Molecular Engineering of Self-Assembling ... - ACS Publications

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Molecular engineering of self-assembling diphenylalanine analogues results in formation of distinctive microstructures Michal Pellach, Sudipta Mondal, Linda Shimon, Lihi Adler-Abramovich, Ludmila Buzhansky, and Ehud Gazit Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01322 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on June 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Molecular engineering of self-assembling diphenylalanine analogues results in formation of distinctive microstructures

Michal Pellach,† Sudipta Mondal,† Linda Shimon,‡ Lihi Adler-Abramovich,§ Ludmila Buzhansky† and Ehud Gazit†,#,*

†

Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life

Sciences, Tel Aviv University, Ramat Aviv 69978, Israel ‡

Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100,

Israel §

Department of Oral Biology, The Goldschleger School of Dental Medicine, Sackler Faculty

of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel #

Department of Materials Science and Engineering, Iby and Aladar Fleischman Faculty of

Engineering, Tel Aviv University, Ramat Aviv 69978, Israel

*Corresponding Author Ehud Gazit E-mail: [email protected] Phone: +972-3-6409030 Fax: +972-3-6409407

1 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Diphenylalanine is one of the most studied building blocks in organic supramolecular chemistry, forming ordered assemblies of unique mechanical, optical, piezoelectric and semiconductive properties. These structures are being used for diverse applications including energy storage, biosensing, light emission, drug delivery, artificial photosynthesis and chemical propulsion. In order to increase the structural diversity of this dipeptide building block, three previously-unreported analogues were synthesised in which the aliphatic chain between the peptide backbone and the phenyl ring was gradually lengthened. Each dipeptide self-assembled into unique microstructures, differing in morphology, which ranged from flat plates to long microrods to flattened microplanks. The structures were also found to possess distinctive optical properties. Furthermore, X-ray crystallography of each of the three diphenylalanine analogues presented distinctive molecular arrangements. The remarkable differences between each dipeptide in the intermolecular interactions they formed give insight into the physicochemical mechanisms of self-assembly, and in addition, indicate the biological significance of the single methylene bridge of phenylalanine.

INTRODUCTION Peptides are excellent molecular entities for the formation of supramolecular nanostructures due to their self-assembling capabilities, as well as biocompatibility and therefore potential utilisation as building blocks for biomaterials. Specifically, since the self-assembly of diphenylalanine (DiPhe) into peptide nanotubes,1 aromatic dipeptides have attracted considerable attention, with different morphologies that have been achieved by variation of fabrication methods as well as chemical modification of the dipeptide.2-7 Aromatic interactions can occur in a face-to-face (centred or offset) or edge-to-face (Y or T shaped) manner,8 and play an important role in self-assembly processes as well as stabilisation of secondary, tertiary and quaternary structure of proteins and peptides containing aromatic sidechains. A well-characterised example of aromatic interactions in


Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peptides is the tryptophan zipper, a short β-hairpin peptide containing cross-strand interactions between indole rings.9 Additional study of cross-strand aromatic interactions in βhairpin peptides has indicated a stabilising effect on secondary structure.10-14 The presence of aromatic residues in α-helices have also shown to provide additional stability.15 Among the four coded aromatic amino acids, phenylalanine has highest propensity for self-aggregation and preference for self-pairing, whereas tryptophan, tyrosine and histidine all prefer heterologous aromatic interactions.16,17 Following the screening of the 400 dipeptides and more recently 8000 tripeptides from naturally occurring amino acids, it was shown that those containing aromatic residues have greatest tendency to form self-assembled nanostructures. Specifically, each of the ten tripeptide building blocks with the highest aggregation propensity contained phenylalanine and five of them actually included the DiPhe motif.18,19 This further emphasises the role of Phe and DiPhe in the construction of ordered peptide assemblies. Originally derived from the amyloid-forming Aβ peptide, DiPhe has been shown to selfassemble into crystalline peptide nanotubes, with the crystal structure displaying head-to-tail hydrogen bonding, and channels within which water molecules assist with its stabilisation via hydrogen bonding with the amino hydrogen atoms.20-22 Molecular dynamics simulations have demonstrated both interpeptide head-to-tail hydrogen bonding and peptide–water hydrogen bonding interactions.23 DiPhe self-assembled structures have presented remarkable stiffness, mainly attributed to intermolecular aromatic interactions.24,25 The nanostructures are chemically and thermally stable,26 have displayed intrinsic luminescence properties,27-29 and in addition have revealed piezoelectric properties.30,31 Formation of DiPhe quantum-dot-like or nanowire morphologies with semiconductive properties have also been demonstrated.32-34 As a result of a vast range of properties, DiPhe structures are being used for diverse applications including energy storage, biosensing, light emission, drug delivery, artificial photosynthesis and chemical propulsion.35,36 Attachment of moieties traditionally utilised as amine protecting groups such as tbutoxycarbonyl (Boc) and fluorenylmethoxycarbonyl (Fmoc) groups have shown to



