Stimuli Induced Structural Changes of Gold Nanoparticle

Masahiro Higuchi*, Kenji Nagata, Sohei Abiko, Masayoshi Tanaka and Takatoshi Kinoshita. Department of Materials Science and Technology, and Department...
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Langmuir 2008, 24, 13359-13363

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Stimuli Induced Structural Changes of Gold Nanoparticle Assemblies Having Sequential Alternating Amphiphilic Peptides at the Surface Masahiro Higuchi,*,† Kenji Nagata,† Sohei Abiko,† Masayoshi Tanaka,†,‡ and Takatoshi Kinoshita†,‡ Department of Materials Science and Technology, and Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-5888, Japan ReceiVed August 4, 2008. ReVised Manuscript ReceiVed September 26, 2008 Gold nanoparticles having sequential alternating amphiphilic peptide chains, Phe-(Leu-Glu)8, on the surface have been prepared. We describe structural control of the amphiphilic peptide coated gold nanoparticle assembly by a conformational transition of the surface peptides. Under the acidic condition, the conformation of the surface amphiphilic peptide was converted to a β-sheet structure from an aggregated R-helix by incubation. Under this condition, the amphiphilic peptide coated gold nanoparticles formed a nanosheet assembly. The plasmon absorption maximum of the gold nanoparticles shifted to a shorter wavelength with the formation of the β-sheet assembly of the surface peptide. This suggests that the structure of the peptide coated gold nanoparticle assembly could be controlled by the conformational transition of the surface peptide. Furthermore, the core gold nanoparticle could be fixed in the β-sheet assembly in the state that stood alone. This system may be useful for novel molecular devices that exhibit quantized properties.

Introduction The design and assembly of ordered molecular nanostructures has become one of the key aims of new materials and molecular devices. Assemblies of nanoscale materials into specific structures composed of semiconductor and/or metal nanoparticles are predicted to exhibit quantized properties and have become a focus for novel functional devices1-7 such as optoelectronics and medical diagnostics. These functions are closely related to the arrangement of nanoparticles in the assemblies. The formation of ordered arrangement of nanoparticles has been vigorously investigated. Nanoparticles having organic molecules such as peptides,8 oligonucleotides,9,10 and oligosaccharides11 as building blocks have been utilized to form ordered and periodicity arrangements of the nanoparticles. Especially, the structure of peptide assemblies can be controlled by external stimuli such as pH.12,13 Therefore, the functions of peptide coated nanoparticle assemblies may be regulated by the stimuli induced rearrangement of the nanoparticles owing to the structural changes of the surface peptide chains. * To whom correspondence should be addressed. [email protected]. † Department of Materials Science and Technology. ‡ Department of Frontier Materials.

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(1) Alivisatos, A. P. Science 1996, 271, 933–937. (2) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (3) Sun, S.; Murry, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (4) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226–229. (5) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1994, 116, 7413–7414. (6) Mirkin, C. A.; Taton, T. A. Nature 2000, 405, 626. (7) Otsuka, H.; Akiyama, Y.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2001, 123, 8226–8230. (8) Higashi, N.; Kawahara, J.; Niwa, M. J. Colloid Interface Sci. 2005, 288, 83–87. (9) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640–4650. (10) Hazarika, P.; Ceyhan, B.; Niemeyer, C. M. Angew. Chem., Int. Ed. 2004, 43, 6469–6471. (11) Bae, A.-H.; Numata, M.; Hasegawa, T.; Li, C.; Kaneko, K.; Sakurai, K.; Shinkai, S. Angew. Chem., Int. Ed. 2005, 44, 2030–2033. (12) Koga, T.; Taguchi, K.; Kobuke, Y.; Kinoshita, T.; Higuchi, M. Chem.sEur. J. 2003, 9, 1146–1156. (13) Higuchi, M.; Inoue, T.; Miyoshi, H.; Kawaguchi, M. Langmuir 2005, 21, 11462–11467.

