Symmetry-Directed Self-Organization in Peptide Nanoassemblies

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Symmetry Directed Self-Organization in Peptide Nano-Assemblies Through Aromatic #-# Interactions Sajitha Sasidharan, Prakash Kishore Hazam, and Vibin Ramakrishnan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b09474 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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The Journal of Physical Chemistry

Symmetry Directed Self-organization in Peptide Nano-assemblies through Aromatic π-π Interactions Sajitha Sasidharan, Prakash Kishore Hazam and Vibin Ramakrishnan* Department of Biosciences & Bioengineering, Indian Institute of Technology Guwahati, Guwahati-781039, INDIA. Email: [email protected]

ABSTRACT Almost all biological systems are assemblies of one or more biomolecules from nano to macro dimensions. Unlike inorganic molecules, peptide systems attune with the conceptual framework of aggregation models while they form nano-assemblies. Three significant recent theoretical models indicate that nucleation, end to end association and geometry of growth are determined primarily by size and electrostatics of individual basic building blocks. In this study, we put to test six model systems, differentially modulating the prominence of three design variables, namely aromatic π-π interactions, local electrostatics and overall symmetry of the basic building unit. Our results indicate that the crucial design elements in a peptide based nanoassembly are a) stable extended π-π interaction network b) size and c) overall symmetry of the basic building blocks. The six model systems represent all the design variables in the best way possible considering the complexity of a bio-molecule. The results provide important directives in deciding the morphology and crystallinity of peptide nano-assemblies.

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INTRODUCTION Almost all biological systems are functional assemblies of one or more fundamental biomolecular unit mediated through non-bonding interactions from nano to macro dimensions. Robustness and efficient functioning of these assembled architectures are already well established. Understanding the algorithm that connects the physico-chemical characteristics of basic building blocks, and stable structure formation at nano and micro level dimensions of these assemblies are a central problem, while we explore the possibility of making use of such biomolecules as fundamental units. Though bio-inspired design is a topic of modern day curriculum, pioneering work on this topic is a book“On Growth and Form” by D’Arcy Wentworth Thompson written almost 100 years ago, attempting to quantitatively assess structural patterns and formations in biological systems.1 Self-organization is envisaged as a spontaneous process which involves development of an order or a recurring pattern as a result of local interactions between smaller subunits of an initially disordered system. A classic case of self-organization is folding and aggregation of protein chain to functional and dysfunctional forms respectively.2 Helmholtz in one of his recorded lectures makes an observation that, ‘the forces so far as they cause chemical and mechanical influence in a living system, must be quite the same character as inorganic forces’.1 Uncertainty and lack of understanding in ascertaining long term physiological effects with nanomaterials of inorganic origin3 are prompting researchers to look up to materials of biological origin such as nucleic acids and proteins as basic building units. Earlier reports suggest that interplay between electrostatic and hydrophobic interactions, modulated by solvent effects result in the ordered aggregation and assembly at nano to micro dimensional range.4-8


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Protein or peptide molecule is a hetero-polymer made of 20 amino acids of L stereochemistry. Due to planarity of peptide bond and L-stereochemistry, conformational possibilities of individual amino-acids forming a protein chain are sterically restricted. This results in a peculiar situation, that only 21% of the total φ, ψ space of Ramachandran map can be accessed for protein main-chain.9 Even then, natural proteins have enormous varieties in their functional space and about 1500 variations in fold space.10 The protein molecule is a long polymeric chain, sometimes even extended beyond 1000 amino-acids. Smaller units or peptides are generally non- functional. The smallest polymer unit is a dipeptide with two amino acids joined together through a peptide bond. But assembly of individual peptides to form potentially functional units was a distinct possibility though unrealized till Ghadiri first reported nanotubes formed out of self-assembling units of short peptides stacked one over the other.11-12 Usage of locking conformational basins by incorporating D amino acids was first employed by Ghadiri. Hierarchical assembly of such conformationally rigid monomeric units forming robust nano and micro scale networks were recently demonstrated by Joshi and coworkers.13A significant observation involving supramolecular self-assembly of amino-acids like leucine, phenylalanine and their combinations were later on reported by Gorbitz.14 Gazit, Rosenmann and coworkers made tremendous advancements in the following years by demonstrating multi-various application fronts to di-phenylalanine (FF) nanotubes.15-16 Design of FF nano-assemblies to generate nanotubes, nanowires, quantum dots and nano-spheres followed, thus establishing an important discipline of peptide nano-assembly (PNA) in the advancement of nanotechnology.17 Though a number of amino-acids and their combinations were in principle possible, only FF nanotubes emerged as the most sought after model system due to its structural simplicity and robustness in assembly formation.15,