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

significantly modify the self-assembly of DiPhe resulting in new nanostructures with altered and even superior mechanical properties. Boc-Phe-Phe self-assembly presents extremely rigid spherical structures,25 whereas Fmoc-Phe-Phe forms fibres that gelate in solution into hydrogels.37-40 The tendency of additional Fmoc-dipeptide supramolecular structures to form hydrogels has been investigated, with self-assembly said to be driven by and stabilized by hydrogen bonding and aromatic interactions between π electrons of the fluorenyl rings.41 Selfassembly and coassembly of DiPhe-based peptides has been further studied by many research groups,4,42 and theoretical calculations of the aromatic intermolecular interactions have given rationalisation for their mechanical and semiconductive properties.24,43 Exploiting Phe-like non-coded amino acids resulted in increased structural diversity. Selfassembly of aromatic homodipeptides have been studied and resulted in formation of interesting micro- and nanostructures. The simplest aromatic homodipeptide, diphenylglycine, was shown to self-assemble into stable close-caged fullerene-like spherical structures.44 Additional study of the self-assembly of various synthetic homodipeptides have been studied, and were discovered to form a variety of tubular structures, spherical structures, fibrillar structures as well as plate-like structures.36,45 Dipeptides of non-coded amino acid 3,4dihydroxyphenylalanine (DOPA) have also been explored as self-assembling building blocks, with fibrillar structures formed by both DOPA-DOPA and hydrogel-forming Fmoc-DOPADOPA.46 In the current work, the unique physical and mechanical properties of DiPhe self-assembled structures, which have given rise to their diverse applications, were used as inspiration for original peptide design. Investigating beyond naturally occurring amino acids, we explored the effect of increasing the distance of the aromatic ring from the peptide backbone of DiPhe. A series of aromatic dipeptides were synthesised comprising of dihomophenylalanine (DiHpa), di-2-amino-5-phenylpentanoic acid (DiApp) and di-2-amino-6-phenylhexanoic acid (DiAph). Their self-assembling propensities were investigated and compared to DiPhe. The consecutive addition of a single CH2 group between the peptide backbone and each of the two


Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


phenyl rings significantly affected the morphology of the supramolecular structures, observed by microscopy, as well as the packing arrangement of the dipeptide molecules, as studied by single crystal X-ray analysis. Notable differences in optical properties of the structures were also observed.

EXPERIMENTAL SECTION Synthesis of diphenylalanine analogues. Aromatic amino acids (Aaa) LHomophenylalanine (Hpa) and Boc-L-Hpa were obtained from commercial sources, and 2amino-5-phenylpentanoic acid (App) and 2-amino-6-phenylhexanoic acid (Aph) were synthesized in a similar manner to as previously described.47,48 (Details of the synthesis are summarised in the Supporting Information.) This was followed by Boc or methyl ester protection. The three DiPhe analogues were synthesized using standard solution phase peptide synthesis. Briefly, dipeptides Boc-Aaa (1.2 mmol) and Aaa-OMe (1.2 mmol) were suspended in ACN (12 mL). HBTU (0.47g, 1.2 mmol) in ACN (5 mL) was added, followed by DIPEA (0.7 mL). The mixture was stirred overnight at room temperature. The mixture was evaporated to dryness, the product was collected with dichloromethane (DCM), and washed with dilute HCl (pH ~2 3x7 mL) followed by NaHCO3 solution (3x7 mL) and then the organic phase was dried over Mg2SO4. The product was evaporated to dryness, and purified by silica gel chromatography (Eluent Hex:EtOAc 75:25). Yield = 75-80%. Removal of protecting groups: The product was dissolved in THF (12mL) and 1M NaOH (1.5 mL) was added. MeOH was added to eliminate phase separation. The mixture was stirred for 3 h at room temperature then evaporated to dryness. The product was redissolved in MeOH and passed through Amberlite resin, evaporated to dryness and dissolved in TFA for Boc removal. ~2 h later the TFA was evaporated followed by repeated evaporations using DCM, hexane and ether. DiHpa obtained was a colourless solid, DiAph was precipitated with DCM, also a colourless solid, and solid DiApp was obtained after lyophilisation. For DiApp and DiAph



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HPLC was used to separate between DL and LL dipeptides (Fig. S3). MS m/z (M+H+) DiHpa 341.4, DiApp 369.3, DiAph 397.3. Self-assembly. Lyophilized dipeptide powder was dissolved in 1,1,1,3,3,3hexafluoroisopropanol (HFIP, 100 mg/mL, 5 µL). The concentrated peptide solutions were then diluted to a total concentration of 5 mg/mL with deionized water. Characterization of self-assembled microstructures. Fluorescent micrographs were obtained with an Olympus BX51 microscope. Scanning electron microscopy was performed following gold coating, using a Jeol JSM 6300 scanning electron microscope. Forced ion beam (FIB) was performed using a RaithionLine FIB with a 35KV gallium ion beam, at a current of 24 pA. The beam was concentrated to a selected area for milling through the structures. Imaging was performed with and without tilting at 25-30°. AFM samples were prepared on a mica surface and images were obtained using a NanoWizard3 (JPK Instruments) system in tapping mode, with a scan rate of 0.5 Hz. Fluorescence spectra were obtained using a Cary Eclipse spectrofluorometer (Agilent Technologies). Crystallization and X-ray crystal structure analysis and crystal data. DiHpa selfassembled structures were crystalline and used for diffraction. DiApp and DiAph were crystallised by dissolving in MeOH, addition of water (20-25% of mixture), followed by slow evaporation of MeOH. Single crystals suitable for X-ray diffraction of DiHpa, DiApp and DiAph were coated with Paratone oil (Hampton Research). Crystals were mounted on loops and flash frozen in LN. Diffraction data measurements for compounds DiHpa and DiApp were done on a Bruker KappaApexII system with MoKα radiation at 100(2)K. Data for DiAph was collected at ESRF beamline ID29 at λ=0.70 Å at 100(2)K with a Detetris 6M detector. The DiHpa and DiApp data were collected and processed with Apex2 Suite. The DiHpa data were collected and processed with MXCube and EDNA. The structures were solved by direct methods using SHELXT-2013. The structures were refined by full-matrix least-squares against F2 with SHELXL-2013. The crystallographic data of DiHpa, DiApp and DiAph are given in Fig. S5, tables SI-SIII. The crystallographic information files have


