3D Molecular Architectures from Self-Assembly of Peptide Foldamers

Article pubs.acs.org/accounts

Foldectures: 3D Molecular Architectures from Self-Assembly of Peptide Foldamers Sung Hyun Yoo and Hee-Seung Lee* Department of Chemistry, KAIST, Daejeon 34141, Korea CONSPECTUS: The wide range of fascinating supramolecular architectures found in nature, from DNA double helices to giant protein shells, inspires researchers to mimic the diverse shapes and functions of natural systems. Thus, a variety of artificial molecular platforms have been developed by assembling DNA-, peptide-, and protein-based building blocks for medicinal and biological applications. There has also been a significant interest in the research of non-natural oligomers (i.e., foldamers) that fold into well-defined secondary structures analogous to those found in proteins, because the assemblies of foldamers are expected not only to form biomimetic supramolecular architectures that resemble those of nature but also to display unique functions and unprecedented topologies at the same time due to their different folding propensities from those of natural building blocks. Foldamerbased supramolecular architectures have been reported in the form of nanofibers, nanochannels, nanosheets, and finite three-dimensional (3D) shapes. We have developed a new class of crystalline peptidic materials termed “foldectures” (a compound of foldamer and architecture) with unprecedented topological complexity derived from the rapid and nonequilibrium aqueous phase self-assembly of foldamers. In this Account, we discuss the morphological features, molecular packing structures, physical properties, and potential applications of foldectures. Foldectures exhibit well-defined, microscale, homogeneous, and finite structures with unique morphologies such as windmill, tooth, and trigonal bipyramid shapes. The symmetry elements of the morphologies vary with the foldamer building blocks and are retained upon surfactant-assisted shape evolution. Structural characterization by powder X-ray diffraction (PXRD) revealed the molecular packing structures, suggesting how the foldamer building blocks assembled in the 3D structure. The analysis by PXRD showed that intermolecular hydrogen bonding connects foldamers in head-to-tail fashion, while hydrophobic attraction plays a role in arranging foldamers in parallel, antiparallel, or cholesteric phase-like manners. Each packing structure from the foldamer building blocks possesses distinct symmetry elements that are directly expressed in the 3D morphologies. Because of their well-ordered molecular packing structures, foldectures exhibit facet-dependent surface characteristics and anisotropic magnetic susceptibility. The facet-dependent surface property was harnessed to synthesize anisotropic metal nanoparticle−foldecture composites, and the anisotropic magnetic susceptibility enables foldectures to undergo real-time alignment and rotating motion in response to an external magnetic field. By means of their unusual shapes and properties, foldectures have been demonstrated to mimic the functionality of natural systems such as magnetosomes or carboxysomes. Further development of foldectures using higher-order building units, complicated packing motifs, and functional moieties could provide a novel biocompatible platform rivaling 3D biological architectures in natural systems.

1. INTRODUCTION The design and synthesis of artificial materials in a f lask, mimicking sophisticated and complex natural systems, have been fundamental challenges in synthetic chemistry and biology. Foldamers1−5 are promising artificial molecular frameworks that mimic the structural behavior of biological components. They can fold into well-defined secondary structures that resemble biopolymers in a process guided by intramolecular hydrogen bonds or electrostatic interactions. A variety of foldamer families including β-peptides,6 γ-peptides,7 δ-peptides,8 peptoids,9 azapeptides,10 oligoureas,11 aromatic oligoamides,12 hybrids of these, and many others have been synthesized, and each backbone displays distinct secondary structures. Their welldefined conformation enables us to precisely modify the spatial arrangements of functional groups on foldamers in a predictable way. Notably, these non-natural synthetic oligomers follow different folding rules and thus form different secondary © 2017 American Chemical Society

structures than natural building blocks. This implies that foldamers can acquire properties not found in natural compounds. Recently, with this knowledge, foldamers have been applied in biological and medicinal fields as substitutes for biological components.13 The next important challenge in this field is the design of more complicated and higher-order architectures (i.e., tertiary or quaternary structures) that mimic large protein complexes such as enzymes and structural proteins. In light of the marvelous functional complexity and morphologies of tertiary and quaternary protein structures such as intracellular microcompartments,14 foldamer-based higher-order architectures should also exhibit considerable functional and structural diversity. Additionally, the structural differences between foldamers and biopolymers suggest that Received: October 31, 2016 Published: February 13, 2017 832