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Attempts to alter the basic model systems like


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combinations of phenylalanine (F) with tryptophan (W),

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or using tri-peptides (FFF)19 etc.

were attempted, resulting in an assembly pattern morphologically different from FF nanotubes. Assembly experiments with stereo-chemical variant ((D)Phe-(D)Phe) resulted in a structure of opposite handedness.15 Apart from basic building blocks, fabrication conditions may also play a crucial role in assembly formation, as demonstrated by Li and coworkers in a recent report.20 Earlier studies have provided ample evidence that FF nano-assembly is mediated through quadrupolar interactions involving phenyl embraces self-arranged in parallel displaced and T shaped configurations for maximum stability.21 Apart from electrostatics, another important variable for any assembly is the symmetry of constituent basic units. Asymmetry in basic unit prohibits the epitaxial growth, forming a stable macromolecular assembly.22 In this study we put to test six model systems, with varying symmetry elements differentially modulating the prominence of other two design variables, namely aromatic π-π interactions and end to end local electrostatics of basic building unit while designing nano-level assemblies of peptides. Our results indicate the contribution of extended π-π interaction and oppositely charged end groups in directing peptide nano-assemblies. Our results further suggest that size and overall symmetry of basic building blocks also have an important role in deciding the morphology and crystallinity of nanostructures. EXPERIMENTAL SECTION Sample Preparation: FF peptides were purchased from Sigma Aldrich Pvt. Ltd. Designed Ff, FBf, fBfBf and FfFfFf peptides were purchased from Mimotopes Pvt. Ltd. (Australia). FF and Ff stock solutions were prepared by dissolving the peptides in 1,1,1,3,3,3-hexafluoro-2- propanol (Sigma Aldrich) at a concentration of 100 mg/mL. FfFfFf and fBfBf were dissolved at a concentration of 50 mg/mL and FBf at a concentration of 25 mg/mL respectively. One day-aged


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peptide solutions which were diluted in dd.H2O to a final concentration of 0.5 mg/mL was used for FESEM and AFM analysis and a concentration of 2 mg/mL was used for TEM analysis. Field Emission Scanning Electron Microscopy: FESEM analyses were performed using Nova Nanosem NPE206 at 15 kV. 20 µL of one day aged peptide samples were loaded on silica wafer for analysis and air dried. Samples were coated with chromium for enhancing the conductivity. Atomic Force Microscopy: AFM images were performed using Agilent Model 5500 series instrument. 20 µL of one day aged peptide samples were loaded on silica wafer for analysis. Air dried samples were analyzed using semi-contact imaging mode. Obtained data’s were processed using WSxM 5.0 Develop 7.0 software.23 Transmission Electron Microscopy: One day aged peptide solution (10 µL) at a concentration of 2 mg/mL was loaded on a carbon300 mesh copper grid covered with strong carbon film. Negative staining was performed by adding 10 µL of 2% uranyl acetate in water. After two minutes, excess fluid was removed from the grid and the samples were dried at room temperature. Negatively stained samples were viewed in JEOL transmission electron microscope, Model: JEM 2100, operating at 200 kV.

RESULTS AND DISCUSSION De novo designs of model molecular systems were based on three criteria; aromatic π-π interactions, local electrostatics and symmetry of basic building blocks. We have designed these systems by placing amino-acids in specific allowed basins in Ramachandran diagram, such that they are conformationally locked assuming a singular structure and may associate further to form


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Figure 1. Chemical structure of diphenylalanine molecule (a). Allowed basins 1, 2 & 3 in Ramachandran map (b) and its respective combinations (c) for FF. Figure (d) shows allowed region for D-Phe(f) in Ramachandran diagram and Figure (e) shows combination of L-Phe(F) and d-Phe(f) basins. Figure (f) and (g) shows the distance between geometric centers of benzene rings in two phenylalanine residues of FF and Ff.

the assembled architecture. This was a primary requirement, while we design such model systems because, peptides are otherwise free to assume any conformations based on the reaction conditions while assembling themselves to a supramolecule. But the specific role that solvent (water) plays in such assembly formation is yet to be fully understood.