Page 6 of 23

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been deposited at the CCDC (Cambridge Crystallographic Data Centre) with numbers 1445190 (DiHpa), 1445191 (DiApp), 1445192 (DiAph). This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

RESULTS AND DISCUSSION Peptide Design. Our approach to peptide design was inspired by DiPhe, which has become an extremely important building block in organic supramolecular chemistry.1,4,36 The unique and diverse properties of DiPhe are largely due to its phenyl rings and the intermolecular aromatic interactions it forms. The peptide design involved gradually increasing the degrees of freedom of the aliphatic chain by consecutive addition of a methylene bridge, to gradually increase the conformational diversity of the molecule, and thus increase possible orientations of intermolecular aromatic interactions. The degrees of freedom and the location of the aromatic ring were expected to have significant implications in the self-assembly process. The dipeptides to be explored were DiHpa, DiApp and DiAph (Fig. 1).

Figure 1. Design of dipeptide analogues of DiPhe: DiHpa, DiApp and DiAph. The sidechain is consecutively lengthened by a methylene group, gradually increasing the distance from the peptide backbone. Peptide Synthesis. The dipeptides were synthesised from their corresponding amino acids. Commercially available Hpa was used without further purification, whereas the amino acids App and Aph were synthesised by methods based on previous synthetic schemes.47,48 Details of the synthesis are described in the Supporting Information. The purity of both App and Aph was >99% by HPLC, and despite the utilisation of a protease intended for isolation of the S



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(L) enantiomer, a ~10% impurity was observed when analysed by chiral HPLC, indicating presence of the second enantiomer (Fig. S1). The ~90% enantiomerically pure App and Aph were used without further purification. Standard solution phase peptide synthesis was performed in acetonitrile with HBTU as the coupling reagent, producing the dipeptides DiHpa, DiApp and DiAph. The LL isomers of DiApp and DiAph were then isolated from the mixture of stereomers by (preparative) HPLC (Fig. S2, S3). Peptide Self-Assembly. The self-assembly of the three closely-related dipeptides was performed by solvent switch from HFIP to water, the same conditions in which DiPhe selfassembles into nanotubes. For each dipeptide immediate precipitation was observed on addition of water, followed by gradual formation of microstructures with defined architecture. While a comprehensive kinetic study of the microstructure formation is beyond the scope of this manuscript, structural transitions in formation of peptide micro- and nanostructures have been previously reported.2,14,49-52 DiHpa, differing from the well-characterised DiPhe by a single methylene bridge, originally formed an opaque precipitate, and then over ~3-3.5 h formed thin, crystalline, plate-like structures, visible to the eye (Fig. 2a). SEM images depicted mostly elongated hexagonal plates, just under 1 µm in thickness, and AFM indicated that these are composed of multiple layers, the thinnest of which was found to be ~1 nm. Compaction of multiple layers of ~2-5 nm in thickness indicate layer-by-layer growth of the plates (Fig S4). DiApp formed rods of ~2-5 µm in diameter (Fig. 2b), and in length reached over 100µm. In order to determine whether the rods were hollow (tubular) or filled, a forced ion beam (FIB) was used for milling through a cross-section of the rod, FIB imaging was performed at a tilt of 25-30°. Indeed, the rods were solid throughout the cross-section (Fig. 3). Self-assembly of DiAph resulted in formation of flattened “planks” (of smaller dimensions compared to the DiApp rods), as well as rods with similar morphology as those of DiApp, but 1-2 µm in diameter (see Fig. 3c). The DiAph planks (Fig. 2c) were the main microstructures observable 24 h after solvent switch and were uniform and unique in shape, with long and flattened


Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


morphology with curved edges. The larger DiAph rods, similarly to the DiApp rods, were found to be solid rather than hollow.

Figure 2. Aromatic dipeptide self-assembled microstructures following solvent switch from HFIP to water. (a) DiHpa flat crystalline plates were formed over approximately 3 h (i), visible to the eye and by microscopy. SEM (ii) revealed their elongated hexagonal structure. (b) Light microscopy (i) and SEM (ii, iii) of DiApp rods, measured to be ~2-5 µm in diameter and observed to possess angular morphology rather than circular. The rods were observed to be filled rather than hollow, with occasional appearance of hollow ends. (c) Light microscopy (i) and SEM (ii, iii) of DiAph planks, up to approximately 10 µm in length, 0.5-1.0 µm in width and 0.2 µm in height, with curved edges. Note differences in SEM scale bars.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Dipeptide microstructures. (a) FIB images of DiApp rods and (b) DiAph rods. The DiApp rods were 2-3 times the diameter of the DiAph rods formed. Milling through the rods revealed presence of solid (non-hollow) structures (aii and bii), and indicated significantly different molecular arrangement and packing compared to DiPhe nanotubes. (c) SEM image of DiAph structures displaying both rods (larger structures) and planks (smaller structures). Crystal Structure Analysis. In order to provide insight into the molecular organization of the synthesized dipeptides, the crystal structure of each peptide was analysed (Fig. 4). The DiHpa crystals grown in 5% HFIP/water were suitable for diffraction and revealed the presence of three asymmetric molecules with P2(1) symmetry. These three molecules adopt a similar backbone conformation to each other with little variation in torsion angles (Fig. 4a, Fig. S5). The two side chains of DiHpa were arranged in opposite directions relative to the amide bond, with an extended head-to-tail backbone network connected by H-bonding. This conformation


