diﬀerential self-assembly behaviors of cyclic and linear...

Differential Self-Assembly Behaviors of Cyclic and Linear PeptidesSung-ju Choi,† Woo-jin Jeong,† Seong-Kyun Kang,‡ Myongsoo Lee,‡ Eunhye Kim,§ Du Yeol Ryu,§

and Yong-beom Lim*,†

†Translational Research Center for Protein Function Control and Department of Materials Science & Engineering and §Departmentof Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea‡Center for Bio-Responsive Assembly and Department of Chemistry, Seoul National University, Seoul 151-747, Korea

*S Supporting Information

ABSTRACT: Here we ask the fundamental questions aboutthe effect of peptide topology on self-assembly. The studyrevealed that the self-assembling behaviors of cyclic and linearpeptides are significantly different in several respects, inaddition to sharing several similarities. Their clear differencesincluded the morphological dissimilarities of the self-assemblednanostructures and their thermal stability. The similaritiesinclude their analogous critical aggregation concentrationvalues and cytotoxicity profiles, which are in fact closelyrelated. We believe that understanding topology-dependentself-assembly behavior of peptides is important for developingtailor-made self-assembled peptide nanostructures.

■ INTRODUCTIONMacromolecules that have cyclic structures (macrocycles) areinteresting molecules not just because they do not have chainends but because their topological features can significantlyaffect their chemical, physical, and biological properties.Macrocycles can more effectively maintain rigid and conforma-tionally constrained structures than their linear counterparts.1−6

Many examples exist in nature in which such conformationalconstraints help to stabilize folded or self-assembled structures.One of the strategies typically used by nature to establishreversible covalent macrocyclic constraints in a linearpolypeptide chain is by making disulfide bonds, which canassist in protein folding by reducing the conformational entropyof the unfolded state.7,8

Peptides have the same chemical structure as proteins but areshorter in length. Peptides have several advantages overproteins, including the fact that they can be synthesized andmass-produced more easily, and they have broader chemicaldiversity.9 However, peptides and peptide epitopes are oftenunstructured because of their short chain length andthermodynamic instability, limiting their usefulness whenprecise and stable conformations are required, as in specificmolecular recognition events. Therefore, many attempts havebeen made to stabilize the peptide conformation usingcyclization strategies.10,11 Several proteins, including cyclotides,topologically unique head-to-tail cyclized proteins in which thecyclic structure offers remarkable thermal stability and proteaseresistance, use a similar strategy.12,13

Self-assembled peptide nanostructures, constructed by thecontrolled self-assembly of peptide molecules, have becomepromising biomaterials.14−27 Self-assembled peptide nanostruc-tures range in size from tens of nanometers to several

micrometers, which are comparable to the sizes of naturalproteins or protein assemblies. Therefore, self-assembledpeptide nanostructures might be regarded as nanosized artificialproteins. Research, even though still highly primitive whencompared with natural proteins, are in progress to devise self-assembled peptide nanostructures that can mimic or displacethe diverse biological functions of natural proteins, possiblywith enhanced properties or with functions unprecedented innature.One of the most simple, widespread, and effective ways of

constructing self-assembled nanostructures is to make use ofamphiphilic building blocks, in which one of the blocks isdesigned to be hydrophilic and the other is designed to behydrophobic. Probably one of the most famous examples ofamphiphilic building blocks for self-assembly is diblockcopolymers. Likewise, many self-assembled peptide nanostruc-tures are also based on the peptide molecules havingamphiphilic character.28−30 Various types of self-assemblingamphiphilic peptides have been developed to date and havereported to be useful in many bioapplications. However, despitethe unique and advantageous properties of cyclic molecules, allof the self-assembling amphiphilic peptides developed to datemake use of linear peptide molecules as building blocks, and theself-assembly behavior of cyclic peptides with two dissimilarand amphiphilic blocks has not been explored so far, to the bestof our knowledge. It should be noted that self-assembling cyclicpeptides with alternating D- and L-amino acids cannot beconsidered as amphiphiles and usually form nanostructures

Received: April 17, 2012Revised: May 25, 2012Published: June 8, 2012

Communication

pubs.acs.org/Biomac

© 2012 American Chemical Society 1991 dx.doi.org/10.1021/bm3005947 | Biomacromolecules 2012, 13, 1991−1995

pubs.acs.org/Biomac

soluble only under hydrophobic lipid bilayer environment.6

Because linear peptides often have unstable conformations(vide ante), conformational stability of surface-exposedpeptides within nanostructures self-assembled from linearpeptides can be limited. In this Communication, we ask thefundamental questions about differential self-assembly behav-iors of amphiphilic peptides with cyclic or linear structures.