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We have broadly defined conformational possibilities of phenylalanine (F) residue in FF (Figure 1), adopting any three φ,ψ basins. First in a beta basin extending up to polyproline II helix (PPII) region, second in a right handed α helical basin and third is a rare left handed α helical basin(Figure 1b). Two di-phenylalanine (FF) monomeric units (Figure 1a) can have roughly nine geometric variant combinations with each phenylalanine residue adopting any of the three conformational basins (Figure 1c). We modeled all geometrical possibilities and calculated the distance between the geometric centers of phenylalanine benzene rings (Figure 1f) (Section 1, Table S1 in the supporting information). Equilibrium distance for aromatic π-π interaction is 4.96 Å, where it has a maximum energetic advantage of 3-4 kcal/mol. However, the distance for favorable quadrupolar interactions involving benzene rings ranges from 4.04 to 6.0 Å.21 Two (φ, ψ) basin combinations of FF falls within this range and out of these two, (13) and (21) combinations were at optimal distance for a possible aromatic π-π interaction (Figure 1c, f). In basin combination (21) the two benzene rings of FF are approximately orthogonal in their orientation, while in (13), it is parallel and at an optimum position for T shaped geometry (Figure S1). Basin 3 is a left handed α helical region and is not a preferred basin for a phenylalanine residue as per statistics obtained from protein databases.24 Interestingly, (13) basin combination (Figure 2a) is specifically the geometry we observe in the crystal structure data of FF nanotubes solved by Gorbitz (CCDC 16340).14The second and probably the only other possibility to design two phenylalanine, residues in the same (13) basin configuration is by using D-phenylalanine (abbreviated as D-Phe or f) as the second residue, and this combination of ‘Ff’ is our second model system. This Ff dipeptide will have a basin combination of (12’), of which basin 3 in LPhe (F) is replaced by 2’ basin of D-Phe (f) (Figure 1d,e). This molecular model also has a


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. Figure 2. Spatial orientations of combinations of diphenylalanine side-chain orientations that are chosen as model systems 1 to 3 in this study. All combinations are shown in supporting information Figure S1. Oval shaped cartoon representations indicate benzene ring. Red and blue lines indicate CO and NH dipole. Peptide models 4 to 6, with their conformations restricted by incorporating Aib in the sequence, FBf (d) and fBfBf (e). FfFfFf peptide based on the design clues from gramicidin and Ghadiri’s nanotubes(f). Gramicidin like beta helical structures results when backbone φ, ψ angle alternates between L and D beta basins of Ramachandran diagram (g).


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comparable distance of 5.39 Å between the geometric centers of its benzene rings (Figure 1g,2b) (Section 1, Table S2 in the supporting information). Our third model system is an equimolar mixture of both FF and Ff, to see whether they form any nano level assembly (Figure 2c). To test the specific effect of intra molecular interaction between benzene rings to determine nano-level architecture, we designed FBf, such that α-amino isobutyric acid (Aib or B) separates two phenylalanine residues (Figure 2d). The advantage of having Aib is that, it restricts the conformational flexibility of the tripeptide to sample only in α-helical conformation due to steric effects, and hence is structurally locked. The exclusive right handed α-helical basin preference separates the two phenyl rings from interacting distance (Section 1, Table S3 in the supporting information). It is logical to assume that the geometry of assemblies is a derivative of the individual geometry or symmetry of the basic constituent, and is especially obvious in inorganic systems. Earlier schools of thought, that nature’s creations are not excluded from the laws of geometry25 and correlation studies between biological forms and mechanical phenomena support this logic.1 Some observations on biomolecular interactions also give confidence to test the effect of individual symmetry of peptidic systems in defining nano-architecture.22A fifth model fBfBf was designed, (Figure 2e) which is considerably asymmetric in all three planes (Figure 3). This will also have restricted right handed helical conformation, (Figure 2e) having an asymmetric spatial orientation. If we assume, that distances along all three directions as vertices of a triangle, we get an asymmetric structure with three faces having three different size proportions, (Figure 3d) thus limiting the possibility of an assembled architecture.