Page 10 of 23

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is completely different from FF, which revealed syn conformation of the side chains.21 The most common structural motif of hydrophobic dipeptides comprises two NH3+…..-OOC headto-tail hydrogen bonds that laminate into two dimensional sheets and the residual amino proton is accepted by acceptor group of solvent molecules.53 However, the DiHpa asymmetric unit is free of solvent molecules and the aromatic side chain arranged in unique conformation to yield NH3+…π interactions (Fig. 4b). The two adjacent DiHpa continuous linear peptide backbones run laterally with respect to each other with parallel β-sheet-like configuration, stabilised by π-stacking and interstrand H-bonding, incorporating both terminal polar groups and internal amide groups (Fig. 4biii). The individual sheets stack to afford layer-by-layer arrangement (Fig. S6). The interfaces of the neighbouring sheets adhere by the zipper-like arrangements of the phenyl rings. The formation of stable zipper-like modules and the presence of unique NH3+…π interactions appear to create a two dimensional sheet, leading to the crystalline plate-like structure viewed by microscopy, and do not allow nanotube formation obtained for DiPhe. The thickness of each individual sheet of the crystal structures is 1.1 nm and this is in good agreement with the height profile obtained by AFM analysis (Fig S4). The dipeptide comprising a further addition of a methylene group to each residue in DiHpa, giving DiApp, crystallised in P212121 space group with four independent molecules per asymmetric unit and showed unique and complex crystal packing (Fig S5, S7). DiApp exhibits two conventional NH3+…..-OOC head-to-tail hydrogen bonding, but unlike DiHpa, the third amine proton acts as a donor moiety to the trifluoroacetic acid cocrystallised with the peptide. As shown in Fig. 4b, three adjacent chains of NH3+…..-OOC head-to-tail hydrogen bonding along the a-axis create a novel packing motif and run antiparallel to each other. Each strand interacts with the neighbouring stand through H-bonding and π-stacking interactions. When viewed along the a-axis, it is evident that the packing motif adopts a cylindrical shape where the hydrogen bonded peptide backbone constitute the core, and the surface of the cylinder is composed of hydrophobic aromatic moieties, several of which appear as free



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hanging or protruding aromatic rings that are available for interaction. The rods viewed by microscopy could be a result of several cylindrical units that interact via “hanging” aromatic rings (Fig S7). As evident from the crystal structure, unlike DiPhe, there was no void space on the crystal lattice and it could be predicted that the solid packing of the DiApp resulted in the formation of rod-like morphology rather than tubular structures. The DiAph peptide crystallised in the P2(1) space group, which is the same as that of DiHpa, and showed several similarities in crystal packing. However, in contrast to the DiHpa, the unit cell of DiAph is composed of eight asymmetric dipeptide molecules and these exhibit extended backbone conformation (Fig. S5, S8), differing from DiHpa. In addition, the crystal lattice also contained cocrystallised TFA molecules. The continuous NH3+…..-OOC head-totail hydrogen bonding arranged the peptides in linear chain geometry and could be compared with the single strand of β-sheet (Fig. 4c). Two such strands run antiparallel to each other and are in sharp disparity to the DiHpa structure where individual strands run parallel to each other. It can be assumed that such antiparallel arrangement of peptide chains modify the electronic nature of the crystal facets and constructive elongation of the one crystal facet leads to the formation of the plank morphology observed for self-assembly of this peptide rather than the hexagonal lattice obtained from DiHpa. Layer-by-layer organization of the antiparallel β-sheet-like motifs furnish the three dimensional packing of the crystal.


Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Selected molecular interactions as observed by single crystal X-Ray analysis. (a) The crystal structure of DiHpa. (i) The asymmetric unit of DiHpa composed of three molecules; (ii) The rare NH…. interaction in the DiHpa crystal; (iii) Head-to-tail hydrogen bonded strands of DiHpa interact with the adjacent strands in the parallel direction resembling parallel β-sheet structure. (b) Groups of three adjacent head-to-tail hydrogen bonded strands of DiApp displayed a unique packing pattern. (i) view along the c-axis, with side chains removed for clarity; (ii) view along the a-axis. (c) Antiparallel β-sheet-like packing afforded by DiAph. The crystal structure data is summarised in Fig. S5. We can infer from a comparative analysis of the three peptide crystals reported here that substitution at the γ carbon atom, as in DiApp, leads to unique three-dimensional molecular arrangement. This may emanate from the incompatibility between head-to-tail hydrogen bonded chain formation and steric crowding; and may account for the absence of natural hydrophobic amino acids with γ-branching. DiAph, however, with substitution at the δ