DOI: 10.1021/acs.accounts.6b00545 Acc. Chem. Res. 2017, 50, 832−841

Article

Accounts of Chemical Research

Figure 1. Foldectures from self-assembly of peptidic foldamers mimic 3D biological architecture. Images of protein structures are reproduced with permission from ref 14. Copyright 2008 AAAS.

architecture construction (Figure 2a). Thus, we opted to use a heptamer of trans-(S,S)-2-aminocyclopentanecarboxylic acid (ACPC) as a building block for initial exploration of the selfassembly of a helical peptidic foldamer (ACPC7, Figure 2b).21 This simple homo-oligomer adopts a stable helical conformation through intermolecular 12-membered hydrogen bonding between CO (i) and N−H (i + 3) (a so-called 12-helix) in both solid state and solution. This building block molecule is highly hydrophobic because the cyclopentyl rings of the peptide residues are displayed on the lateral exterior of the helix and the N- and C-termini are protected by a tert-butyloxycarbonyl (Boc) group and a benzyl group, respectively. Based on the packing motifs of analogous compounds,27 both lateral hydrophobic attraction between helical faces and head-to-tail intermolecular hydrogen bonds are likely to organize into self-assembled structures. Self-assembly of ACPC7 was accomplished in aqueous environments under solvophobic conditions. When ACPC7 was exposed to distilled water, it formed homogeneous windmill-shaped supramolecular architectures, observed using scanning electron microscopy (SEM) (Figure 2c). The SEM analysis revealed a microsized self-assembled structure with four sails, which had D4h symmetry. (Since this foldecture is composed of homochiral molecules, it cannot have inversion or mirror symmetry. Note that the point groups given in this Account only describe the macroscopic morphologies of the foldectures.) Dramatic morphological evolution was observed (Figure 2d−f) when self-assembly experiments were performed in the presence of the nonionic surfactant P123 (Pluronic P123, (ethylene glycol)20-(propylene glycol)70-(ethylene glycol)20), a widely used shape-guiding reagent for colloidal nanocrystals.28 As the P123 concentration in the self-assembly media increased, the resulting shapes increased in vertical length while their four sails decreased in length (or width), thus forming rectangular rod-shaped structures. The uniqueness and high homogeneity of these shapes, in comparison with other peptide-based materials, imply that the limited number of molecular packing motifs, resulting from the conformational rigidity of the building blocks, compels them to follow a restricted set of pathways to construct

foldamer-based higher-order architectures should carry unique properties unprecedented in nature. For example, foldamerbased enzyme-mimetic architectures can serve as substratespecific, regioselective, biocompatible catalysts whose catalytic activity is tunable at atomic resolution and exceeds that of enzymes. However, the difficulties in synthesizing giant architectures via covalent bonds between multiple secondary structures have hampered direct imitation of such structures. A self-assembly process that exploits noncovalent attractions between building blocks may be a plausible strategy to address this goal. In recent decades, higher-order structures from selfassembly of foldamers have been reported15 in the form of helical bundles,16 nanofibers,17 nanochannels,18 nanosheets,19 liquid crystals,20 and 3D molecular architectures.21−26 The 3D molecular architectures from self-assembly of foldamers, termed “foldectures”, are among the most distinctive biomimetic materials because of their unusual 3D shapes. Their unique morphologies and well-defined interior and exterior structures imply that foldectures could serve as artificial systems rivaling 3D biological architectures such as viral capsids, intracellular microcompartments, and even globular proteins (Figure 1). In addition, foldecture research could provide a comprehensive understanding of the fundamental principles of formation and function of natural complexes. Here, we focus on the synthesis, characterization, and related functions of foldectures and discuss future perspectives on the use of foldectures for biomimicry.