Here, we describe the preparation of peptide coated gold nanoparticles and structural control of the peptide-gold nanoparticle assembly by conformational transition of the surface peptides. We chose a sequential alternating amphiphilic peptide, whose amino acid sequence is Phe-(Leu-Glu)8, as the surface peptide chain on the gold nanoparticle. The sequential alternating amphiphilic peptides have been observed with the reversible helix-sheet conformational transition.14-16 Under the basic condition, the sequential alternating amphiphilic surface peptide took an R-helical conformation containing a considerable amount of random coil and β-sheet structures. On the other hand, the conformation of the surface peptide was converted to a β-sheet structure from an aggregated R-helix by incubation under the acidic condition. The conformational transition of the surface peptide induced the morphological changes of the peptide coated gold nanoparticle assembly from the globular to sheet structure. Interestingly, the plasmon absorption maximum of the gold nanoparticles shifted to a shorter wavelength with the formation of the nanosheet assembly. This suggested that the core gold nanoparticle could be fixed in the assembly in the state that stood alone. This system may be useful for the novel molecular devices that exhibit quantized properties.

Experimental Section Materials. Amhiphilic Sequential Peptide. The Phe-(Leu-Glu)8 sequence was chosen as a β-sheet forming element. And thioctic acid for the gold binding site was introduced at the amino terminal of the sequential alternating amphiphilic peptide (Chart 1). The peptide, Phe-(Leu-Glu)8-S, having thioctic acid was synthesized by the conventional solid-phase method.17 A Fmoc-Phe loaded CLEARAcid-Resin (0.4 mequiv/g, Peptide Institute Inc.) was used as the resin for peptide synthesis. A total of 0.5 g of the resin was swelled by 5 mL of dichloromethane for 1 day in the reaction vessel, and then the resin was rinsed by 5 mL of pure dimethylformamide (DMF) (14) Hong, D.-P.; Hoshino, M.; Kuboi, R.; Goto, Y. J. Am. Chem. Soc. 1999, 121, 8427–8433. (15) Fukushima, Y. Chem. Lett. 1999, 28, 157–158. (16) Sugimoto, N.; Zou, J.; Kazuta, H.; Miyoshi, D. Chem. Lett. 1999, 637, 638. (17) Ramirez-Goodwin, K. A.; Rowlen, K. L. Langmuir 1998, 14, 2562– 2566.

10.1021/la802527n CCC: $40.75  2008 American Chemical Society Published on Web 11/07/2008

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Chart 1. Sequential Alternating Amphiphilic Peptide, Phe-(Leu-Glu)8-S

