Sunbum Kwon

Unprecedented Molecular Architectures by the Controlled Self‐Assembly of a β‐Peptide Foldamer

During past decades, several types of peptide-based scaffolds, ranging from simple aromatic dipeptide to small protein fragments, have been studied to understand the underlying mechanism and mimic to create artificial nano/microstructures. However, a limited number of design principles have still been reported in peptidic scaffolds allowing well-defined self-assembled structure formation, presumably due to the intrinsic large conformational flexibility of natural peptides. In this presentation, we report the first example of highly homogeneous, well-defined and finite architectures by the β-peptide self-assembly.

Self-assembled peptide architecture with a tooth shape: folding into shape.

Molecular self-assembly is the spontaneous association of molecules into structured aggregates by which nature builds complex functional systems. While numerous examples have focused on 2D self-assembly to understand the underlying mechanism and mimic this process to create artificial nano- and microstructures, limited progress has been made toward 3D self-assembly on the molecular level. Here we show that a helical β-peptide foldamer, an artificial protein fragment, with well-defined secondary structure self-assembles to form an unprecedented 3D molecular architecture with a molar tooth shape in a controlled manner in aqueous solution. Powder X-ray diffraction analysis, combined with global optimization and Rietveld refinement, allowed us to propose its molecular arrangement. We found that four individual left-handed helical monomers constitute a right-handed superhelix in a unit cell of the assembly, similar to that found in the supercoiled structure of collagen.

Microtubes with Rectangular Cross-Section by Self-Assembly of a Short β-Peptide Foldamer

In nature, complex and well-defined structures are constructed by the self-assembly of biomolecules. It has been shown that β-peptide foldamers can mimic natural peptides and self-assemble into three-dimensional molecular architectures thanks to their rigid and predictable helical conformation in solution. Using shorter foldamers, which can be prepared more easily than longer ones, to form such architectures is highly desirable, but shorter foldamers have been overlooked due to the seemingly inferior number of intramolecular hydrogen bonds to stabilize a folded state in solution. Here we report that a β-peptide tetramer, although it lacks full helical propensity in solution, does self-assemble to form well-defined microtubes with rectangular cross-section by evaporation-induced self-assembly.

Magnetotactic molecular architectures from self-assembly of β-peptide foldamers

The design of stimuli-responsive self-assembled molecular systems capable of undergoing mechanical work is one of the most important challenges in synthetic chemistry and materials science. Here we report that foldectures, that is, self-assembled molecular architectures of β-peptide foldamers, uniformly align with respect to an applied static magnetic field, and also show instantaneous orientational motion in a dynamic magnetic field. This response is explained by the amplified anisotropy of the diamagnetic susceptibilities as a result of the well-ordered molecular packing of the foldectures. In addition, the motions of foldectures at the microscale can be translated into magnetotactic behaviour at the macroscopic scale in a way reminiscent to that of magnetosomes in magnetotactic bacteria. This study will provide significant inspiration for designing the next generation of biocompatible peptide-based molecular machines with applications in biological systems.

A Hollow Foldecture with Truncated Trigonal Bipyramid Shape from the Self-Assembly of an 11-Helical Foldamer

The creation of self-assembling microscale architectures that possess new and useful physical properties remains a significant challenge. Herein we report that an 11-helical foldamer self-assembles in a controlled manner to form a series of 3D foldectures with unusual three-fold symmetrical shapes that are distinct from those generated from 12-helical foldamers. The foldamer packing motif was revealed by powder X-ray diffraction technique, and provides an important link between the molecular-level symmetry and the microscale morphologies. The utility of foldectures with hollow interiors as robust and well-defined supramolecular hosts was demonstrated for inorganic, organic, and even protein guests. This work will pave the way for the design of functional foldectures with greater 3D shape diversity and for the development of biocompatible delivery vehicles and containment vessels.