2. FOLDECTURE DISCOVERY Intermolecular interactions including hydrogen bonds, hydrophobic attraction, and aromatic and electrostatic interactions play key roles in supramolecular self-assembly. If one could manipulate their strength and directionality in 3D space, it could be possible to design and synthesize atomically defined supramolecular structures with specific morphologies. We hypothesized that the foldamer self-assembly process could be understood via the predictability of the conformational features of the building blocks and that knowledge of the blueprints would help elucidate the process of controlled supramolecular833


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blocks was performed by exposing them to an aqueous environment in the absence or presence of P123 surfactant. Square plate (F11) and molar tooth (F12) shaped foldectures were obtained from self-assembly of 1 in distilled water and P123(aq), respectively (Figure 3a). F11, resembling Mesoamerican pyramids, is a C4v-symmetric square plate with a 4-fold dendritic skeleton on its face. F12, featuring a tooth shape consisting of a crown and roots, is also C4v-symmetric. The two different shapes of foldectures F11 and F12, built from identical foldamer building blocks, share morphologically concomitant features such as 4-fold symmetry and noncentrosymmetry. Self-assembly of 2 in the absence or presence of P123 resulted in two distinct shapes of foldectures, a rhombic plate (F21) and a rhombic rod (F22), respectively (Figure 3b). As in the case of F1n, there are common morphological features between F21 and F22. Both are Ci-symmetric structures and have two rhombic facets with identical internal angles. The large morphological difference between F1n from 1 and F2n from 2 indicates that variations in the molecular structures of foldamer building units are directly expressed in the shapes of foldectures and their selfassembly behavior. The foldectures F31 from 3 in the absence of P123 showed a tripod shape and C3v symmetry (Figure 3c). As the concentration of P123 in the self-assembly media was increased, shape evolution was observed, from the primitive shape F31 to caudate trigonal bipyramids (F32), trigonal bipyramids (F33), truncated trigonal bipyramids (F34), and truncated trigonal bipyramids with a concave basal facet (F35) (Figure 3c). F32−F35 share a common trigonal bipyramid skeleton. In F32, a tail protrudes from the apex of the trigonal bipyramid whereas the compressed shapes and presence of basal faces in F34 and F35 were presumed to result from the inhibition of growth at the same apex. From the self-assembly of 4 in distilled water or P123(aq), we obtained microrods (F41) or C2-symmetric parallelogram plates (F42), respectively (Figure 3d). Considering that the morphologies of F3n and F4n contain different symmetry elements, it appears that the helical conformation (in this case an 11-helix) is not the dominant parameter in 3D shape development. This implies that the morphologies of foldectures are determined not only by the helical conformation of the building blocks but also by other parameters including length and order of sequence. The self-assembled structure of 5 at a P123 concentration of 0.1 g L−1 was obtained as a form of Ci-symmetric elongated hexagonal plate, F51 (Figure 3e). As the P123 concentration increased, the midline length of the foldectures decreased. The SEM and atomic force microscopy (AFM) analysis of F52, formed from self-assembly of 5 at a P123 concentration of 1 g L−1, revealed its microscale length and nanoscale thickness. F5n all have Ci symmetry, in common with F2n, implying that the molecular packing motifs of both F5n and F2n share a similar strategy.

Figure 2. (a) Schematic representation of 3D shape formation of foldectures via controlled self-assembly, (b) chemical structure of ACPC7 (left) and molecular model of the 12-helical ACPC7 viewed perpendicular to the helix axis (right, intramolecular hydrogen bonds are shown as dotted lines, hydrogen atoms are omitted for clarity), and SEM images of the self-assembled structures of ACPC7 prepared from (c) distilled water and (d−f) various P123(aq) in increasing order of P123 concentration (scale bars 1 μm). Panels b−f are reproduced with permission from ref 21. Copyright 2010 Wiley-VCH.

the final morphology. This initial study on foldectures demonstrated the following unique features. Foldectures exhibit unique morphologies (which are unlike those of other peptidebased materials), unrivaled uniformity in shape and size, and surfactant-assisted morphological evolution with retention of symmetry elements.