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Figure 3. Approximate projections of three dimensional spatial arrangements of five model systems. Radius of gyrations along X, Y, Z are assumed as vertices of a triangle, to present a qualitative impression of its topology. Comparative analysis of Area (in Å2) of each triangle will provide a qualitative impression of the extent of symmetry in the distribution of constituent atoms in space.

The conformational preference of short peptides and ‘disordered’ segments to assume polyproline II conformation has already been established by various groups using poly-alanines as model systems. Solvation effects and inter-atomic electrostatic interactions within a peptide were proposed as the driving force for this preference.26 We have shown by Molecular Dynamics Simulations, the existence of specific folds such as helix and hairpins in octa-alanine conformational ensemble.27 We have further shown that the countervailing local and global electrostatic interactions may be attributed to the complex conformational folding behavior of peptide sequences, distinctly different from other hetero- polymers.28 Recent protein aggregation models by research groups of Vito Foderà, Anthony L. Fink, Lisa M. Gloss and Athene M.


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Donald predicts the formation of multi-fractal structures, with geometry of growth, primarily directed by electrostatic interactions.29-33 Peptide systems 1 to 5 has unfavorable intra-molecular local electrostatics compensated by favorable inter-peptide interactions through partially charged end groups. The sixth molecular model system is an exception to this with intra as well as interpeptide interactions are in a favorable orientation (Figure 2f). The FfFfFf peptide segment, with alternating L and D chiral phenylalanine residues, resemble earlier reported Ghadiri’s nanotubes (Figure 2g).11-12The most optimized structure out of 36 = 729 total conformational possibilities feasible for this sequence is shown in figure 2f, yet other conformations are not sterically prohibited in FfFfFf unlike other five structurally locked model systems (Section 1, Table S5 in the supporting information). This peptide is designed to investigate whether a stable architecture is feasible with assembly of structurally unlocked basic units satisfying quadrupolar interactions between benzene rings of phenylalanine residues alone.

Topology of two benzene rings facilitating intra-molecular π-π interactions are optimum in FF dipeptide nanotube crystal structure data by Gorbitz.14 In the crystal structure reported, first phenylalanine residue is in the β basin and the second one in the left handed (LH) α helical basin, thus conforming to the critical distance requirement for optimal T shaped phenyl embraces, mediated through quadrupolar interactions. The end residues of FF are positively and negatively charged at least partially. This facilitates the end to end propagation of basic FF units. Apart from this, two phenyl rings are protruded out of the main chain Cα atom with a beta carbon (CH2) in between. The chi1, and chi2 side chain dihedral angles can facilitate optimal π-π interactions. Overall, local electrostatics of opposite polarity at the ends, symmetrical positioning


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Figure 4. Micrographs of the nanotubes formed by the first two model systems of diphenylalanine peptides, FF and Ff in the first and second column respectively. FESEM (a) and AFM (b) images displaying the nanotubular structures of FF and Ff. (c) TEM images of the negatively stained nanotubes FF and Ff.

of two phenyl rings, and optimum inter ring distance between phenyl groups facilitate formation of assembled architecture mediated through favorable electrostatics. However, the adoption of not so favorable LH α-helical region by second F unit, appears to be a compulsive necessity for FF nanotube formation. The designed Ff dipeptide with second phenylalanine (f) in 2’ basin,


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Figure 5. FESEM and AFM images of (a) nanocombs formed by FBf, (b) random nanoassemblies formed by fBfBf and (c) nanofibres formed by FfFfFf.

forming (12’) configuration (Figure 1d, e) was hypothesized to have almost identical nanotubular structure. This experiment worked remarkably well with Ff forming nanotubes of