Structure Modulation of Silica Microspheres in Bio-Inspired Silicification: Effects of TEOS Concentration

Silica nanoand microspheres have applications in various areas, such as in photonic crystals, catalysis, biosensors, bioassay, and drug delivery. Numerous synthetic methods have been developed and been modified to meet the demands of the applications mentioned above. In particular, the Stcber method is considered as a basic platform for the chemical synthesis of silica spheres: it generally uses ammonia as a catalyst and silicon alkoxide as a precursor in the water/ethanol co-solvent system. Although the conventional Stcber method is quite useful for sizeand shape-control of silica structures, the reaction conditions are harsh owing to the use of ammonia (pH>12) and cannot be applied to biological systems, such as living cells. In this respect, biomimetic (or bio-inspired) silicification, inspired by diatom and glass sponges, was suggested as an alternative approach for the mild and biocompatible formation of silica structures, because it proceeds under physiologically mild conditions (i.e., neutral pH, room temperature, and ambient pressure). The bio-inspired silicification has successfully been applied to the coating of individual living cells without deterioration of cell viability by us. We also have recently reported that individually separated silica microspheres were formed under relatively mild conditions in the presence of cetyltrimethylammonium bromide (CTAB) by using cysteamine (HSCH2CH2NH2) as a biomimetic hydrolysis catalyst, inspired by silicatein, a silica-forming protein, found in glass sponge. It is noteworthy that the diameter of the formed silica spheres was on the micrometer-scale, because microspheres had barely been found in both Stcber and modified Stcber methods. Understanding how the bioinspired silicification was affected by reaction parameters in the cysteamine/CTAB system, such as reactant concentrations and solvents, was required for the detailed elucidation of mechanisms and the morphological control of silica. However, there have been few reports on the effects of the concetrations of silica precursors on silica morphogenesis. Herein, we systematically investigated the effects of the concentration of tetraethyl orthosilicate (TEOS) in detail, along with the ratio of water and ethanol. The synthetic procedure was as follows (Figure 1). The final concentrations of cysteamine and CTAB were fixed to be 50 mm and 5 mm, respectively, after optimization for silica formation. We varied the water/ethanol ratio from 0.6:1 to 1:1 (v/v), and the concentration of TEOS from 80 to 140 mm in 20 mm-intervals. Although the observable changes in the reaction could be seen after 45 minutes, the silicification was performed for 3 hours for comparative studies. The resulting silica precipitates were washed with ethanol several times using centrifugation, dispersed in ethanol, and characterized by attenuated total reflectance infrared (ATR-IR) spectroscopy and field-emission scanning electron microscopy (FE-SEM). The IR spectra showed the characteristic peaks at 1049 (Si O Si asymmetric stretching), 955 (Si O stretching), and 784 cm 1 (Si O Si symmetric stretching) after silicification (for the representative IR spectrum, see the Supporting Information, Figure S1). To investigate the effects of TEOS concentration, we first varied only the concentration of TEOS in the 0.65:1 water/ ethanol system, while keeping the other parameters (the concentrations of cysteamine and CTAB) the same. Of interest, the silica morphology was found to be affected greatly by the concentration of TEOS, as shown in the FE-SEM micrographs (Figure 2a). Specifically, interconnected aggre[a] J. H. Park, J. Y. Choi, T. Park, S. H. Yang, S. Kwon, Prof. Dr. H.-S. Lee, Prof. Dr. I. S. Choi Molecular-Level Interface Research Center Department of Chemistry KAIST Daejeon 305-701 (Korea) Fax: (+82)42-350-2810 E-mail : ischoi@kaist.ac.kr hee-seung_lee@kaist.ac.kr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100265.

Heterogeneous photonic crystal light sources

A heterogeneous photonic crystal cavity structure based on the band edge mode is reviewed. The outer photonic bandgap region is introduced to contain the central band edge mode in order to minimize in-plane losses. Lasing modes are identified from the spectral position and polarization characteristics, and compared with theoretical calculations based on the three-dimensional finite-difference time-domain method.

Photonic crystal lasers for nano-photonics

Characteristics of low threshold, high-index contrast, 2-D InP slab triangular- and square-lattice photonic crystal band edge lasers operating near 1,550 nm are summarized. Surface-emitting lasing actions are observed near the third /spl Gamma/ points and the band edges. A very low threshold of 35 /spl mu/W is observed from the laser operating near the third TE-like mode /spl Gamma/ point. The lasing occurs from a very small pump area of 6 /spl mu/m in diameter.

Low threshold 2-D photonic crystal lasers

Several feasible forms of low threshold 2-D slab photonic crystal lasers such as nondegenerate unit-cell lasers and band edge lasers are discussed. Through the existence of very small, very high-Q resonant modes in the square lattice, we argue the effectiveness of the photonic bandgap in photon confinement. However, the issues still needing to be solved are those of electrical pumping and surface recombination. We believe the photonic crystal will help us to find the route to the ultimate thresholdless lasers that have extremely small mode volume and high-Q values. In addition, the demonstration of the cavity-less small volume band edge laser can lead us to a new form of very small practical lasers.