3. DIVERSE MORPHOLOGIES OF FOLDECTURES We envisioned that the morphologies of foldectures could be attributed to the structural features of the foldamer building blocks. The conformational information on the foldamer building blocks is encoded and regulated by various residues and sequences of peptides. With this hypothesis, we designed several helical foldamer building units including 12-helical peptides (1, BocNH-ACPC6-OBn, and 2, BocNH-ACPC6OH), 11-helical peptides (3, BocNH-(Aib-ACPC)3-Aib-OBn, and 4, BocNH-(Aib-ACPC)3-OBn), and a graft peptide (5, BocNH-ACPC6-Leu2-OBn, which adopts a 12-helix through the ACPC region but forms an 11-membered hydrogen-bonded ring at the C-terminus) (Figure 3).22−26 Self-assembly of the building

4. MOLECULAR PACKING STRUCTURES OF FOLDECTURES The specific morphological features of foldectures arise from the symmetry elements present within the molecular packing motifs. However, unambiguously ascertaining the molecular packing structures of foldectures has proven difficult because of their microscale dimensions, which hamper the use of experimental characterization techniques such as single-crystal X-ray diffraction (SC-XRD). We have thus tried to identify the molecularlevel structural characteristics of foldectures using PXRD. An analysis of the PXRD data by pattern indexing, Le Bail fitting, and 834


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Figure 3. (a−e) Chemical structures of helical foldamers and SEM images of foldectures: (a) F1n of 1 prepared from distilled water (F11) and P123(aq) (F12); (b) F2n of 2 prepared from distilled water (F21) and P123(aq) (F22); (c) F3n of 3 prepared from distilled water (F31) and various P123(aq) (F32, F33, F34, and F35 in increasing order of P123 concentration) (inset, TEM images (scale bars 1 μm) of each foldecture), (d) F4n of 4 prepared from distilled water (F41) and P123(aq) (F42); (e) F5n of 5 prepared from 0.1 g L−1 of P123(aq) (F51) and 8 g L−1 of P123(aq) (F52) and AFM image of F52 (right). Panel a adapted with permission from ref 22. Copyright 2011 American Chemical Society. Panel b adapted with permission from ref 23. Copyright 2015 American Chemical Society. Panel c adapted with permission from ref 24. Copyright 2015 Wiley-VCH. Panel d adapted with permission from ref 25. Copyright 2015 Elsevier Masson SAS. Panel e adapted with permission from ref 26. Copyright 2016 The Royal Society of Chemistry.

parallel by hydrophobic attraction, thus creating a twodimensional (2D) sheet parallel to the ab plane. Interestingly, the sheets pack along the c-axis, thus forming a cholesteric phaselike layered structure along a 3-fold screw axis parallel to the caxis. We determined that the tail of F32 extends along the [001] direction, based on the refined March−Dollase preferred orientation parameter in the Bragg−Brentano geometry. Therefore, the triangular molecular packing motif is responsible for the trigonal symmetry of F3n. F4n, with C2 symmetry, has a monoclinic unit cell and the P21 space group (Figure 4d).25 Intermolecular hydrogen bonds, (CO)i,4 → (H−N)j,1 and (CO)i,5 →(H−N)j,3, conjoin the foldamers in head-to-tail fashion to form a one-dimensional wire. The layered structures formed from parallel arrangements of wires are stacked antiparallel along the b-axis. The 2-fold antiparallel packing is responsible for the C2 symmetry of F4n. F5n, which form elongated hexagonal plates with Ci symmetry, have an orthorhombic unit cell and the P21212 space group (Figure 4e).26 An intermolecular hydrogen bond ((CO)i,7 → (H−N)j,2) conjoins the foldamers in head-to-tail fashion, creating a zigzag-type network along the b-axis. Adjacent networks along the bc plane are arrayed parallel, creating a 2D sheet, by both hydrophobic attraction and intermolecular hydrogen bonding ((CO)i,6 → (H−N)k,1) between foldamers and the sheets stacked antiparallel along the a-axis. Interestingly, these antiparallel sheets are arrayed along a 2-fold screw axis parallel to the c-axis. Following further analysis,26 the hexagonal facets of F52 facing each other were assigned as the bc planes from which the 2D sheet forms. To summarize, we determined that the well-defined secondary structures of foldamer building blocks were retained during foldecture formation, and the geometric elements of the resulting molecular packing structures were directly expressed as the morphological features of diverse cases of foldectures. Together, intermolecular hydrogen bonds and hydrophobic attraction between foldamer building blocks are the primary interactions in foldecture construction. Intermolecular hydrogen bonds connect foldamer building units along their helical axis, forming head-totail structures, which extend to form superhelices (F1n), zigzag-