three times. A solution of Fmoc-amino acid (0.7 mmol) in 3 mL of DMF, 1,3-diisopropylcarbodiimide (0.7 mmol) in 1 mL of DMF, and 1-hydroxy-7-azabenzotriazole (0.7 mmol) in 1 mL of DMF was added to the resin, and then the suspension was shaken for 2 h to attach the Fmoc-amino acid to the amino group on the resin. After the reaction, the resin was rinsed by pure DMF and then the DMF solution containing 20 vol % piperidine was added to the vessel to remove the amino terminal Fmoc-protecting group for 1 h. After the reaction, the resin was rinsed by pure DMF until the piperidine was completely removed. This reaction cycle was repeated successively to obtain the desired sequence. The thioctic acid was then attached to the amino terminal of the peptide on the resin by the same protocol described above. After the coupling reactions were completed, the peptide-resin was dried under vacuum and then cooled in an ice bath. A total of 10 mL of cooled aqueous solution containing 95 vol % trifluoroacetic acid (TFA) was added to the cooled peptide-resin to remove the OBut protecting groups of Glu side chains and to cleavage the peptide from the resin support for 1 h at room temperature. After the reaction, the reaction mixture was filtrated to separate the peptide solution from the resin support. The TFA solution of the peptide was concentrated to a volume of approximately 1-2 mL, and then 100 mL of cooled ether was added to the TFA solution to precipitate the peptide. Identification of the peptide was made by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy (SHIMADZU /KRATOS KOMPACT II, Kratos Analytical). The molecular weight of Phe-(LeuGlu)8-S was 2288. This value was in fair agreement with calculated value, 2292. Gold Nanoparticle HaVing Amphiphilic Sequential Peptide on the Surface. Phe-(Leu-Glu)8-S was dissolved in 40 mL of DMF. Hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4 · 4H2O) was dissolved in 10 mL of purified water. The HAuCl4 aqueous solution was then slowly added to the Phe-(Leu-Glu)8-S solution in a vessel to make the desired concentration of Phe-(Leu-Glu)8-S and HAuCl4. The molar ratio of HAuCl4 to Phe-(Leu-Glu)8-S was fixed to 2.3. The solution was vigorously stirred during the addition of 10 mL of ice-chilled aqueous sodium borohydride (NaBH4) (10 times the molar concentration of HAuCl4) at room temperature for 1 h. The final concentration of HAuCl4 was 0.2 mM. After the reaction, this reaction mixture was dialyzed overnight against 1 L of distilled water using a Spectra/Pore molecular porous membrane tube (Spectrum Medical Industries, Inc., MWMC 3500). After the dialysis, the solution was lyophilized to obtain the gold nanoparticles having Phe-(Leu-Glu)8 on the surface (Phe-(Leu-Glu)8-Au). Methods. Spectroscopic Measurements. Adsorption and circular dichroism (CD) spectra of Phe-(Leu-Glu)8 coated gold nanoparticles were recorded on a Jasco V-550 UV/vis spectrophotometer and Jasco J-820 spectropolarimeter, respectively. Experiments were performed in a quartz cell with a 5 mm path length at ambient temperature. The concentration of Phe-(Leu-Glu)8-Au nanoparticles in aqueous solution containing 40 vol % trifluoroethanol (TFE) was fixed at 0.25 mg/mL. Microscopic Measurements. The size of gold core of the Phe(Leu-Glu)8-Au nanoparticle was estimated from a transmission electron micrographs. Samples were prepared by slow evaporation of one drop of a Phe-(Leu-Glu)8-Au nanoparticle suspension in 40 vol % TFE aqueous solution on a poly(vinyl formal) coated copper mesh grid. Electron microscopy was carried out using a Hitachi H-7000 electron microscope operating at 100 kV. Initial magnification was ×200 000.

Figure 1. UV/vis spectrum of Phe-(Leu-Glu)8-Au nanoparticles in TFE.

The morphology of the Phe-(Leu-Glu)8-Au nanoparticle was observed with an atomic force microscope (NanoScope IV, Veeco Instruments) using the tapping mode at ambient temperature. An aliquot of Phe-(Leu-Glu)8-Au nanoparticle suspension was placed on freshly cleaved mica, allowing the Phe-(Leu-Glu)8-Au nanoparticle to be absorbed on its surface at room temperature in a clean chamber. After the absorption, the excess suspension was removed by absorption onto filter paper. Atomic force microscopy (AFM) images were done in “height” mode using silicon cantilevers (125 µm, tip radius < 10 nm). The amplitude ratio of tip oscillation of 0.8 and higher was employed to avoid sample damage. The scanning speed was at a line frequency of 1 Hz. High-resolution images were obtained by contact mode using oxide sharpened silicon nitrate cantilevers (120 µm, tip radius < 10 nm). The scanning speed was at a line frequency of 5 Hz. All original images were sampled at a resolution of 512 × 512 points.

