopic grade benzene and ethanol (Wako Pure Chem.) were used as the spreading solvents. Gold (99.999%) and chromium (99.99%) were used for the surface modification of the substrates. Synthesis. The chemical shifts of the 1H NMR spectra were recorded using a JEOL JNM-LA400 (400 MHz) spectrometer, and reported relative to chloroform (δ 7.26) or tetramethylsilane (δ 0.00). Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were measured using a Voyger DESTR (PerSective Biosystems) with a dithranol matrix, and the gel permeation chromatography (GPC) measurement was carried out using an SCL-10Avp (Shimadzu) with a TSK-gel R-3000 column (Tosoh). Elemental analyses (C, H, and N) were performed at the Microanalysis Center of Kyushu University. Surface Pressure-Area (π-A) Isotherms. π-A Isotherms were measured using a FSD-300 computer-controlled film balance system (USI System). A mixture of benzene/ethanol (80/20) was used as the spreading solvent. Compression was started about 10 min after spreading at a rate of 0.4 mm‚s-1 (or 60 mm2‚s-1 based on area). The subphase temperature was maintained at 10.0 ( 0.2 °C. FT-IR Measurements. Infrared spectra of a single monolayer on a gold-deposited glass slide, which were prepared with a VPC260 vapor-deposition apparatus (ULVAC), were obtained using an FT-IR spectrometer Magna 560 (Nicolet) equipped with an MCT detector. The monolayer was transferred at 50 mN m-1 by
10.1021/la000901l CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000
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Figure 2. Structures of dendrimers used in this study.
Figure 1. Schematic illustrations of (A) design of the unit amphiphile and (B) dendrimer growth. Plausible conformational models of the G3 dendrimer (see Figure 2) are shown (C) at the phase transition point and (D) in the condensed phase. Peptide chains are denoted by lines to make the images clearer. the horizontal drawing-up method.4 All data were collected in the reflection absorption spectral (RAS) mode at a spectral resolution of 4 cm-1. The measurement was performed under sufficiently flowing dried air.
Results and Discussion Molecular Design and Synthesis. Structures of the synthesized dendrimers are shown in Figure 2. A unit (4) Oishi, Y.; Kuri, T.; Takashima, Y.; Kajiyama, T. Chem. Lett. 1994, 1445.
molecule having a dioctadesylamino tail and an acetyl head (G0) was first synthesized. The glutamate side chain of the unit molecule was deprotected, and linked by ethylenediamine to give the first generation molecule (G1). Connecting the glutamate-deprotected G0 to the lysinedeprotected G1 resulted in the second generation molecule (G2). The third generation molecule (G3) was similarly synthesized. The lysine side chains in these dendrimers are protected by the hydrophobic Boc group. Since peak broadening in 1H NMR spectra was significant for the larger dendrimers, the obtained compounds were identified with the aid of MALDI-TOF-MS, GPC, and elemental analysis.5 Monolayer Properties. Figure 3A shows the π-A isotherms of the unit molecule (G0) and the dendrimers (G1-G3) at 10 °C. A transition behavior was clearly observed, and the molecular areas increased as the dendrimer size increased. For a precise evaluation of the
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Figure 3. π-A isotherms of the dendrimers and their analyses. (A) π-A Isotherms of (a) G0, (b) G1, (c) G2, and (d) G3 at 10 °C. (B) Compressibility isotherm of G3 monolayer at 10 °C. The corresponding π-A isotherm is also shown. From points a and b, the phase transition point and most condensed point were estimated. (C) Molecular areas in expanded phase at (a) 10, (b) 20, and (c) 30 mN m-1 are plotted versus the number of units. (D) Molecular areas at the phase transition point and the most condensed state are plotted versus the number of units: 9, molecular area at the phase transition point; b, molecular area at the most condensed state; 0, cross-sectional area calculated for alkyl chains + Boc groups; O, cross-sectional area calculated for alkyl chains. (E) Hysteresis π-A isotherms of the (a) G2 and (b) G3 monolayers at 10 °C. The first compression was returned at 50 mN m-1.