Rietveld refinement provided robust insights into the molecular packing strategies of foldamer building blocks to form foldectures, which in turn provided theoretical support for other characterization tools including electron diffraction analysis (Figure 4). F11 and F12 from 1, which are square plate- and tooth-shaped, respectively, have C4v-symmetry, with a tetragonal unit cell and the P41 space group (Figure 4a).22 The helical axes of the four 1 foldamer building blocks in the unit cell are nearly parallel to the c-axis, and all the N-termini and C-termini of the building blocks are aligned in the [001̅] and [001] directions, respectively. Two intermolecular hydrogen bonds, (CO)i,5 → (H−N)j,1 and (CO)i,6 → (H−N)j,1 are observed between consecutive building units in the c-direction. Interestingly, the intermolecularly hydrogen-bonded foldamers form a right-handed supercoiled helical structure. The superhelix is packed with a parallel × parallel motif, thus forming noncentrosymmetric superstructures. This molecular packing motif, which features 4-fold symmetry and is noncentrosymmetric along the c-axis, is evidently responsible for the morphologies of F1n. F22, the rhombic rod-shaped foldecture from foldamer 2 with Ci symmetry, has an orthorhombic crystal system and the P212121 space group (Figure 4b).23 The helical axes of the constituent foldamers are aligned along the c-axis of the unit cell. Two pairs of intermolecular hydrogen bonds ((CO)i,5 → (H−N)j,1 and (CO)i,6→ (H−N)j,2) connect the helices in a zigzag-type network along the c-axis. The zigzag networks are packed parallel to the a-axis and antiparallel to the b-axis. This antiparallel stacking of the intermolecularly hydrogen-bonded peptide networks confers identical surface chemistry on the ab planes. Through a detailed analysis,23 we have assigned the rhombic faces of F22 as ab planes, which strongly supports ascribing the Ci symmetric shape of F22 to the antiparallel molecular packing motif. F3n, with C3v symmetry, has a triclinic unit cell and the P32 space group (Figure 4c).24 A sequence of intermolecular hydrogen bonds ((CO)i,5 → (H−N)j,1), connecting the foldamer building blocks in head-to-tail manner, creates a onedimensional foldamer wire. Adjacent foldamer wires are stacked 835


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Figure 4. Molecular packing strategies of (a) F1n, (b) F2n, (c) F3n, (d) F4n, and (e) F5n based on PXRD analysis. Individual foldamers are shown in red, blue, green, and yellow. Colored arrows reflect the directions (N to C) of individual helices, and circular arrows represent the helical handedness of the foldamers or superhelices. The dashed line represents the (b) ab or (e) bc plane corresponding to the (b) rhombic facets or (e) hexagonal facets of F2n and F5n, respectively.

type networks (F2n and F5n), and wires (F3n and F4n). By hydrophobic attraction, meanwhile, one-dimensional directional superstructures stack parallel to form anisotropic sheets, which pack parallel (F1n), antiparallel (F2n, F4n, and F5n), or with a rotational mode (F3n). These packing strategies, using intermolecular hydrogen bonding for head-to-tail alignment and hydrophobic attraction between the helical faces, are similar to the crystal packing modes of analogous compounds.27 As of now, however, it remains uncertain which structural features within the self-assembling units account for the observed differences in macroscopic morphology of the resultant assemblies. Nevertheless, we believe these issues will be clarified by the further systematic and fundamental studies that are still ongoing in our group (and others). Thus, the governing parameters of self-assembly, including subtle nondirectional van der Waals forces as well as the other driving forces, will eventually be disentangled, and a quantitative understanding of

the energetics will provide answers to the fundamental questions in the near future.

5. ANISOTROPIC SURFACE CHARACTERISTICS OF FOLDECTURES Facet-dependent properties, which are often found in crystalline materials (e.g., inorganic nanoparticles),29 were also anticipated in foldectures because of their well-ordered anisotropic molecular arrangements. The morphological evolution of foldectures upon different self-assembly media is evidence for their having facet-dependent characteristics. Therefore, the preferential interaction of surrounding molecules, such as surfactants, with specific facets of the nascent foldecture is responsible for the evolution of anisotropic morphologies. To experimentally confirm the anisotropic surface characteristics of foldectures, we investigated facet-selective metal deposition on foldectures in which specific facets were preferentially masked by 836