Results and Discussion Characterization of the Gold Nanoparticles Having Sequential Alternating Amphiphilic Peptide on the Surface. The gold nanoparticles having sequential alternating amphiphilic peptide on the surface were prepared by reduction of HAuCl4 with NaBH4 in the presence of Phe-(Leu-Glu)8-S. The Phe-(LeuGlu)8-Au nanoparticles obtained were well dispersed in TFE, and the suspension was pinkish-red with an adsorption band at 512 nm, assigned to a gold nanoparticle plasmon band (Figure 1). This indicated the formation of Phe-(Leu-Glu)8 coated gold nanoparticles. The amount of surface peptide on the Au nanoparticle was determined from the absorbance at 250 nm of the Phe-(Leu-Glu)8-Au suspension in TFE on the basis of the molar extinction coefficient of the Phe residue of Phe-(LeuGlu)8-S in TFE. The amount of fixed sequential alternating amphiphilic peptide on the gold surfaces obtained was 0.18 mmol per 1 g of gold core nanoparticle. The size of the gold core of the Phe-(Leu-Glu)8-Au nanoparticle was estimated by transmission electron microscopic (TEM) observation. Figure 2 shows TEM image and histogram of diameter distribution for the Phe-(Leu-Glu)8-Au nanoparticles. The Phe-(Leu-Glu)8-Au suspension was diluted by Milli-Q water (TFE content: 40 vol %) to avoid damage of the poly(vinyl formal) support film. The spherical particles, whose mean diameters of gold cores were 3.5 nm, were observed in the TEM image. We calculated the surface density of Phe-(Leu-Glu)8 on gold nanoparticle surfaces using eq 1.

1 dFnNA × 10-21 (molecules ⁄ nm2) 6

(1)

where d, F, and n represent the diameter of the gold nanoparicles obtained by TEM measurement (d ) 3.5 nm), the density of gold (F ) 19.3 g/cm3), and the fixed amount of Phe-(Leu-Glu)8 molecules on the gold nanoparticle surface (n ) 0.18 mmol/g),

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Figure 2. (a) TEM image of gold nanoparticles having Phe-(Leu-Glu)8. (b) Histogram of diameter distribution for gold cores of Phe-(Leu-Glu)8Au nanoparticles. Sample was prepared by casting on poly(vinyl formal) coated copper mesh grid from aqueous dispersion containing 40 vol % TFE. Initial magnification ×200 000. Figure 4. AFM image of Phe-(Leu-Glu)8-Au nanoparticles in TFE (750 nm × 750 nm).

Figure 3. CD spectrum of Phe-(Leu-Glu)8-Au nanoparticles in TFE.

respectively. NA is Avogadro’s number. We obtained 1.2 molecules/nm2 as the surface density of Phe-(Leu-Glu)8 molecules on a gold nanoparticle surface. This value corresponded to 47 Phe-(Leu-Glu)8 molecules fixed on one gold nanoparticle surface. In the TEM image, relatively large particles, whose diameters were ∼9 nm, were observed. These large particles are the aggregates of the Phe-(Leu-Glu)8-Au nanoparticles, which are formed during the solvent evaporation in the sample preparation process. The conformation of Phe-(Leu-Glu)8 on the gold nanoparticles was investigated by means of circular dichroism (CD) spectroscopy. The CD spectrum of Phe-(Leu-Glu)8-Au dispersion in TFE (Figure 3) showed a negative maximum at 222 and 208 nm, indicating the existence of a stable right-handed R-helix structure. The helicity of the Phe-(Leu-Glu)8 on the Au nanoparticle was calculated to be 97% from the observed molar ellipticity at 208 nm. To elucidate the size of the Phe-(Leu-Glu)8-Au nanoparticles in TFE, we performed atomic force microscopy (AFM). Figure 4 shows an AFM image of the adsorbed Phe-(Leu-Glu)8-Au nanoparticles on mica from TFE suspension. The homogeneously dispersed globular particles were clearly observed in the AFM image. The observed average width of the particle was estimated to be 18 nm. It is well-known that the convolution of the scanning tip leads to an overestimation of the sample’s width.17 Fung et al. proposed the calculation equation18 to obtain the real width from the observed value. According to Fung’s equation, the average width of the particle was corrected to be 8 nm. From the results of both TEM and AFM observation, the mean thickness of the coated Phe-(Leu-Glu)8 layer could be roughly estimated to be 2 nm. Using the occupied length, 0.15 nm, of one amino acid residue along the helix axis, the helical length of the Phe(Leu-Glu)8 could be estimated to be 2.7 nm. This value is comparable with the coated peptide layer of the Au nanoparticle evaluated by TEM and AFM. (18) Fung, S. Y.; Keyes, C.; Duhamel, J.; Chen, P. Biophys. J. 2003, 85, 537–548.

