Byungcheol Ahn

pH-dependent structures of ferritin and apoferritin in solution: disassembly and reassembly.

The pH-dependent structures of the ferritin shell (apoferritin, 24-mer) and the ferrihydrite core, under physiological conditions that permit enzymatic activity, were investigated by synchrotron small-angle X-ray scattering (SAXS). The solution structure of apoferritin was found to be nearly identical to the crystal structure. The shell thickness and hollow core volumes were estimated. The intact hollow spherical apoferritin was stable over a wide pH range, 3.40-10.0, and the ferrihydrite core was stable over the pH range 2.10-10.0. The apoferritin subunits underwent aggregation below pH 0.80, whereas the ferrihydrite cores aggregated below pH 2.10 as a result of the disassembly of the ferritin shell under the strongly acidic conditions. As the pH decreased from 3.40 to 0.80, apoferritin underwent stepwise disassembly by first forming a hollow sphere with two holes, then a headset-shaped structure, and, finally, rodlike oligomers. As the pH was increased from pH 1.96, the disassembled rodlike oligomers recovered only to the headset-shaped structure, and the disassembled headset-shaped intermediates recovered only to the hollow spherical structure with two hole defects. The apoferritin hole defects that formed during the disassembly process did not heal as the pH was increased to neutral or slightly basic conditions. The pH-induced apoferritin disassembly and reassembly processes were not fully reversible, although they were pseudoreversible over a limited pH range, between 10.0 and 2.66.

Order-order and order-disorder transitions in thin films of an amphiphilic liquid crystalline diblock copolymer.

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors of thin films of an interesting amphiphilic diblock copolymer, poly(ethylene oxide)-b-poly(11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate) (p(EO)-b-p(MAAZ)) and the resulting morphological structures by using synchrotron grazing incidence X-ray scattering (GIXS) and differential scanning calorimetry. The quantitative GIXS analysis showed that the diblock copolymer in the homogeneous, isotropic melt state undergoes phase-separation near 190 degrees C and then forms a body-centered cubic (BCC) structure of spherical p(EO) domains in the p(MAAZ) matrix, at which point the p(EO) domains and the p(MAAZ) matrix are both in amorphous, liquid states. The BCC structure of spherical p(EO) domains is converted to a hexagonal cylinder structure near 120 degrees C, which is induced by the transformation of the isotropic phase of the p(MAAZ) matrix to the smectic A phase, which is composed of a laterally ordered structure of p(MAAZ) blocks with fully extended side groups. The resulting hexagonal cylinder structure is very stable below 120 degrees C. This microscopic hexagonal cylinder structure is retained as the smectic A phase of the p(MAAZ) matrix undergoes further transitions to smectic C near 104 degrees C and to a smectic X phase near 76 degrees C, while the amorphous, liquid phase of the p(EO) cylinders undergoes crystallization near -15 degrees C. These complicated temperature-dependent disorder-order and order-order phase transitions in the films were found to take place reversibly during the heating run. A face-centered orthorhombic structure of p(EO) domains was also found during the heating run and is an intermediate structure in the hexagonal cylinder structure to BCC structure transformation. We use these structural analysis results to propose molecular structure models at various temperatures for thin films of the diblock polymer.

pH-Dependent Structures of an i-Motif DNA in Solution

We have investigated for the first time the structure of i-motif DNA in solution at various pH conditions by using synchrotron small-angle X-ray scattering technique. To facilitate direct structural comparison between solution structures of i-motif DNA at various pH values, we created atomic coordinates of i-motif DNA from a fully folded to unfolded atomic model. Under mild acidic conditions, the conformations for i-motif DNA appeared to be similar to that of the partially unfolded i-motif atomic model in overall shape, rather than the fully folded i-motif atomic model. Collectively, our observations indicate that i-motif DNA molecule is structurally dynamic over a wide pH range, adopting multiple conformations ranging from the folded i-motif structure to a random coil conformation. As the i-motif structure has been used as an important component in nanomachines, we can therefore believe that the structural evidence presented herein will promote the development of future DNA-based molecular-actuator devices.

High-Performance Triisopropylsilylethynyl Pentacene Transistors via Spin Coating with a Crystallization-Assisting Layer

The effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, because slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three-dimensional (3D) growth of the TIPS-PEN crystals, because the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm(2) V(-1) s(-1), which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs.

Biaxially extended quaterthiophene-thiophene and -selenophene conjugated polymers for optoelectronic device applications

New biaxially extended quaterthiophene (4T) conjugated polymers, including poly(5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′4′′,2′′′]quaterthiophene) (P4T) and their copolymers with thiophene(P4TT), bithiophene (P4T2T), selenophene(P4TSe) and biselenophene (P4T2Se) were synthesized by Stille coupling reactions under microwave heating. The effects of the ring number of thiophene and selenophene moieties on the physical properties and polymer structures were systematically investigated experimentally and theoretically. With the increased ring number of the unsubstituted thiophene and selenophene moieties, the band gaps and the main-chain torsional angles were reduced. However, the side-chain torsional angles were increased with increasing the ring number, and thus significantly affected the carrier transporting characteristics. Among these studied conjugated polymers, the field-effect transistor (FET) based on P4TSe showed the highest hole mobility of up to 4.28 × 10−2 cm2 V−1 s−1 and an on/off ratio of 1.12 × 104. The photovoltaic device prepared from P4TSe/PC71BM exhibited the highest power conversion efficiency (PCE) of 2.6%, which resulted from more balanced hole/electron mobility and a smaller band gap. The above results revealed that the conformation, charge-transporting and optoelectronic device characteristics of biaxially extended 4T-based conjugated copolymers could be manipulated by incorporating the heteroaromatic ring spacer.