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Figure 5. (a) Scheme of the facet-selective Au NP deposition on F22 and thiolation experiments. (b) STEM image and EDS mapping (inset, blue C; green Au) of the Au NP−F22 composite. (c) SERS spectra of thiolated Au NP−F22 composite and a Au NP−F22 composite. The characteristic Raman signals of benzenethiol are highlighted with arrows. (d) Schematic illustration of synthesis (left) and SEM image (right) of the anisotropic Au NP−F32 composite. Panels a−c adapted with permission from ref 23. Copyright 2015 American Chemical Society. Panel d adapted with permission from ref 24. Copyright 2015 Wiley-VCH.

surfactants (Figure 5a).23,24 In situ gold nanoparticle (Au NP) deposition on foldectures was performed in the presence of P123. Scanning transmission electron microscopy (STEM), electron dispersive spectroscopy (EDS), and SEM images showed that the Au NPs selectively adhered onto specific facets: two rhombic facets of F22 (Figure 5b) and four triangular facets of F32 (Figure 5d). The facet-selective attachment of the Au NPs was not observed in control experiments performed in the absence of P123. The anisotropic surface properties originated from the different chemical composition of each surface. For example, the carbamate group at the N-terminus and the carboxylic acid group at the C-terminus of foldamer 2 are on the (001) rhombic facet of F22 whereas the cyclopentyl rings at the helix barrel of foldamer 2 are on the rectangular facets of F22. These facetdependent chemical environments can induce differential interaction between the foldecture surface and surrounding molecules such as water or surfactants. Theoretical analysis using molecular dynamics simulations showed that the stabilization energy from water solvation differed among facets of F22.23 The (001) surface, where hydrogen bond donors and acceptors are exposed, was much more stable than other facets.

Furthermore, we were able to modify the resulting composite face of F22 with thiols by dispersing the composites in an aqueous solution of benzenethiol. The unique surface-enhanced Raman scattering (SERS) signals of benzenethiol confirmed the successful facet-specific thiolation (Figure 5c). This modification methodology on anisotropic metal NP−foldecture composites can be used to construct higher-order superstructures for optical and biochemical applications.

6. MAGNETIC RESPONSE OF FOLDECTURES One of the most fascinating properties of foldectures is their magnetic responsiveness.30 As expected, foldectures consisting of organic molecules show diamagnetism, the contribution of which to the material’s magnetic response is negligible compared with other forms of magnetism such as ferromagnetism or paramagnetism. Although the effect of diamagnetism at the singlemolecular level is imperceptible, the collective diamagnetic response from the sum of contributions from the individual constituents in a structurally well-ordered arrangement is expected to be measurable. In this regard, foldectures are suitable materials for amplifying diamagnetic responses at the molecular level to exhibit macroscopic behavior because of their high crystallinity and monodispersed morphologies. 837


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Figure 6. (a) Chemical structure (left) of foldamer 6 and SEM image (right) of rectangular plate foldecture F6 from 6. Scale bar, 5 μm. (b,c) SEM images of the macroscopic alignment of (b) F22 and (c) F6 under in-plane magnetic field (left) and out-of-plane magnetic field (right). Scale bars, 5 μm. (d) Optical microscopy images showing rotation of F22 suspended in water under a rotating magnetic field. Scale bars, 10 μm. Arrows and circles indicate the direction of the magnetic field. Adapted with permission from ref 30. Copyright 2015 Macmillan Publishers Limited.

F6, the minor axis of the rectangular plate, were estimated to be the axes of the largest diamagnetic susceptibility, with calculated values of −1837.7 × 10−6 cm3 mol−1 and −2640.1 × 10−6 cm3 mol−1, respectively. The theoretical prediction thus coincided with the direction of magnetic alignment observed in the experiments. The diamagnetic susceptibilities of organic molecules are generally attributed to delocalized electrons in the resonance structures. In our peptidic foldamer scaffolds, amide groups are expected to predominantly contribute to the diamagnetic response. The amide resonance planes align nearly parallel to the helical axes of the foldamer building blocks and the average orientation of the helical axes in F22 (or F6) is parallel to the caxis (or b-axis), which tends to be aligned along the direction of the applied magnetic field. This interpretation is consistent with the many helical peptides known to orient parallel to an applied field and can be used to anticipate anisotropic responses to an external magnetic field.32 More importantly, the amplified magnetic susceptibility from spatially well-ordered arrangements has enabled foldectures to respond to magnetic fields much weaker than that generated by a conventional MRI machine.