■ EXPERIMENTAL SECTIONGeneral. Fmoc-amino acids and coupling reagents were purchased

from Novabiochem (Germany) and Anaspec (USA). Generalchemicals were obtained from Sigma-Aldrich (USA) and Merck(Germany). HPLC solvents were purchased from Fisher Scientific(USA). Tissue culture reagents were obtained from Invitrogen (USA).Although we did not perform detailed kinetic experiment, we foundthat spectroscopic signals and nanostructural morphologies are quitestable and reproducible soon after the dissolution of the peptides. Tomake it quite sure that the peptide assemblies were in thermodynamicequilibrium state, we incubated all samples for several days beforetaking measurements. Repeated experiments revealed that steady statehas been reached after the incubation period.Cyclic Peptide Synthesis. The 2-Chlorotrityl resin was first

preloaded with Fmoc-Gly-OH. Further couplings of amino acids wereperformed on a Tribute peptide synthesizer on 0.1 mmol scale(Protein Technologies). Standard amino acid protecting groups wereemployed for the synthesis. For the head-to-tail cyclization reaction,N-terminal Fmoc-group was deprotected following the completion ofthe final amino acid coupling. The protected peptide fragment (20μmol) was liberated from the resin by AcOH/2,2,2-trifluoroethanol(TFE)/methylene chloride (MC) (2:2:6) treatment. After anappropriate time (∼1 to 2 h), the resin was removed by filtration,and the filtrate was recovered (4 mL × 2). Finally, the resin waswashed three times with the cleavage mixture. Hexane was added tothe filtrate to remove acetic acid as an azeotrope with hexane. Theprotected peptide fragment was obtained as a white powder followingrepeated evaporation cycles (the dissolution of the peptide fragment inMC, hexane addition, and evaporation). For cyclization, typically 5μmol of the protected peptide fragment and 12 μmol DIPEA weredissolved in DMF (10 mL) and transferred into a syringe. To achievepseudo-high dilution condition, this solution was added to a stirredsolution of 5 μmol PyBOP and 5 μmol HOBt in DMF (10 mL) at arate of 0.05 mL/min by using a syringe pump. Once the addition wascompleted, the reaction mixture was further stirred for ∼5 h.Following DMF evaporation, the residue was dissolved in MC, andthen tert-butyl methyl ether/hexane was added to triturate the cyclizedand protected peptide fragment (three times). The peptide fragmentwas treated with cleavage cocktail (TFA/TIS/water; 95:2.5 2.5) for 3 hand was triturated with tert-butyl methyl ether. The peptides werepurified by reverse-phase HPLC (water−acetonitrile with 0.1% TFA).The molecular weight was confirmed by MALDI-TOF massspectrometry. The purity of the peptides was >95%, as determinedby analytical HPLC. Concentration was determined spectrophoto-metrically in water/acetonitrile (1:1) using a molar extinctioncoefficient of tryptophan (5502 M−1cm−1) at 280 nm. Yield: cyPA-1(3.5%), liPA-1 (6.7%), cyPA-2 (2.6%), liPA-2 (4.5%).Transmission Electron Microscopy (TEM). Two μL of sample

(typically, 5−50 μM) was placed onto a carbon-coated copper grid anddried completely. Then, 2 μL of water was added for 1 min to dissolveand remove unbound peptide and was wicked off by filter paper. Thesample was stained with 1% uranyl acetate for negative staining. Thespecimen was observed with a JEOL-JEM 2010 instrument operatingat 120 kV. The data were analyzed with DigitalMicrograph software.Atomic Force Microscopy. For atomic force microscopy (AFM),

1 μL of the sample in water was deposited onto a freshly cleaved micasurface for 1 min and dried in air. The images were obtained in tappingmode with a Nanoscope IV instrument (Digital Instruments). AFMscans were taken at set point of 0.8 to 1 V, and scanning speed was 1to 2 Hz.

Circular Dichroism. Circular dichroism (CD) spectra weremeasured using a Chirascan CD spectrometer equipped with peltiertemperature controller (Applied Photophysics). Spectra were recordedfrom 260 to 190 nm using a 2 mm path-length cuvette. Scans wererepeated five times and averaged. Molar ellipticity was calculated peramino acid residue.