Effect of C60 Fullerene on the Duplex Formation of i-Motif DNA with Complementary DNA in Solution

The structural effects of fullerene on i-motif DNA were investigated by characterizing the structures of fullerene-free and fullerene-bound i-motif DNA, in the presence of cDNA and in solutions of varying pH, using circular dichroism and synchrotron small-angle X-ray scattering. To facilitate a direct structural comparison between the i-motif and duplex structures in response to pH stimulus, we developed atomic scale structural models for the duplex and i-motif DNA structures, and for the C(60)/i-motif DNA hybrid associated with the cDNA strand, assuming that the DNA strands are present in an ideal right-handed helical conformation. We found that fullerene shifted the pH-induced conformational transition between the i-motif and the duplex structure, possibly due to the hydrophobic interactions between the terminal fullerenes and between the terminal fullerenes and an internal TAA loop in the DNA strand. The hybrid structure showed a dramatic reduction in cyclic hysteresis.

Comprehensive synchrotron grazing-incidence X-ray scattering analysis of nanostructures in porous polymethylsilsesquioxane dielectric thin films

A complete grazing-incidence X-ray scattering (GIXS) formula has been derived for nanopores buried in a polymer dielectric thin film supported by a substrate. Using the full power of the scattering formula, GIXS data from nanoporous polymethylsilsesquioxane dielectric thin films, a model nanoporous system, have successfully been analysed. The nanopores were found to be spherical and to have a certain degree of size distribution but were randomly dispersed in the film. In the film, GIXS was confirmed to arise predominantly via the first scattering process in which the incident X-ray beam scatters without reflection; the other scattering processes and their contributions were significantly dependent on the grazing angle. This study also confirmed that GIXS scattering can be analysed using only independent scattering terms, but this simple approach can only provide structural parameters. The cross terms were found to make a relatively small contribution to the intensity of the overall scattering but were required for the complete characterization of the measured two-dimensional scattering data, in particular the extracted out-of-plane scattering data, and their inclusion in the analysis enabled film properties such as film thickness, critical angle (i.e. electron density), refractive index and the absorption term to be determined.

Synchrotron X-ray scattering characterization of the molecular structures of star polystyrenes with varying numbers of arms.

We have synthesized well-defined multiarmed star polystyrenes, with 6, 9, 17, 33, and 57 arms, and studied their molecular shapes and structural characteristics in a good solvent (tetrahydrofuran at 25 degrees C) and in a theta (Theta) solvent (cyclohexane at 35 degrees C) by small-angle X-ray scattering (SAXS) using a synchrotron radiation source. Analysis of the SAXS data provided a detailed characterization of the molecular shapes, including the contributions of the blob morphology of the arms, the radius of gyration, the paired distance distribution, the radial electron density distribution, and the Zimm-Stockmayer and Roovers g-factor, for the multiarmed star polystyrenes. In particular, the molecular shapes of the star polystyrenes were found to change from a fuzzy ellipsoid, for the 6-armed polystyrene, to a fuzzy sphere, for the 57-armed polystyrene, with an increasing number of arms. The ellipsoidal character of the star polystyrenes with fewer arms may originate from the extended anisotropically branched architecture at the center of the molecule. The arms of the star polystyrenes were found to be more extended than those of the linear polystyrenes. Furthermore, the degree of chain extension in the arms increased with the number of arms.

Physical mixtures of small-molecule and polymeric organic semiconductors: comparing thermodynamic behavior and thin-film structure

Physical mixtures of organic semiconductors are increasingly used for the development of new materials in thin film, organic electronic applications and their electronic properties are strongly affected by their morphology. Here, we report on studies of blends of an electron-donating small molecule, BTD-DTP, with the electron-acceptor polymer PNDI-2T and the correlations between their thermal behaviour, intermixing and thin film structure. A significant depression of the PNDI-2T melting point (ΔT = 111 °C) is observed upon increasing the small molecule content. Grazing incidence X-ray scattering (GIXS) and scanning probe microscopy (SPM) of thin films of varying composition show an increase in the small molecule crystalline phase and reduction in the crystallite orientation distribution, as the small molecule to polymer ratio reaches ∼50 : 50 wt. The domain sizes of the small molecule and polymer crystalline phases reach a minimum at the 50 : 50 wt ratio as well, suggesting the formation of the phases leads to mutual limitation of their crystalline domain size. Comparison of the bulk and thin film properties shows a divergence in behaviour of the small molecule, which in the bulk exhibits only a monotonic decrease in melting point with addition of polymer, but which has an increase in crystallinity, from 20 to 50 wt% PNDI-2T content.

2.16 – Reflectivity, GI-SAS and GI-Diffraction: X-Ray

One of the features of X-rays is the penetration power into matter, revealing the invisible interior of complex objects. Moreover, X-rays are a critical tool to investigate material structures and properties because of the electromagnetic radiation that is able to interact with matter having an electromagnetic nature. In particular, X-ray reflectivity (XR) and grazing incidence X-ray scattering (GIXS) are powerful, nondestructive techniques for examining the structure and properties of materials including various kinds of polymers and their nanostructures and nanosize specimens. These techniques have become more powerful for understanding nanostructures and objects in nanosize by using synchrotron radiation sources. Their fundamental theories and applications in polymer science are given and discussed.