The macroscopic alignment and rotational motion of foldectures were observed under static and dynamic magnetic fields, respectively. The SEM images showed that the rhombic rods F22 aligned with their longitudinal axes, that is, crystallographic c-axes of F22, parallel to the direction of the external static magnetic field (Figure 6b). In the case of the rectangular plates F6 from self-assembly of 6 (BocNH-ACPC8-OBn, Figure 6a)30 the minor axis of the rectangular face, that is, crystallographic baxes of F6, aligned parallel to the direction of the magnetic field (Figure 6c). The remaining axes of F22 and F6 were not spatially fixed, displaying a random orientation. When a horizontal rotating magnetic field was applied to an aqueous suspension of F22, the foldectures rotated to maintain a parallel orientation with respect to the magnetic field (Figure 6d). According to the theoretical description by Fujiwara et al.,31 the axis of the largest magnetic susceptibility tends to align along the direction of the applied magnetic field to minimize anisotropic magnetic energy. Therefore, we calculated each diamagnetic susceptibility along the crystallographic a-, b-, and caxes of F22 and F6 using density functional theory to determine the order of values (Figure 7). The c-axis of F22, which corresponds to the longitudinal axis of the rod, and the b-axis of 838


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act similarly to a compass needle to orient the bacteria in the Earth’s magnetic field (Figure 8a).33 With this inspiration, we envisioned that the synchronized rotating motion of an ensemble of foldectures could be translated into the dynamic motion of a macroscopic object at the millimeter scale. To achieve this goal, we designed and synthesized a macroscopic host object containing a myriad of foldectures with uniform orientations that were tightly fixed by the hard surrounding medium (Figure 8b).30 A rhombus-shaped hydrogel enveloping the aligned foldectures was synthesized by photopolymerization of poly(ethylene glycol)diacrylate (PEGDA) containing F22 while an external static magnetic field was applied. The stiff, rhombusshaped hydrogel container instantaneously rotated to follow the direction of the rotating magnetic field (Figure 8c). In summary, the macroscopic mechanical manipulation of an object lacking a magnetic response was successfully guided by the diamagnetic response of enveloped foldectures; this implies that foldectures playing a similar role to that of magnetosomes may be designable. 7.2. Foldectures for Microscale Compartmentalization

Biological containers, including viral capsids and bacterial microcompartments, enclose diverse functional guests such as genomes and proteins (Figure 9a).14 The major role of these biological containers is defending their contents against external shocks. Because of their thermal and aqueous stability and unrivaled homogeneity in size and shape, hollow foldectures would be promising candidates for microscale compartmentalization. Among the foldectures described above, F34 possesses a hollow interior. This unexpected cavity probably emerged from competition between the thermodynamically favored structure and the kinetically favored structure during morphogenesis. The encapsulation capability of F34 was investigated by self-assembly studies performed in the presence of three classes of fluorescent

Figure 7. Molecular packing arrangement of (a) F22 and (b) F6. The illustrated orientations of foldectures along the magnetic field B represent the observed alignment direction. The calculated diamagnetic susceptibilities along the crystallographic axes of the foldectures are shown in the table (right). Adapted with permission from ref 30. Copyright 2015 Macmillan Publishers Limited.

7. FOLDECTURES AS BIOMIMICS 7.1. Foldectures as Magnetosome-Inspired Artificial Organelles

Magnetosomes, unique intracellular organelles from magnetotactic bacteria, contain superparamagnetic magnetite crystals and

Figure 8. (a) TEM image of a single cell of Magnetospirillum magnetotacticum. Scale bar, 1 μm. (b) Schematic representation of magnetosome-inspired rhombus-shaped hydrogel container enveloping aligned F22. (c) Optical microscopy images of the hydrogel container showing magnetotactic behavior under a dynamic magnetic field. Scale bars, 1 mm. Panel a is reproduced with permission from ref 33. Copyright 1998 AAAS. Panels b and c are adapted with permission from ref 30. Copyright 2015 Macmillan Publishers Limited. 839


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interior, in which inorganic, organic, and protein guests are protected by a robust peptidic wall, can be used to mimic biocompartments for various possible applications.