Fluorescence Spectroscopy. The steady-state fluorescencespectra were recorded using a Hitachi F-4500 fluorescencespectrophotometer in 1 cm path length quartz cuvettes. To measurefluorescence from tryptophan residues, we excited samples at 280 nm.Excitation and emission slits with a nominal bandpass of 5 nm wereused for the measurements.

Cytotoxicity Assay. HeLa cells were grown in DMEMsupplemented with 10% fetal bovine serum (FBS). For the cytotoxicityassay, cells were seeded in a 96-well plate at a density of 104 cells/wellin 100 μL of culture medium and grown at 37 °C, 5% CO2 to reach60−70% confluence. The culture medium was removed, and DMEM(90 μL) was added. Then, the varying concentrations of peptides (10μL) were added to each well. Following 4 h of incubation at 37 °C, 5%CO2, 10 μL of WST-1 solution was added. The cells were furtherincubated for 4 h, and the absorbance of each sample was measured ata wavelength of 450 nm using a microplate reader.

■ RESULTS AND DISCUSSIONAs a simplified model system, we designed several amphiphilicpeptide building blocks, in which oligo-arginines and oligo-tryptophans represented hydrophilic and hydrophobic blocks,respectively (Figure 1). Linear peptides were synthesized using

standard Fmoc protocols on 2-chlorotrityl chloride resin.Cyclization of a protected peptide fragment was performedunder pseudo-high-dilution conditions to prevent intermolec-ular multimerization reactions (Supporting Information).31 Thepeptides were purified by HPLC (>95%). All amphiphilicpeptides were soluble in water.As an initial study, the peptide building blocks were dissolved

in water (5−50 μM), and their nanostructural morphologieswere investigated by TEM. As shown in Figure 2a, TEMrevealed irregularly shaped nanoaggregates of the linear peptideliPA-1. By contrast, regularly shaped spherical objects of ∼10nm in diameter were observed for the cyclic peptide cyPA-1(Figure 2b). To address the question of whether the chainlength affects the self-assembly behavior, we prepared peptidebuilding blocks with longer hydrophilic blocks (liPA-2 andcyPA-2). A similar trend was observed during the self-assembly

Figure 1. Chemical structures and amino acid sequences of cyclic andlinear peptides. Arginines (hydrophilic and charged amino acids) andtryptophan (hydrophobic and aromatic amino acids) are shown inblue and red, respectively.

Biomacromolecules Communication

dx.doi.org/10.1021/bm3005947 | Biomacromolecules 2012, 13, 1991−19951992

of liPA-2 and cyPA-2 (Figure 2c,d). The aggregates of liPA-2exhibited irregular morphologies, whereas those of cyPA-2contained a fairly homogeneous population of spherical objects(diameter; ca. 11 nm). Investigation with AFM furthercorroborated the spherical morphologies and homogeneity(Figure 2e). Dynamic light scattering (DLS) examinationsrevealed that the average hydrodynamic radius (RH) of cyPA-2supramolecular aggregates is 5.89 nm (so the diameter is 11.78nm), which correlates well with the TEM result (Figure 2f). Itshould be noted that the nanostructure formation wasinstantaneous and prolonged storage of up to several monthsdid not change the size and shape of the nanostructures.Therefore, all of these results clearly indicate that the self-assembly behaviors of linear and cyclic peptides are significantlydifferent. In these specific examples of peptides, the linearpeptides tended to aggregate into irregular objects, whereascyclic peptides preferred to form regular spheres.Intrigued by these results, we further scrutinized the

differential self-assembly behaviors of linear and cyclic peptidesusing liPA-2 and cyPA-2. First, the conformation of thepeptides was probed with CD spectroscopy (Figure 3). TheCD spectra of both liPA-2 and cyPA-2 displayed a distinctminimum of ellipticity around 200−203 nm, indicating that thepeptides exist mostly in random coil states and do not haveclearly defined secondary structures. In addition, both peptidesshowed the strong negative bands around 223 to 224 nm.These bands are the behaviors of tryptophan expected fromexciton coupling produced by the aromatic chromophores ofindoles stacking against one another.32−34 Therefore, theseresults indicate that the primary driving force for the self-assembly of both linear and cyclic peptides is the purehydrophobic and π−π stacking interactions between trypto-phan residues and that secondary structure formation, such asβ-sheets, is not involved in the self-assembly process.