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

carbons, greater hydrophobicity and longer side chains, does not present the same steric crowding, and Aph could possibly be investigated as an isostere of Phe. To further gain insight into the molecular arrangement of the peptides within the selfassembled structures, their FTIR spectra were recorded (Fig. S9). DiHpa showed a strong amide I absorption peak at ~1675 cm-1 indicating the presence of hydrogen bonded β-sheet structure, and this correlates with reported IR spectra of DiPhe fibres.54 A similar spectral pattern with the corresponding IR signal at 1642 cm-1 for DiAph also indicated β-sheet formation. However, FTIR spectra do not conclusively differentiate between parallel and antiparallel β-sheets, and the different IR peak positions for DiHpa and DiAph may have resulted from the overall difference in molecular packing.55,56 Interestingly, DiApp revealed an amide I absorbance band at ~1675 cm-1 and the distinct spectral pattern can be ascribed to a completely different three-dimensional packing, as observed in the crystal structure.

Optical properties. The aromatic dipeptide self-assembled microstructures exhibited complex optical properties, with several emission peaks, influenced by the excitation wavelength. DiPhe spectra and optical effects for aligned self-assembled nanotubes have been previously characterised.27 Here we have compared DiPhe spectra to our additional dipeptide structures as a dispersion in aqueous medium. Besides the more prominent emission peaks observed for DiPhe at approximately 290 and 570 nm, an additional lower-intensity peak at approximately 412 nm was observed, with the corresponding excitation peak at approximately 400 nm. These relatively low-intensity peaks are observed for all four dipeptide self-assembled structures, at similar wavelengths (solid light grey line and purple line, Fig. 5). This excitation-emission pair is possibly typical of certain electronic states formed by the intermolecular interaction and aggregation of aromatic residues. A relatively broad emission peak was observed when each of the dipeptide microstructures was excited at 300 nm (Fig. S10). The DiPhe emission peak, at 364 nm was very weak in intensity, compared to the corresponding peak observed for DiHpa, also at 364 nm. The


Page 14 of 23

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DiApp emission peak appears to be shifted to approximately 380 nm whereas excitation of the DiAph microstructures at 300 nm presented an emission peak at ~370 nm. With excitations at different wavelengths DiApp also exhibited emission peaks at ~310 and ~375 nm, and DiAph presented additional emission peaks appeared at ~360 and ~390 nm. Interesting variations in the fluorescence spectra are shown in figure 5 and figure. As described above, the DiAph forms both self-assembled smaller “planks” as well as larger microrods. It was not unanticipated that its crystal structure is most complex demonstrating several different possible intermolecular interactions, which in turn affect the electronic states of the molecules.

Figure 5. Selected fluorescence spectra of DiPhe and DiPhe analogues. Excitation and emission spectra vary for each of the dipeptides, and wavelength-dependent shifts in peak luminescence are observed. Common to all four aromatic peptides is a narrow excitation peak and emission peak at ~400 and ~415 nm respectively. More detailed spectra can be found in the supporting information (Fig S10).

Fluorescence microscopy was performed for the DiApp and DiAph rods using standard UV/blue, blue/green and green/red filters (Fig. 7). Standard filters available were used to detect the intrinsic fluorescence of the structures, which could be observed using all three filters. The fluorescence microscopy was not successful for the DiHpa plates, which are flat



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and >1 µm in thickness and therefore difficult to capture, and the fluorescence of the DiAph planks appeared weaker compared to that of the larger rods.

Figure 7. Optical microscopy of aromatic dipeptide self-assembled structures. (a) DiApp and (b) DiAph light microscopy (i) and fluorescence microscopy of self-assembled microstructures, with visible luminescence using UV-blue (ii), blue-green (iii) and green-red (iv) filters. CONCLUSION Self-assembly of aromatic dipeptides has become a major field of research, with each study revealing new perceptions regarding the autonomous formation of macromolecular assemblies. We synthesised three variations of diphenylalanine, gradually extending the distance between the peptide backbone and the phenyl ring by consecutive insertion of an additional methylene, resulting in three previously unreported chemical entities. Increasing the degrees of freedom also increased the possibilities of intermolecular arrangements and interactions. The longer chain appeared to increase the complexity of the molecular organisation and intermolecular interactions that form, and increase the number of molecules and stabilising forces observed in the crystal structures. The differing intermolecular interactions that form affect the self-assembly process and result in distinctive macromolecular structures, varying in morphology and dimensions. The combination of single crystal analysis, FTIR and self-assembly studies demonstrated important aspects of amino acid side chains in molecular orientation and packing.


Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The significantly different properties of each of these dipeptide self-assembled structures give insight into the significance of nature’s selection of the single methylene bridge of phenylalanine. Variations in the optical spectra and fluorescence properties of the dipeptide nanostructures show that the extension of the alkyl chain of the dipeptides affects the electronic states of the self-assembled structures. These optical properties observed also show potential of the dipeptides as building blocks for organic light-emitting materials.

ASSOCIATED CONTENT Supporting Information Available: Supporting figures S1-S10; Supplementary experimental data including synthesis of amino acids and crystal structure data tables (PDF). X-ray crystallographic file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author: Ehud Gazit. Email: [email protected]; Phone: +972-3-6409030; Fax: +972-3-6409407. Note: The authors declare no competing financial interest. ACKNOWLEGEMENTS The authors would like to thank Dr. Yigal Lilach for the FIB structure analysis and imaging. The authors also thank Chiral Technologies, Europe for their analytical chiral HPLC. REFERENCES 1.