8. CONCLUSION AND FUTURE PERSPECTIVES In this Account, we demonstrated the diverse morphologies, molecular packing strategies, and anisotropic characteristics of 3D molecular architectures formed from self-assembly of helical peptide foldamers. Highly ordered anisotropic molecular packing motifs, which are encoded in the foldamer building blocks, are responsible for their unique shapes and functional anisotropies. Furthermore, their unique shapes and functions allow us to mimic the roles of natural systems. As structure and function are intimately linked, foldectures can mimic naturally occurring 3D architectures in terms of both morphology and function. However, development is still in its infancy and fundamental challenges remain. (1) The structural features and packing strategies of foldectures could be widened by using higher-order building units such as helix-turn-helix, using different types of driving forces (e.g., coordination bonds, electrostatic interaction, or aromatic stacking), or coassembly of heterogeneous foldamer mixtures. (2) If functionalized foldectures could be created from building blocks with functional moieties such as catalytic triads, this would allow a new approach to building foldectures with desired functions. (3) A comprehensive understanding of the factors that determine the packing strategies of foldamers (e.g., their noncovalent interactions or surface topologies) would enable us to construct rationally designed foldectures with high resolution. We hope that these studies will pave the way to the creation of biocompatible 3D molecular architectures with diverse functions and morphologies beyond those found in nature.

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Figure 9. (a) TEM image of carboxysomes from Halothiobacillus neapolitanus cells. (b−d) CLSM images of F34 encapsulating fluorescent materials: (b) Green fluorescent quantum dot, (c) GFP, and (d) rhodamine B. (e) SEM image (inset schematic illustration F34, the hydrolyzed facet is highlighted in green) and (f) CLSM image of rhodamine B released from F34 after treatment with 2 N HCl for 1 day. Panel a is adapted with permission from ref 14. Copyright 2008 AAAS. Panels b−f are adapted with permission from ref 24. Copyright 2015 Wiley-VCH.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hee-Seung Lee: 0000-0003-0004-1884 Author Contributions

guest materials: quantum dots as inorganic nanoparticles, green fluorescent protein (GFP) as a protein, and rhodamine B as an organic molecule. Confocal laser scanning microscopy (CLSM) revealed that the fluorescent guests were successfully encapsulated in F34 (Figure 9b−d). The morphologies of F34 formed in the presence or absence of the guests were identical, that is, the guests had a negligible effect on the self-assembly. Both the encapsulation tests and control experiments showed that the foldamer walls surrounding the compartments were impermeable, thus protecting the guests from external shocks. We envisioned that release of the guest molecules could be accomplished in acidic media, in which the Boc group of the foldamer building blocks is labile. When we dispersed F34 containing rhodamine B in 2 N HCl solution, the basal facets of F34 disintegrated over 1 day (Figure 9e, f). The difference in the decomposition rate between the facets demonstrates that facet-dependent characteristics arose from the anisotropic molecular arrangement. Although the exact formation mechanism and morphogenesis of the void space remain elusive, we expect that the hollow

All authors contributed to writing the manuscript. All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Sung Hyun Yoo is a Ph.D. candidate in the biomimetic organic laboratory at KAIST under the supervision of Prof. Hee-Seung Lee. Hee-Seung Lee received his B.S.(1990), M.S.(1992), and Ph.D.(1996) degrees at the Department of Chemistry, KAIST (Prof. Sung Ho Kang). After working at the R&D center of Samsung Fine Chemicals (1996− 1999), he joined the group of Prof. Samuel H. Gellman at the University of WisconsinMadison as a postdoctoral associate (1999−2003). He started his independent academic career in 2004 as an Assistant Professor at the Department of Chemistry at KAIST, and currently he is a Full Professor. His research interest is to develop biomimetic functional organic molecular systems for diverse applications in biological and material science. 840


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ACKNOWLEDGMENTS The authors appreciate many colleagues who contributed to this work over the years. This research was supported by Samsung Science and Technology Foundation under Project Number SSTF-BA1301-08 and the National Research Foundation (NRF) of Korea grant funded by the Ministry of Science, ICT, and Future Planning (2016R1A2A1A05005509).

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3D Molecular Architectures from Self-Assembly of Peptide Foldamers

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