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comparable dimension as that of FF nanotubes, observed from FESEM, AFM and TEM experiments (Figure 4). Melting points of FF and Ff also were in a comparable range of 592 and 551K for FF and Ff respectively. This prompted us to check the assembly of equimolar mixture of FF and Ff as the third composition, but no significant assembled architecture was observed (Section S2, Figure S2 in the supporting information), an observation similar to an earlier reports, with napthalene conjugated dipeptides by Yang et al.34 We modified the configuration of phenyl embraces by adding α-amino isobutyric acid (Aib) as the middle residue between F and f. Exclusive RH α-helical basin adoption is the hallmark of αamino-isobutyric acid. Aib also forces the neighboring residues to be in an RH α-helical conformation, restricting intra-molecular π-π interactions. This changed the assembly from nanotubes to nano comb (Figure 5a). Same constraint was further extended by designing an asymmetrical fBfBf peptide (Figure 3d). The hypothesis that asymmetry in the basic unit may have prohibitory effects in nano level assembly also worked remarkably well, with fBfBf model system not showing any consistent pattern in their nano assembly (Figure 5b). The sixth and final model system FfFfFf as basic building block, can form channel forming helical conformation much like what we see in gramicidin35or Ghadiris nanotubes.11-12 However, individual residues of FfFfFf molecule can sample any three allowed basins, and can in principle form 36 =729 conformational variants. Nevertheless, its nano-assembly is consistent forming /non-crystalline (Section S3, Figure S3, S4 in the supporting information) amorphous stable nano-fibres (Figure 5c). Epitaxial growth of individual building blocks which has lesser degrees of freedom is the key for the design of molecular architectures at nanolevel. Symmetry and electrostatics expectedly play a crucial role in such assembly buildups. The (13) and (12’) basin combinations of FF and


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Ff presents almost the same electrostatics, symmetry and geometry of phenyl embraces. Consequently, FF and Ff resulted in an almost identical assembled architecture (Figure 4). From a design perspective, this experiment confirms that the nano-assembly is indeed a function of symmetry and electrostatics of fundamental units, even in the case of peptide systems, where conformational flexibility and its effect is much more pronounced compared to inorganic systems. We observed a difference of 41K in melting point between FF and Ff, and this difference may be attributed to the observation that Ff is comparatively less symmetric than FF (Figure 3a and 3b). Though this asymmetry is not significant to prohibit the assembly, but may affect the overall stability of a well packed structure. Effect of asymmetry is, but very well evident in fBfBf, with no pronounced assembly formation noted by microscopic methods (Figure 5b). Change in chemical constitution along with lack of intra-molecular π-π interaction between two benzene rings of adjacent phenylalanine residues prohibited nanotube formation of conformationally locked FBf basic unit, but resorted to a nano-comb like architecture (Figure. 5a) by inter-molecular association. The amorphous but stable nano-fibres formed by FfFfFf (Figure 5c) may be attributed to favorable local electrostatics and intra-molecular π-π interactions between phenylalanine residues with adjacently positioned side-chains. Recent coarse grained MD simulation results by Schatz and coworkers attribute nano fiber formation of such short peptide amphiphiles to the micellar and van der Waals interactions at various stages of assembly formation.36 Intermolecular phenyl embraces can also form stable assemblies as evidenced from FBf and fBfBf, but are relatively less stable compared to diphenylalanine nanotubes, an observation based on the beam damage caused by TEM experiment. From our experiments, we found that the most important design element in a peptide based nano-assembly is a stable extended π-π interaction network. In the case of FF assembly, the two phenyl rings are


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placed at optimal distances for maximum stability. The two oppositely polar ends facilitate intermolecular extended network formation necessary to assume tube like structure. The φ dihedral angle range in basin 1 of first residue, from -150 to -90 ensures that nanotubes with wide variety of diameters are possible. Diameter of nanotube decreases as φ angle gets closer to PPII basin. This explains the wide range of FF nanotube diameter reported by Gazit, and coworkers ranging from 20-300 nm.15 Basic configuration of two phenyl rings in FF and Ff are the same and so are their nano-level assemblies. We found that quadrupolar interactions between phenyl ring, within and between FF molecules play a key role in their assembly. We designed FBf, such that both phenyl rings are at 7.08Å apart in a sterically locked conformation, having all amino acids adopting RH α-basin. The polar ends are relatively symmetric in its positioning. In this structure, it helps the molecule to form comb like architecture, though intra-molecular phenyl embraces are absent, but inter-molecular interactions are still possible. We could not find any significant nano level assembly in the case of fBfBf. Understandably, molecules with an inherent asymmetry cannot undergo symmetric assembly. In the presented work, we tried to optimize inter and intra molecular quadrupolar interactions between phenyl rings as in FfFfFf to get some design directives that can be employed while designing PNA’s. FF is the core recognition motif of Alzheimer’s β amyloid peptide. Therefore, the mechanism of self-assembly of phenylalanine based systems may be more close to peptide/protein aggregation models than inorganic systems.37 Recent study by Athene M Donald and coworkers have presented a convincing model for protein aggregation arguing that the geometry of growth is principally determined by electrostatic interactions between basic units.33 Donald and coworkers argue that peptidic systems containing polar groups unevenly distributed on the molecular surface will generate multi-pole moments affecting inter-peptide association.