Reches, M.; Gazit, E., Casting Metal Nanowires within Discrete Self-Assembled

Peptide Nanotubes. Science 2003, 300, 625-627. 2.

Mandal, D.; Nasrolahi Shirazi, A.; Parang, K., Self-Assembly of Peptides to

Nanostructures. Org. Biomol. Chem. 2014, 12, 3544-3561. 3.

Gazit, E., Self-Assembled Peptide Nanostructures: The Design of Molecular Building

Blocks and Their Technological Utilization. Chem. Soc. Rev. 2007, 36, 1263-1269.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.

Yan, X.; Zhu, P.; Li, J., Self-Assembly and Application of Diphenylalanine-Based

Nanostructures. Chem. Soc. Rev. 2010, 39, 1877-1890. 5.

Gour, N.; Barman, A. K.; Verma, S., Controlling Morphology of Peptide-Based Soft

Structures by Covalent Modifications. J. Pept. Sci. 2012, 18, 405-412. 6.

Kim, S.; Kim, J. H.; Lee, J. S.; Park, C. B., Beta-Sheet-Forming, Self-Assembled

Peptide Nanomaterials Towards Optical, Energy, and Healthcare Applications. Small 2015, 11, 3623-3640. 7.

Yan, X.; He, Q.; Wang, K.; Duan, L.; Cui, Y.; Li, J., Transition of Cationic Dipeptide

Nanotubes into Vesicles and Oligonucleotide Delivery. Angew. Chem. Int. Ed. 2007, 46, 2431-2434. 8.

Martinez, C. R.; Iverson, B. L., Rethinking the Term "Pi-Stacking". Chemical Science

2012, 3, 2191-2201. 9.

Cochran, A. G.; Skelton, N. J.; Starovasnik, M. A., Tryptophan Zippers: Stable,

Monomeric Β-Hairpins. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 5578-5583. 10.

Rajagopal, A.; Aravinda, S.; Raghothama, S.; Shamala, N.; Balaram, P., Aromatic

Interactions in Model Peptide Β-Hairpins: Ring Current Effects on Proton Chemical Shifts. Biopolymers 2012, 98, 185-194. 11.

Tatko, C. D.; Waters, M. L., Selective Aromatic Interactions in Β-Hairpin Peptides. J.

Am. Chem. Soc. 2002, 124, 9372-9373. 12.

Roy, R. S.; Gopi, H. N.; Raghothama, S.; Gilardi, R. D.; Karle, I. L.; Balaram, P.,

Peptide Hairpins with Strand Segments Containing Α- and Β-Amino Acid Residues: CrossStrand Aromatic Interactions of Facing Phe Residues. Biopolymers 2005, 80, 787-799. 13.

Makwana, K. M.; Mahalakshmi, R., Asymmetric Contribution of Aromatic

Interactions Stems from Spatial Positioning of the Interacting Aryl Pairs in Β-Hairpins. Chembiochem 2014, 15, 2357-2360. 14.

Pellach, M.; Atsmon-Raz, Y.; Simonovsky, E.; Gottlieb, H.; Jacoby, G.; Beck, R.;

Adler-Abramovich, L.; Miller, Y.; Gazit, E., Spontaneous Structural Transition in Phospholipid-Inspired Aromatic Phosphopeptide Nanostructures. ACS Nano 2015, 9, 40854095. 15.

Butterfield, S. M.; Patel, P. R.; Waters, M. L., Contribution of Aromatic Interactions

to Α-Helix Stability. J. Am. Chem. Soc. 2002, 124, 9751-9755. 16.

Marsili, S.; Chelli, R.; Schettino, V.; Procacci, P., Thermodynamics of Stacking

Interactions in Proteins. Phys. Chem. Chem. Phys. 2008, 10, 2673-2685. 17.

Gazit, E., Self Assembly of Short Aromatic Peptides into Amyloid Fibrils and

Related Nanostructures. Prion 2007, 1, 32-35.


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18.

Frederix, P. W. J. M.; Ulijn, R. V.; Hunt, N. T.; Tuttle, T., Virtual Screening for

Dipeptide Aggregation: Toward Predictive Tools for Peptide Self-Assembly. J. Phys. Chem. Lett. 2011, 2, 2380-2384. 19.

Frederix, P. W. J. M.; Scott, G. G.; Abul-Haija, Y. M.; Kalafatovic, D.; Pappas, C.

G.; Javid, N.; Hunt, N. T.; Ulijn, R. V.; Tuttle, T., Exploring the Sequence Space for (Tri)Peptide Self-Assembly to Design and Discover New Hydrogels. Nat. Chem. 2015, 7, 30-37. 20.

Görbitz, C. H., The Structure of Nanotubes Formed by Diphenylalanine, the Core

Recognition Motif of Alzheimer's Β-Amyloid Polypeptide. Chem. Commun. 2006, 2332-2334. 21.

Görbitz, C. H., Nanotube Formation by Hydrophobic Dipeptides. Chem. Eur. J. 2001,

7, 5153-5159. 22.