isotherms, the neighboring nine points in a π-A isotherm were fitted to the second-order equation using the Savitzky-Golay method6 and the mathematically obtained slope of the isotherms was converted to compressibility ((-dA/dπ)/A). The compressibility isotherm of G3 is shown in Figure 3B as an example. A transition point was determined from a local minimum (a in Figure 3B) near an inflection point. A molecular area at the most condensed state was also obtained from a point with the lowest compressibility (b in Figure 3B). A linear relationship between the molecular area and the number of units can be seen in the expanded phase (Figure 3C), indicating that the fluctuation in density is not significant between the generations. The molecular areas at the phase transition point were also analyzed (Figure 3D). The obtained areas linearly increased as the number of amphiphile units increased and are in a good agreement with the area calculated for the alkyl chains + Boc groups based on the assumption of cross-sectional areas of 0.20 and 0.25 nm2 for the single alkyl chain and the Boc group,3b respectively. The areas observed for the most condensed state also linearly increased and agreed with the area estimated for only the alkyl chains (Figure 3D). These behaviors suggest the following image of the assembling process. Both the alkyl chains and the Boc groups are present at the air-water interface in the expanded phase, and they are well packed at the transition point. A further compression induces the escape of the Boc group from the two-dimensional plane probably due to the lower packing ability. A dense packing between the alkyl chains can be achieved in the condensed phase where the Boc groups are probably concealed between the array of the underneath peptide moieties. These structural
motifs are depicted in parts C and D of Figure 1 as plausible models. The replacement of the Boc group by another group would provide useful information on this interpretation. However, the π-A behaviors of G2 and G3 are not completely the same. For example, the area of the most condensed state of the G2 monolayer deviates from the linear relationship and is apparently larger than the summation area of the alkyl chains. A difference also appears in the hysteresis isotherms of the G2 and G3 monolayers (Figure 3E). The compression and expansion curves of the G2 monolayer with a turning pressure of 50 mN m-1 completely traced the same route, implying that the adhesive force between the dendrimer molecules is weak even in the condensed phase. In contrast, the monolayer of G3 showed a difference between the compression and expansion isotherms. Therefore, some strong interactions occur between and/or within the G3 dendrimers in the condensed phase. The FT-IR spectrum of the G2 LB film (Figure 4A) has ν(CdO) and ν(NH) bands at 1667 and 3315 cm-1, respectively, indicating that the peptide moieties are not very strongly hydrogen-bonded. This probably leads to the lower packing nature of G2 in the condensed phase. In contrast, ν(CdO) and ν(NH) bands were observed at 1628 and 3287 cm-1 for the FT-IR spectrum of the G3 LB film (Figure 4B), respectively. Especially, the apparently lower shift in the ν(CdO) frequencies suggests a stronger hydrogen bonding between the peptide moieties. This fact well explains the smaller area in the condensed phase and the strong hysteresis behavior. The hydrogen bonding formation might originate from the increased local density of the units located at the outer shell of G3. The difference between the dendrimers also appears in the π-A behavior.
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and Culture. We appreciate Professor Yoshio Hisaeda of Kyushu University for the elemental analyses. We thank Professor Yoshio Okahata of Tokyo Institute of Technology for advise on the MALDI-TOF-MS measurements. Supporting Information Available: MALDI-TOF-MS data for G0, G1, G2, and G3. This material is available free of charge via the Internet at http://pubs.acs.org. LA000901L
Figure 4. FT-IR spectra in RAS mode of a single monolayer on a gold-deposited glass slide: (A) G2; (B) G3.