Given the results described above, intensity of the band at223 to 224 nm can be used as a measure of peptideaggregation.34 As shown in the inset of Figure 3a, temper-ature-dependent CD spectra of liPA-2 showed a rapid andlinear decrease in mean residue ellipticity at 224 nm. At a hightemperature (84 °C), the normalized degree of aggregation wasdecreased to 62% compared with that at 4 °C. In contrast, theellipticity at 223 nm for cyPA-2 stopped decreasing above 54°C, and the degree of aggregation was maintained at around82%, even at a high temperature of 84 °C (an inset in Figure3b). Taking all of these results into account, the cyclic peptideforms more robust and thermally stable nanostructures than thelinear peptide, even though they make use of the sameaggregation mechanism (i.e., pure hydrophobic and π−πstacking interactions, vide ante).It is well known that the indole chromophore in tryptophan

is highly sensitive to the nature of its local environment, whichhas led to the wide use of tryptophan’s intrinsic fluorescence inprobing protein conformational changes and interactions withother molecules.35,36 As shown in Figures 3c,d, a clearconcentration-dependent increase in tryptophan fluorescencewas observed at concentrations ranging from 0.1 to 20 μM forboth liPA-2 and cyPA-2. Plots of the fluorescence intensity at350 nm as a function of peptide concentrations are shown inthe insets of Figure 3c,d. Above certain concentrations, therewere sudden increases in fluorescence intensity for bothpeptides. These discontinuous changes in intensity most likelyreflect the onset of aggregation and critical aggregationconcentration (CAC). The point of intersection of the

Figure 2. Nanostructural morphologies of the self-assembling peptidesupramolecular aggregates. TEM images of (a) liPA-1, (b) cyPA-1, (c)liPA-2, and (d) cyPA-2. (e) AFM image of cyPA-2. (f) DLS result forcyPA-2 (average RH = 5.89 nm).

Figure 3. Characterization of the self-assembly behavior of linearversus cyclic peptides. Temperature-dependent CD spectra of (a)liPA-2, 10 μM and (b) cyPA-2, 10 μM (4−84 °C, 10 °C interval).Insets: normalized mean residue ellipticity at 224 and 223 nm for liPA-2 and cyPA-2, respectively. Concentration-dependent fluorescenceemission spectra of (c) liPA-2 and (d) cyPA-2. Insets: plots offluorescence intensity as a function of peptide concentrations (logscale).



extrapolated linear regression lines was used to calculate theCACs of the peptides. The calculation shows that CACs forliPA-2 and cyPA-2 are 5.7 and 3.3 μM, respectively. Althoughthe CAC value was slightly lower for the cyclic peptide than forthe linear counterpart, the difference was rather small.Taken together, these results allow us to propose a model for

the spherical nanostructures (Figure 4). The spherical nano-

object should be either a spherical micelle or a vesicle.37,38 Inthe case of cyPA-2, the length of the molecule determined byCorey−Pauling−Koltun space-filling modeling (CPK model-ing) was ca. 2.5 to 3.0 nm, depending on the peptideconformation. Fitting such a molecular length into a sphericalmicelle model would result in micelles of roughly 6 nm indiameter (Figure 4a), which is far smaller than the diameter ofthe spherical nano-object observed (∼11 nm). Instead, thevesicle model agrees nicely with the peptide’s molecular length,wherein the cyclic peptides form an extended conformationalong the axis, connecting the hydrophilic and hydrophobicblocks and indole rings of tryptophans, which are stackedagainst each other in a bilayered structure (Figure 4b). Thestacking of the indole residues is in good agreement with theCD data (vide ante). Likewise, the diameter of the self-assembled spherical nano-object of cyPA-1 also fits well withthe vesicle model. Therefore, the results suggest that the cyclicpeptides are likely to self-assemble into bilayered vesicularnanostructures, which are unusually small considering theirmolecular length. The unique self-assembly behaviors of thecyclic peptides, their high thermal stability, and their formationof remarkably small vesicular structure likely result from theirconstrained structures and the entropic advantage of their cyclictopology during the self-assembly process.39 It would beinteresting to more systematically investigate the intrinsicdriving force underlying unique self-assembly behavior of cyclicpeptides, possibly using self-assembling peptides with differentamino acid composition and molecular weight; this will be thesubject of future study.After establishing the self-assembly behavior of linear and