Figure 5. (a) pH-induced CD spectral changes of Phe-(Leu-Glu)8-Au in aqueous solution containing 40 vol % TFE. (b) pH-dependence of [θ]222/[θ]208 and fraction of second-order structure of the surface Phe(Leu-Glu)8 chains estimated from the CD curve-fitting method. The CD measurements were carried out immediately after adjustment of the pH for the sample solution.

pH-Induced Conformational Transition of the Surface Peptide on the Au Nanoparticle. We demonstrated the regulation of the surface peptide conformation on the gold nanoparticle. Figure 5 shows pH-induced CD spectral changes of Phe-(LeuGlu)8-Au in aqueous solution containing 40 vol % TFE and the fraction of the secondary structure, which was calculated by a curve-fitting method19 using a linear combination of typical CD spectra for R-helical, β-sheet, and random coil conformations. The CD measurements were carried out immediately after adjustment of the pH of the sample dispersion. At pH 9.2, the surface Phe-(Leu-Glu)8 chains on the gold nanoparticle showed a negative maximum at 223 and 206 nm, indicating the existence of a right-handed R-helical conformation with a considerable amount of random coil and β-sheet structures. It is clear that the random coil conformation of the surface Phe-(Leu-Glu)8 chain changed to the R-helical conformation by decreasing the pH of the aqueous solution. On the other hand, CD spectra of Phe(Leu-Glu)8-Au in acidic aqueous solution below pH 5 did not fit by a linear combination of typical CD spectra for dispersed R-helical, β-sheet, and random coil conformations. The ratio of molar ellipticity of the CD band at 222 nm to that at 208 nm, [θ]222/[θ]208, was increased by decreasing the pH under the acidic condition below pH 5. It is well-known that the CD spectra of aggregated R-helical peptides were red-shifted of a 222 nm band and flattened of a 208 nm band,20 compared with typical dispersed R-helical peptides. This result implied that surface Phe-(LeuGlu)8 chains took an R-helical conformation and the surface peptide chains formed aggregates under the acidic condition owing (19) Greenfield, N.; Fasman, G. D. Biochemistry 1969, 8, 4108–4116. (20) Maeda, H.; Kato, H.; Ikeda, S. Biopolymer 1984, 23, 1333–1346.

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Figure 6. pH-dependence of the plasmon absorption maximum of Phe(Leu-Glu)8-Au in aqueous solution containing 40 vol % TFE. The measurements were carried out immediately after adjustment of the pH for the sample solution.

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Figure 8. Time-dependence of molar ellipticities at 222 and 208 nm, and CD negative maximum of Phe-(Leu-Glu)8-Au in aqueous solution containing 40 vol % TFE after the pH of the suspension was rapidly changed from pH 9.5 to 3.7.

Figure 7. CD spectral changes of Phe-(Leu-Glu)8-Au in aqueous solution containing 40 vol % TFE after the pH of the suspension was rapidly changed (a) to pH 9.5 and (b) from pH 9.5 to 3.7.