Mason, T. O.; Chirgadze, D. Y.; Levin, A.; Adler-Abramovich, L.; Gazit, E.;

Knowles, T. P. J.; Buell, A. K., Expanding the Solvent Chemical Space for Self-Assembly of Dipeptide Nanostructures. ACS Nano 2014, 8, 1243-1253. 23.

Guo, C.; Luo, Y.; Zhou, R.; Wei, G., Probing the Self-Assembly Mechanism of

Diphenylalanine-Based Peptide Nanovesicles and Nanotubes. ACS Nano 2012, 6, 3907-3918. 24.

Azuri, I.; Adler-Abramovich, L.; Gazit, E.; Hod, O.; Kronik, L., Why Are

Diphenylalanine-Based Peptide Nanostructures So Rigid? Insights from First Principles Calculations. J. Am. Chem. Soc. 2013, 136, 963-969. 25.

Adler-Abramovich, L.; Kol, N.; Yanai, I.; Barlam, D.; Shneck, R. Z.; Gazit, E.;

Rousso, I., Self-Assembled Organic Nanostructures with Metallic-Like Stiffness. Angew. Chem. Int. Ed. 2010, 49, 9939-9942. 26.

Adler-Abramovich, L.; Reches, M.; Sedman, V. L.; Allen, S.; Tendler, S. J. B.; Gazit,

E., Thermal and Chemical Stability of Diphenylalanine Peptide Nanotubes: Implications for Nanotechnological Applications. Langmuir 2006, 22, 1313-1320. 27.

Amdursky, N.; Molotskii, M.; Aronov, D.; Adler-Abramovich, L.; Gazit, E.;

Rosenman, G., Blue Luminescence Based on Quantum Confinement at Peptide Nanotubes. Nano Lett. 2009, 9, 3111-3115. 28.

Kim, J. H.; Lee, M.; Lee, J. S.; Park, C. B., Self-Assembled Light-Harvesting Peptide

Nanotubes for Mimicking Natural Photosynthesis. Angew. Chem. Int. Ed. 2012, 51, 517-520. 29.

Semin, S.; van Etteger, A.; Cattaneo, L.; Amdursky, N.; Kulyuk, L.; Lavrov, S.;

Sigov, A.; Mishina, E.; Rosenman, G.; Rasing, T., Strong Thermo-Induced Single and TwoPhoton Green Luminescence in Self-Organized Peptide Microtubes. Small 2015, 11, 11561160. 30.

Kholkin, A.; Amdursky, N.; Bdikin, I.; Gazit, E.; Rosenman, G., Strong

Piezoelectricity in Bioinspired Peptide Nanotubes. ACS Nano 2010, 4, 610-614.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

31.

Gan, Z.; Wu, X.; Zhu, X.; Shen, J., Light-Induced Ferroelectricity in Bioinspired

Self-Assembled Diphenylalanine Nanotubes/Microtubes. Angew. Chem. Int. Ed. 2013, 52, 2055-2059. 32.

Amdursky, N.; Molotskii, M.; Gazit, E.; Rosenman, G., Elementary Building Blocks

of Self-Assembled Peptide Nanotubes. J. Am. Chem. Soc. 2010, 132, 15632-15636. 33.

Lee, J. S.; Yoon, I.; Kim, J.; Ihee, H.; Kim, B.; Park, C. B., Self-Assembly of

Semiconducting Photoluminescent Peptide Nanowires in the Vapor Phase. Angew. Chem. Int. Ed. 2011, 50, 1164-1167. 34.

Ryu, J.; Park, C. B., Synthesis of Diphenylalanine/Polyaniline Core/Shell Conducting

Nanowires by Peptide Self-Assembly. Angew. Chem. 2009, 121, 4914-4917. 35.

Adler-Abramovich, L.; Badihi-Mossberg, M.; Gazit, E.; Rishpon, J., Characterization

of Peptide-Nanostructure-Modified Electrodes and Their Application for Ultrasensitive Environmental Monitoring. Small 2010, 6, 825-831. 36.

Adler-Abramovich, L.; Gazit, E., The Physical Properties of Supramolecular Peptide

Assemblies: From Building Block Association to Technological Applications. Chem. Soc. Rev. 2014, 43, 6881-6893. 37.

Raeburn, J.; Pont, G.; Chen, L.; Cesbron, Y.; Levy, R.; Adams, D. J., Fmoc-

Diphenylalanine Hydrogels: Understanding the Variability in Reported Mechanical Properties. Soft Matter 2012, 8, 1168-1174. 38.

Smith, A. M.; Williams, R. J.; Tang, C.; Coppo, P.; Collins, R. F.; Turner, M. L.;

Saiani, A.; Ulijn, R. V., Fmoc-Diphenylalanine Self Assembles to a Hydrogel Via a Novel Architecture Based on Π–Π Interlocked Β-Sheets. Adv. Mater. 2008, 20, 37-41. 39.

Mahler, A.; Reches, M.; Rechter, M.; Cohen, S.; Gazit, E., Rigid, Self-Assembled

Hydrogel Composed of a Modified Aromatic Dipeptide. Adv. Mater. 2006, 18, 1365-1370. 40.