cyclic peptides, it was of interest to investigate the manner inwhich these amphiphilic peptides interact with mammalian cellsand to obtain their differential cytotoxicity profiles for potentialcellular applications. To this end, a comparative toxicity assaywas performed in HeLa cells using the WST-1 cell proliferation

assay. As shown in Figure 5, both liPA-2 and cyPA-2 are nottoxic within the concentration range tested, and they exhibited

similar levels of cytotoxicity. It has been shown that stablemolecular assembly prevents free diffusion of the individualamphiphilic components to the cell surface and lipid bilayerinterior, and the cytotoxicity of amphiphilic peptides is largelyproportional to their CAC values.29,40 Therefore, the similarcytotoxicity profiles of the linear and cyclic peptides are mostlikely due to their analogous CAC values.

■ CONCLUSIONSIn this work, we have shown that the properties of self-assembling cyclic and linear peptides are significantly differentin several respects, in addition to sharing several similarities.Their clear differences included the morphological dissim-ilarities of the self-assembled nanostructures and their thermalstability. Moreover, the study has revealed that cyclic peptidescan make unique and unusual assemblies because of theirconstrained structure. The similarities include their analogousCAC values and cytotoxicity profiles, which are likely to beclosely related. Although it is an open question whether thesetrends can be applied to self-assembling peptides with differentstructures and molecular weights, it is clear that the topology ofamphiphilic peptides profoundly affects their self-assemblybehaviors. Understanding this principle is important fordeveloping tailor-made self-assembled peptide nanostructures.

■ ASSOCIATED CONTENT*S Supporting InformationSpectral data. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

Figure 4. Models of self-assembled cyPA-2 supramolecular aggregates.(a) Left, a micelle model; right, a vesicle model. (b) Detailed view ofthe vesicle model. Modeling was performed using Materials Studio(Accelrys).

Figure 5. Cytotoxicity profiles of the self-assembling peptides in HeLacells (WST-1 cell proliferation assay). liPA-2 (filled circle) and cyPA-2(open circle). Mean ± SD (n = 4).



http://pubs.acs.orgmailto:[email protected]

■ ACKNOWLEDGMENTSWe thank National Research Foundation (NRF) of Korea forfunding through the Future-based Technology DevelopmentProgram (Nano Fields; 2011-0019125), the Basic ScienceResearch Program (2011-0003540, 2011-0003065, and2012R1A1A2006453), the Translational Research Center forProtein Function Control, Yonsei University (2012-0000888),and Seoul R&BD program (ST110029M0212351).