to the decreasing electrostatic repulsion between the Glu residues. Figure 6 shows the pH-dependence of the plasmon absorption maximum of Phe-(Leu-Glu)8-Au in aqueous solution containing 40 vol % TFE. The UV-vis spectra were measured immediately after adjustment of the pH of the sample solution. The absorption maximum of Phe-(Leu-Glu)8-Au, whose surface peptide took an aggregated R-helical structure under the acidic solution, was shifted to a higher wavelength. It has been reported that the formation of gold nanoparticle assemblies causes a red-shifting of the peak wavelength of the plasmon absorption spectrum.21 This result suggested that the R-helical surface peptide on the gold nanoparticle, which was formed under the acidic condition, connected the peptide coated gold nanoparticles owing to the peptide-peptide interaction. We investigated the dynamics of the pH-induced conformational transition for the Phe-(Leu-Glu)8 surface peptide of Phe(Leu-Glu)8-Au in aqueous solution containing 40 vol % TFE. First, the pH of Phe-(Leu-Glu)8-Au in aqueous solution was adjusted to basic (pH 9.5; R-helical conformation with a considerable amount of random coil and β-sheet structures, Figure 5). Under this condition, the CD spectra of the Phe-(Leu-Glu)8Au did not change after incubation for 3 days (Figure 7a). The pH of the Phe-(Leu-Glu)8-Au solution rapidly changed to acidic (pH 3.7; aggregated R-helical structure, Figure 5), and then the CD spectral changes of the Phe-(Leu-Glu)8-Au with incubation time were measured (Figure 7b). Under this condition, pH 3.7, the CD spectrum of the Phe-(Leu-Glu)8-Au was rapidly altered to the typical pattern of a β-sheet structure, with a single negative maximum around 218 nm from the pattern of the aggregated R-helical conformation. The single negative maximum of the (21) Nath, N.; Chilkoti, A. J. Am. Chem. Soc. 2001, 123, 8197–8202.

Figure 9. AFM images (2 µm × 2 µm) showing the time-dependence of Phe-(Leu-Glu)8-Au nanoparticles in aqueous suspension containing 40 vol % TFE. (a) Observation was carried out after incubation for 3 days at pH 9.5. (b,c) Observations were carried out (b) immediately and (c) after incubation for 3 days and after the pH of the suspension was rapidly changed from pH 9.5 to 3.7.

CD spectra was shifted to a shorter wavelength, and the value of [θ]208 decreased toward 0 with progress in incubation time (Figure 8). These CD spectral changes of Phe-(Leu-Glu)8-Au under acidic conditions implied that the aggregated R-helical rods of surface Phe-(Leu-Glu)8 chains on the gold nanoparticles changed in the β-sheet structure. The intramolecular hydrogen bonding in the aggregated R-helix is likely to be rearranged into the stable intermolecular one during the incubation, which induces the conformational transition of surface peptides on the gold nanoparticles. Morphological Changes of Phe-(Leu-Glu)8 Coated Gold Nanoparticle Assembly. The pH-induced morphological changes of assemblies composed of Phe-(Leu-Glu)8-Au nanoparicles in the aqueous solution containing 40 vol % TFE were investigated directly by using the AFM technique. Figure 9a shows the AFM image of Phe-(Leu-Glu)8-Au nanoparicles after incubation for 3 days at pH 9.5. In this image, dispersed globular particles are clearly observed. The corrected diameters of the particles were 30-70 nm. These values were larger than that of the Phe-(LeuGlu)8-Au nanoparicles in TFE. Under this condition, a quantitative

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Figure 10. Time-dependence of the plasmon absorption maximum of Phe-(Leu-Glu)8-Au. The measurements were carried out after the pH of the solution was rapidly changed from pH 9.5 to 3.7.