Orbach, R.; Mironi-Harpaz, I.; Adler-Abramovich, L.; Mossou, E.; Mitchell, E. P.;

Forsyth, V. T.; Gazit, E.; Seliktar, D., The Rheological and Structural Properties of FmocPeptide-Based Hydrogels: The Effect of Aromatic Molecular Architecture on Self-Assembly and Physical Characteristics. Langmuir 2012, 28, 2015-2022. 41.

Jayawarna, V.; Ali, M.; Jowitt, T. A.; Miller, A. F.; Saiani, A.; Gough, J. E.; Ulijn, R.

V., Nanostructured Hydrogels for Three-Dimensional Cell Culture through Self-Assembly of Fluorenylmethoxycarbonyl–Dipeptides. Adv. Mater. 2006, 18, 611-614. 42.

Yuran, S.; Razvag, Y.; Reches, M., Coassembly of Aromatic Dipeptides into

Biomolecular Necklaces. ACS Nano 2012, 6, 9559-9566. 43.

Akdim, B.; Pachter, R.; Naik, R. R., Self-Assembled Peptide Nanotubes as Electronic

Materials: An Evaluation from First-Principles Calculations. Appl. Phys. Lett. 2015, 106, 183707.


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44.

Reches, M.; Gazit, E., Formation of Closed-Cage Nanostructures by Self-Assembly

of Aromatic Dipeptides. Nano Lett. 2004, 4, 581-585. 45.

Reches, M.; Gazit, E., Designed Aromatic Homo-Dipeptides: Formation of Ordered

Nanostructures and Potential Nanotechnological Applications. Phys. Biol. 2006, 3, S10-S19. 46.

Fichman, G.; Adler-Abramovich, L.; Manohar, S.; Mironi-Harpaz, I.; Guterman, T.;

Seliktar, D.; Messersmith, P. B.; Gazit, E., Seamless Metallic Coating and Surface Adhesion of Self-Assembled Bioinspired Nanostructures Based on Di-(3,4-Dihydroxy-L-Phenylalanine) Peptide Motif. ACS Nano 2014, 8, 7220-7228. 47.

Shahripour, A.; Kaltenbronn, J. S.; Lunney, E. A.; Steinbaugh, B. A.; Hamby, J. M.;

Hamilton, H. W.; Sawyer, T. K.; Humblet, C.; Doherty, A. M., Design and Synthesis of Renin Inhibitors: Incorporation of Transition-State Isostere Side Chains That Span from the S1 to the S3 Binding Pockets and Examination of P3-Modified Renin Inhibitors. J. Med. Chem. 1995, 38, 2893-2905. 48.

Amblard, M.; Rodriguez, M.; Lignon, M. F.; Galas, M. C.; Bernad, N.; Artis-Noel, A.

M.; Hauad, L.; Laur, J.; Califano, J. C., Synthesis and Biological Evaluation of Cholecystokinin Analogs in Which the Asp-Phe-Nh2 Moiety Has Been Replaced by a 3Amino-7-Phenylheptanoic Acid or a 3-Amino-6-(Phenyloxy)Hexanoic Acid. J. Med. Chem. 1993, 36, 3021-3028. 49.

Ziserman, L.; Lee, H.-Y.; Raghavan, S. R.; Mor, A.; Danino, D., Unraveling the

Mechanism of Nanotube Formation by Chiral Self-Assembly of Amphiphiles. J. Am. Chem. Soc. 2011, 133, 2511-2517. 50.

Adamcik, J.; Castelletto, V.; Bolisetty, S.; Hamley, I. W.; Mezzenga, R., Direct

Observation of Time-Resolved Polymorphic States in the Self-Assembly of End-Capped Heptapeptides. Angew. Chem. Int. Ed. 2011, 50, 5495-5498. 51.

Pashuck, E. T.; Stupp, S. I., Direct Observation of Morphological Transformation

from Twisted Ribbons into Helical Ribbons. J. Am. Chem. Soc. 2010, 132, 8819-8821. 52.

Kornmueller, K.; Letofsky-Papst, I.; Gradauer, K.; Mikl, C.; Cacho-Nerin, F.;

Leypold, M.; Keller, W.; Leitinger, G.; Amenitsch, H.; Prassl, R., Tracking Morphologies at the Nanoscale: Self-Assembly of an Amphiphilic Designer Peptide into a Double Helix Superstructure. Nano Research 2015, 8, 1822-1833. 53.

Görbitz, C. H., Structures of Dipeptides: The Head-to-Tail Story. Acta Crystallogr.

Sect. B 2010, 66, 84-93. 54.

Zhu, P.; Yan, X.; Su, Y.; Yang, Y.; Li, J., Solvent-Induced Structural Transition of

Self-Assembled Dipeptide: From Organogels to Microcrystals. Chem. Eur. J. 2010, 16, 31763183.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

55.

Fleming, S.; Frederix, P. W. J. M.; Ramos Sasselli, I.; Hunt, N. T.; Ulijn, R. V.;

Tuttle, T., Assessing the Utility of Infrared Spectroscopy as a Structural Diagnostic Tool for Β-Sheets in Self-Assembling Aromatic Peptide Amphiphiles. Langmuir 2013, 29, 9510-9515. 56.

Khurana, R.; Fink, A. L., Do Parallel Β-Helix Proteins Have a Unique Fourier

Transform Infrared Spectrum? Biophys. J. 78, 994-1000.


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic:


Molecular Engineering of Self-Assembling ... - ACS Publications

Recommend Documents