■ REFERENCES(1) Lim, Y. B.; Moon, K. S.; Lee, M. Angew. Chem., Int. Ed. 2009, 48,1601−1605.(2) Choi, S. J.; Jeong, W. J.; Kim, T. H.; Lim, Y. B. Soft Matter 2011,7, 1675−1677.(3) Hackenberger, C. P. R. Angew. Chem., Int. Ed. 2008, 47, 5984−5988.(4) Mylne, J. S.; Colgrave, M. L.; Daly, N. L.; Chanson, A. H.; Elliott,A. G.; McCallum, E. J.; Jones, A.; Craik, D. J. Nat. Chem. Biol. 2011, 7,257−259.(5) Zhang, K.; Lackey, M. A.; Wu, Y.; Tew, G. N. J. Am. Chem. Soc.2011, 133, 6906−6909.(6) Hartgerink, J. D.; Granja, J. R.; Milligan, R. A.; Ghadiri, M. R. J.Am. Chem. Soc. 1996, 118, 43−50.(7) Betz, S. F. Protein Sci. 1993, 2, 1551−1558.(8) Zavodszky, M.; Chen, C. W.; Huang, J. K.; Zolkiewski, M.; Wen,L.; Krishnamoorthi, R. Protein Sci. 2001, 10, 149−160.(9) Marx, V. Chem. Eng. News 2005, 83, 17−24.(10) Hili, R.; Rai, V.; Yudin, A. K. J. Am. Chem. Soc. 2010, 132,2889−2891.(11) Kawakami, T.; Ohta, A.; Ohuchi, M.; Ashigai, H.; Murakami, H.;Suga, H. Nat. Chem. Biol. 2009, 5, 888−890.(12) Gustafson, K. R.; Sowder, R. C., II; Henderson, L. E.; Parsons, I.C.; Kashman, Y.; Cardellina, J. H.; Mcmahon, J. B.; Buckheit, R. W., Jr;Pannell, L. K.; Boyd, M. R. J. Am. Chem. Soc. 1994, 116, 9337−9338.(13) Poth, A. G.; Colgrave, M. L.; Lyons, R. E.; Daly, N. L.; Craik, D.J. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 10127−10132.(14) Lim, Y. B.; Moon, K. S.; Lee, M. Chem. Soc. Rev. 2009, 38, 925−934.(15) Banwell, E. F.; Abelardo, E. S.; Adams, D. J.; Birchall, M. A.;Corrigan, A.; Donald, A. M.; Kirkland, M.; Serpell, L. C.; Butler, M. F.;Woolfson, D. N. Nat. Mater. 2009, 8, 596−600.(16) Gazit, E. Chem. Soc. Rev. 2007, 36, 1263−1269.(17) Jeong, W. J.; Lim, Y. B. Macromol. Biosci. 2012, 12, 49−54.(18) König, H. M.; Kilbinger, A. F. Angew. Chem., Int. Ed. 2007, 46,8334−8340.(19) Lim, Y. B.; Lee, E.; Lee, M. Angew. Chem., Int. Ed. 2007, 46,3475−3478.(20) Lim, Y. B.; Lee, E.; Yoon, Y. R.; Lee, M. S.; Lee, M. Angew.Chem., Int. Ed. 2008, 47, 4525−4528.(21) Kwon, S.; Shin, H. S.; Gong, J.; Eom, J.-H.; Jeon, A.; Yoo, S. H.;Chung, I. S.; Cho, S. J.; Lee, H.-S. J. Am. Chem. Soc. 2011, 133, 17618−17621.(22) Mart, R. J.; Osborne, R. D.; Stevens, M. M.; Ulijn, R. V. SoftMatter 2006, 2, 822−835.(23) Percec, V.; Dulcey, A. E.; Balagurusamy, V. S. K.; Miura, Y.;Smidrkal, J.; Peterca, M.; Nummelin, S.; Edlund, U.; Hudson, S. D.;Heiney, P. A.; Hu, D. A.; Magonov, S. N.; Vinogradov, S. A. Nature2004, 430, 764−768.(24) Nagy, K. J.; Giano, M. C.; Jin, A.; Pochan, D. J.; Schneider, J. P.J. Am. Chem. Soc. 2011, 133, 14975−14977.(25) Shao, H.; Parquette, J. R. Angew. Chem., Int. Ed. 2009, 48, 2525−2528.(26) Bowerman, C. J.; Nilsson, B. L. J. Am. Chem. Soc. 2010, 132,9526−9527.(27) Matsuura, K.; Murasato, K.; Kimizuka, N. J. Am. Chem. Soc.2005, 127, 10148−10149.(28) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294,1684−1688.

(29) Lim, Y. B.; Lee, E.; Lee, M. Angew. Chem., Int. Ed. 2007, 46,9011−9014.(30) Tovar, J. D.; Claussen, R. C.; Stupp, S. I. J. Am. Chem. Soc. 2005,127, 7337−7345.(31) Malesevic, M.; Strijowski, U.; Bachle, D.; Sewald, N. J.Biotechnol. 2004, 112, 73−77.(32) Dempsey, C. E.; Mason, P. E.; Jungwirth, P. J. Am. Chem. Soc.2011, 133, 7300−3.(33) Grishina, I. B.; Woody, R. W. Faraday Discuss. 1994, 245−262.(34) Ladokhin, A. S.; Selsted, M. E.; White, S. H. Biochemistry 1999,38, 12313−12319.(35) Chen, Y.; Barkley, M. D. Biochemistry 1998, 37, 9976−9982.(36) Robbins, R. J.; Fleming, G. R.; Beddard, G. S.; Robinson, G. W.;Thistlethwaite, P. J.; Woolfe, G. J. J. Am. Chem. Soc. 1980, 102, 6271−6279.(37) Lim, Y. B.; Moon, K. S.; Lee, M. J. Mater. Chem. 2008, 18,2909−2918.(38) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005,105, 1401−1443.(39) Honda, S.; Yamamoto, T.; Tezuka, Y. J. Am. Chem. Soc. 2010,132, 10251−10253.(40) Pakstis, L. M.; Ozbas, B.; Hales, K. D.; Nowak, A. P.; Deming,T. J.; Pochan, D. Biomacromolecules 2004, 5, 312−318.



diﬀerential self-assembly behaviors of cyclic and linear...

Documents