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Consistent with this argument, our systems (except FF and Ff), have similar molecular structure, and therefore have almost similar assembled architectures. FF and Ff, though stereo-chemically different, have identical charge distribution, when they assume identical basins in Ramachandran map resulting in nanotube like morphology. Knowles, and coworkers in a recent work highlights the role of filamentous structures undergoing nucleation and end to end association in the dynamic behavior of the growth of linear protein assemblies. Adapting this framework, we can assume that partially charged end groups provide a directive influence in nucleation and association,38 and such elongated systems can further self-catalyze to form extended architectures, a phenomenon predicted by Fodera and coworkers in a recently published 2D model for aggregation.39 As stated earlier, we put to test three design elements namely benzene quadrupolar interactions, peptide backbone electrostatic interactions and overall symmetry for indicative results, so that researchers involved in creation of peptide based nano assembled architecture will get a supportive design guideline. The sample set chosen and the results are sufficient to provide clear directives, though more detailed study would further enrich this line of enquiry. Though modular design attempts with peptides have earlier been reported, this could probably be the first attempt to modulate nano-assembly from molecular size and symmetry.40It will also complement the mechanistic investigations on the driving forces involved in protein and peptide aggregation. CONCLUSIONS Organic and bio-organic molecules have variables like non-bonded interactions, stereochemistry and symmetry playing a larger role, in the stability and morphology of assembly formation. These factors are often detrimental in directing their assembled architecture and consequently their function. Through this manuscript, we tried to extract indicative information


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about their relative prominence of symmetry, π-π interaction and overall size with six model molecular systems (Table S6). We observe that morphology and stability of this electrostatic interaction mediated assembly is largely decided by their individual symmetry. Smaller molecules with greater number of symmetry elements tend to form more ordered crystalline assemblies, whereas larger molecules especially asymmetric ones are more prone to be amorphous. From these experiments, we could make a clear suggestion that overall size, symmetry and charge distribution of fundamental unit dictates the topology of peptide nanoassembly. However, a comprehensive mechanistic picture, especially role of water in modulating assembly formation is yet to be understood fully and may require elaborate investigation.

ASSOCIATED CONTENT Supporting Information. Supporting Information for this article has details of distance calculations between benzene rings of phenylalanine residues (section 1), FESEM image of equimolar mixture of FF and Ff (section 2), electron microscopic images and PXRD of FfFfFf (section 3) and a table summarizing the effect of three design variables on six peptide model systems which are not explicitly detailed in the main text. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Authors deeply acknowledge Dr. M.M. Shaijumon, IISER Thiruvananthapuram and Instrumentation facility of IISER; Dr. Nitin Chaudhary, IIT Guwahati for useful discussions. We acknowledge, Department of Biotechnology, Govt. of India, Grant No.BT/350/NE/TBP/2012 and IITG startup grant for funding. We thank Central Instruments Facility, IIT Guwahati for instrument support.

ABBREVIATIONS ‘F’ – (L) Phenylalanine, ‘f’– (D) Phenylalanine, ‘Aib’-alpha-Aminobutyric acid, ‘FESEM’Field Emission Scanning Electron Microscopy, ‘AFM’ – Atomic force Microscopy, ‘TEM’ – Transmission Electron Microscopy.

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The specific role that molecular symmetry plays in the assembly and architecture of peptide based systems forming extended stable structures at nano-micro level dimensions have been investigated. Results underline the prominence of molecular symmetry in crafting pi-pi interactions resulting in specific topology, stability and crystallinity. 50x50mm (300 x 300 DPI)


Symmetry-Directed Self-Organization in Peptide Nanoassemblies

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