The first-order transition would be observed in the G3 monolayers, while G2 seems to have the second-order transition (see Figure 3A). In this Letter, we present a straightforward method for applying dendrimer chemistry to a two-dimensional system. This method allows us to introduce hydrophobic tails and a polar head to dendrimer monolayers with desirable sequences and combinations. For example, it will provide media suitable for direction-specific energy or electron transfer7 and defined sugar clusters useful for the investigation of lectin recognition on a membrane surface.8 Experiments along this line including molecular image observations are now in progress in our laboratory. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Area (No. 282) from the Ministry of Education, Science, Sports,
(5) G0. Mp 74.4-75.0 °C. 1H NMR (400 MHz, DMSO-d6): δ ) 0.85 (t, J ) 6.8 Hz, 6H, CH3(CH2)16), 1.16-1.26 (m, 60H, CH3(CH2)15), 1.251.65 (m, 34H, (CH3)3C + (CH2)15CH2CH2N + β-CH2 (Lys) + γ-CH2 (Lys) + δ-CH2 (Lys)), 1.70-1.86 (m, 5H, CH3CO + β-CH2 (Glu)), 2.38 (m, 2H, γ-CH2 (Glu)), 2.86 (m, 4H, -CH2 (Lys)), 3.09-3.31 (m, 4H, (CH2)16CH2N), 4.21 (m, 2H, R-CH2 (Lys)), 4.73 (m, 1H, R-CH2 (Glu)), 5.08 (s, 2H, PhCH2O), 6.69 (m, 2H, NH (Lys side chain)), 7.34 (m, 5H, Ph), 7.907.98 (m, 3H, CONH (peptide main chain)). MS (MALDI-TOF; m/z): Calcd for C72H130N6O10 1239.0; Found 1262.0 (M + Na)+. GPC (TSKgel R-3000, THF): Mw/Mn ) 1.02. Anal. Calcd for C72H130N6O10: C, 69.75; H, 10.57; N, 6.78. Found: C, 69.84; H, 10.56; N, 6.83. G1. Mp 170.1-171.3 °C. 1H NMR (400 MHz, CDCl ): δ ) 0.87 (t, J ) 6.8 Hz, 12H, CH (CH ) ), 3 3 2 16 1.24-1.26 (m, 120H, CH3(CH2)15), 1.26-1.45 (m, 60H, (CH3)3C + (CH2)15CH2CH2N + γ-CH2 (Lys) + δ-CH2 (Lys)), 1.72 (m, 12H, β-CH2 (Lys) + β-CH2 (Glu)), 2.05 (s, 6H, CH3CO), 2.14 (m, 4H, γ-CH2 (Glu)), 2.75-3.20 (m, 12H, -CH2 (Lys) + NHCH2CH2NH), 3.29 (m, 4H, (CH2)16CH2N), 3.45 (m, 4H, (CH2)16CH2N), 4.47 (m, 4H, R-CH (Lys)), 4.84 (m, 2H, R-CH (Glu)), 5.01 (s, 2H, NHCH2CH2NH), 6.16-6.08 (m, 4H, NH (Lys side chain)), 7.84-8.45 (m, 6H, CONH (peptide main chain)). MS (MALDI-TOF; m/z): calcd for C132H252N14O18 2323.0; found 2346.2 (M + Na)+. GPC (TSKgel R-3000, THF): Mw/Mn ) 1.02. Anal. Calcd for C132H252N14O18: C, 68.23; H, 10.93; N, 8.44. Found: C, 67.96; H, 10.89; N, 8.34. G2. Mp 150.9-151.7 °C. 1H NMR (400 MHz, CDCl3): δ ) 0.88 (t, J ) 6.8 Hz, 36H, CH3(CH2)16), 1.15-1.32 (m, 360H, CH3(CH2)15), 1.32-1.47 (m, 144H, (CH3)3C + (CH2)15CH2CH2N + γ-CH2 (Lys) + δ-CH2 (Lys)), 1.57 (m, 36H, β-CH2 (Lys) + β-CH2 (Glu)), 2.00 (s, 24H, CH3CO), 2.14 (m, 12H, γ-CH2 (Glu)), 2.95-3.15 (m, 28H, -CH2 (Lys) + NHCH2CH2NH), 3.27 (m, 12H, (CH2)16CH2N), 3.45 (m, 12H, (CH2)16CH2N), (Peaks of amide protons and R-CH protons were too broad to be assigned). MS (MALDI-TOF; m/z): calcd for C372H708N38O46: 6449.8; found: 6473.1 (M + Na)+. GPC (TSKgel R-3000, THF): Mw/Mn ) 1.03. Anal. Calcd for C372H708N38O46‚2H2O: C, 68.89; H, 11.06; N, 8.21%. Found: C, 68.70; H, 10.95; N, 8.21%. G3. Mp 79.279.8 °C. 1H NMR (400 MHz, CDCl3): δ ) 0.87 (t, J ) 6.8 Hz, 84H, CH3(CH2)16), 1.16-1.31 (m, 840H, CH3(CH2)15), 1.31-1.45 (m, 312H, (CH3)3C + (CH2)15CH2CH2N + γ-CH2 (Lys) + δ-CH2 (Lys)), 1.59 (m, 84H, β-CH2 (Lys) + β-CH2 (Glu)), 2.01 (s, 42H, CH3CO), 2.14-2.37 (m, 28H, γ-CH2 (Glu)), 2.95-3.11 (m, 12H, -CH2 (Lys) + NHCH2CH2NH), 3.17-3.21 (m, 28H, (CH2)16CH2N), 3.43-3.48 (m, 28H, (CH2)16CH2N), 4.39 (m, 28H, R-CH (Lys)), 4.89 (m, 32H, R-CH (Glu) + part of amide protons) (Peaks of amide protons were too broad to be assigned). MS (MALDI-TOF; m/z): calcd for C852H1620N86O102: 14703; found: 14726.8 (M + Na)+. GPC (TSKgel R-3000, THF): Mw/Mn ) 1.03. Anal. Calcd for C852H1620N86O102‚25H2O: C, 67.53; H, 11.11; N, 7.95. Found: 67.89; H, 10.88; N, 7.55 (hygroscopic sample). (6) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1964, 36, 1627. (7) (a) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. (b) Selby, T. D.; Blackstock, S. C. J. Am. Chem. Soc. 1998, 120, 12155. (c) Schenning, A. P. H.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 4489. (d) Vo¨gtle, F.; Gestermann, S.; Kauffmann, C.; Ceroni, P.; Vicinelli, V.; De Cola, L.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 12161. (e) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (8) Mammen, M.; Choi, S.-K.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 2754.