curve-fitting analysis of the CD spectrum (Figure 7a) gave the following percentage of secondary structure of Phe-(Leu-Glu)8 on the Au nanoparticle: 42% random coil, 30% β-sheet, and 28% R-helix. This result suggested that the hydrophobic interaction between leucine residues and the intermolecular hydrogen bonding among the β-sheets on the Au nanoparticle induced the aggregation of Phe-(Leu-Glu)8-Au nanoparticles. However, formation of larger precipitates did not occur because of the electrostatic repulsion among the negatively charged glutamic acid residues of the surface peptides. The morphology of the assemblies composed of Phe-(LeuGlu)8-Au nanoparicles was drastically changed under the acidic condition. An AFM image obtained immediately after the pH of the dispersion steeply changed from pH 9.5 to 3.7, in which the surface peptide took an aggregated R-helical conformation (Figure 7b), revealed dispersed distorted globular particles (Figure 9b). After incubation for 3 days, the distorted globular particles were transformed to a sheet structure (Figure 9c) with the conformational transition of the surface peptide from aggregated R-helix to β-sheet structure (Figure 7c). Interestingly, the wavelength of the plasmon absorption maximum was decreased with incubation time under the acidic condition (Figure 10). In other words, the conformational transition to the β-sheet structure from the aggregated R-helix of the surface peptide of the Phe-(Leu-Glu)8Au induced the blue-shift of the plasmon absorption maximum. This implies that the interaction among the core gold nanoparticles decreases in the β-sheet assembly. This result could be explained as follows. The molecular length of the Phe-(Leu-Glu)8, which took a β-sheet structure, could be estimated to be 5.4 nm using the occupied length 0.32 nm of one amino acid residue along the molecular axis of the β-sheet. This value is greater than the helical length of the peptide, 2.7 nm. That is to say, the individual core gold nanoparticle in the β-sheet assembly is separated compared with a state in the assembly formed by an aggregated R-helical peptide. This result suggests that the core gold nanoparticles could be fixed in the state that stood alone in the nanosheet assembly composed of the sequential alternating amphiphilic peptides that took a β-sheet conformation. Figure 11 shows the proposed schematic picture for the assembly behavior of Phe-(Leu-Glu)8-Au under different pH conditions. We attempted the fabrication of a nanosheet assembly composed of Phe-(Leu-Glu)8-Au nanoparticles. A freshly cleaved mica plate was immersed into the Phe-(Leu-Glu)8-Au aqueous dispersion containing 40 vol % TFE at pH 3.7. The concentration of the Phe-(Leu-Glu)8-Au nanoparticles was 0.25 mg/mL. After the incubation for 5 days, the mica plate was rinsed with the aqueous solution containing 40 vol % TFE at pH 3.7 for several

Figure 11. Schematic picture for the assembly behavior of Phe-(LeuGlu)8-Au under different pH conditions.

Figure 12. AFM images of Phe-(Leu-Glu)8-Au nanoparticle assemblies. Sample was prepared by adsorption on mica from aqueous dispersion containing 40 vol % TFE at pH 3.7 for 5 days. (a) Tapping mode AFM image (5 µm × 5 µm). (b) High-resolution image obtained by contact mode (500 nm × 500 nm).

times. Figure 12a shows a tapping mode AFM image for the assembly of adsorbed Phe-(Leu-Glu)8-Au nanoparticles on the mica surface. The homogeneous nanosheet assembly composed of Phe-(Leu-Glu)8-Au nanoparticles was clearly observed over an area of a few square micrometers. In the high-resolution AFM image obtained by contact mode, individual nanoparticles were observed in the nanosheet assembly (Figure 12b). This implies the possibility that the nanosheet assembly, in which the nanoparticles are fixed in the state that stands alone, can be easily fabricated by the self-assembly technique. In conclusion, the conformation of the sequential alternating amphiphilic peptide, Phe-(Leu-Glu)8, on the gold nanoparticle could be controlled by the external stimuli. Under the acidic condition, the surface peptide formed the aggregated R-helical structure immediately after the pH adjustment of the solution. The conformation of the surface peptide on the gold nanoparticle changed to the β-sheet structure and the peak wavelength of the plasmon adsorption spectrum shifted to shorter wavelength with progress in incubation time. Under this condition, the peptide coated nanoparticles formed nanosheet assemblies owing to the intermolecular hydrogen bonding among the surface peptides. This suggests that the core gold nanoparticles could be fixed in the state that stood alone in the β-sheet assembly of the surface peptides. This system may be useful for novel molecular devices that exhibit quantized properties. LA802527N