Jehan Kim

Current Status of the Synchrotron Small-Angle X-ray Scattering Station BL4C1 at the Pohang Accelerator Laboratory

The small-angle X-ray scattering (SAXS) beamline BL4C1 at the 2.5 GeV storage ring of the Pohang Accelerator Laboratory (PAL) has been in its first year of operation since August 2000. During this first stage it could meet the basic requirements of the rapidly growing domestic SAXS user community, which has been carrying out measurements mainly on various polymer systems. The X-ray source is a bending magnet which produces white radiation with a critical energy of 5.5 keV. A synthetic double multilayer monochromator selects quasi-monochromatic radiation with a bandwidth of ca. 1.5%. This relatively low degree of monochromatization is sufficient for most SAXS measurements and allows a considerably higher flux at the sample as compared to monochromators using single crystals. Higher harmonics from the monochromator are rejected by reflection from a flat mirror, and a slit system is installed for collimation. A charge-coupled device (CCD) system, two one-dimensional photodiode arrays (PDA) and imaging plates (IP) are available as detectors. The overall performance of the beamline optics and of the detector systems has been checked using various standard samples. While the CCD and PDA detectors are well-suited for diffraction measurements, they give unsatisfactory data from weakly scattering samples, due to their high intrinsic noise. By using the IP system smooth scattering curves could be obtained in a wide dynamic range. In the second stage, starting from August 2001, the beamline will be upgraded with additional slits, focusing optics and gas-filled proportional detectors.

Synchrotron Small-Angle X-ray Scattering Studies of the Structure of Porcine Pepsin under Various pH Conditions

Structural characteristics of various conformational states of porcine pepsin in solution under different pH conditions were investigated in terms of size and shape by small-angle X-ray scattering (SAXS). Low-resolution structural models of porcine pepsin were reconstructed from SAXS data, which were made inside the search volume of maximum dimension (Dmax), calculated from the pair distance distribution function p(r). The reconstructed structural models were obtained without imposing any restrictions on the symmetry or anisometry of the pepsin molecule. Under conditions emulating those for physiological activity of the enzyme, the reconstructed structural models exhibited a more extended C-terminal domain compared to the crystal structure. The differences between the solution and crystal structures of pepsin can be explained by inherent conformations of the flexible subdomain in the C-terminal domain under the solution pH conditions. Under mild acidic conditions where the enzyme is inactive, the reconstructed structural models revealed a compact globular conformation similar in overall shape to the crystal structure. These results indicate that the changes in fluorescence and circular dichroism curves observed under acidic conditions could also arise from the inherent conformation of the flexible subdomain, which has a tendency to roll into a sphere in the overall structure, but without affecting the stability of internal structure. Furthermore, the conformational changes in the subdomain might explain the inactivity of pepsin under mildly acidic conditions. Finally, compared to neutral denaturing conditions, pepsin under alkaline denaturing conditions had a larger expanded vertical conformation in the reconstructed model, as a consequence of alkaline denaturation of the N-terminal domain and a fully extended conformation of the C-terminal domain. The structural evidence presented here may have important implications for understanding the relationship between the structure of porcine pepsin and enzymatic function.

Detailed analysis of gyroid structures in diblock copolymer thin films with synchrotron grazing-incidence X-ray scattering

In this study, a grazing-incidence X-ray scattering (GIXS) formula was derived for gyroid structures formed in thin films supported on substrates. Two-dimensional GIXS patterns were measured for gyroid structures formed in polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer nanometre-scale thin films supported on silicon substrates, and a quantitative analysis of the obtained two-dimensional GIXS data was conducted with the scattering formula. This analysis provided details (lattice parameter, width of the PS phase, positional distortion factor, orientation and orientation distribution) of the gyroid structures developed in the diblock copolymer thin films that are not easily obtained using conventional techniques. Moreover, it was possible to simulate complete and detailed two-dimensional GIXS patterns with the determined structure parameters.

Al+Y codeposition using EB-PVD method for improvement of high-temperature oxidation resistance of TiAl

Abstract An “Al+Y codeposition” on TiAl by the single EB-PVD method has been developed to improve the oxidation resistance of TiAl. To determine the optimum codeposition condition, the Al+Y codepositions with various ratios of Al and Y are evaluated through the isothermal and cyclic oxidation tests. The oxidation resistance of TiAl can be improved extensively by the Al+Y codeposition due to the formation of a gradient coating of Al and Y. The Al+Y codeposition with the ratio of Al:2Y for evaporation source material is proved to be the best. With a proper ratio of Al:Y, the Al+Y codeposition forms two distinctive layers of the oxides during high-temperature oxidation; Al 2 O 3 in the inner layer and (Y, Al)O type oxide in the outer layer. In addition to the inner Al 2 O 3 layer, the formation of the outer (Y, Al)O type oxide layer further improves the stability of the coatings. The stability of the Al+Y codeposition greatly depends upon the alloying element of TiAl substrate or oxidation resistance of the TiAl substrate alloy. The non-alloyed TiAl shows a poor coating stability, whereas TiAl–2.8Nb and Alloy K5 show a good coating stability under severe thermal stresses during cyclic oxidation since a stable Al 2 O 3 can form on the surface of these alloys.

Polystyrene‐b‐polyisoprene thin films with hexagonally perforated layer structure: quantitative grazing‐incidence X‐ray scattering analysis

Grazing-incidence X-ray scattering (GIXS) formulas for hexagonally perforated layer (HPL) structures with ABC and AB stacking sequences were derived, and used in the quantitative analysis of the two-dimensional GIXS patterns of polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer thin films supported on silicon substrates. This quantitative analysis provided detailed information (shape, size and size distribution, packing order, layer packing sequence, and orientation) about the HPL structure of the diblock copolymer films that cannot be easily obtained with conventional X-ray and neutron scattering techniques or with conventional microscopic methods.

Two-Dimensionally Well-Ordered Multilayer Structures in Thin Films of a Brush Polypeptide

In this study, we report the first production of two-dimensionally well-ordered molecular multilayers (i.e., with a well-defined molecular lamellar structure) based on the antiparallel beta-sheet chain conformation in thin films of a brush polypeptide, poly(S-n-hexadecyl-dl-homocysteine) (PHHC), through the use of a simple spin-coating process and the quantitative structural and property analysis of the thin films using a grazing incidence X-ray scattering technique combined with Fourier transform infrared spectroscopy and differential scanning calorimetry. These analyses provide detailed information about the structure and molecular conformation of the self-assembled lamellae in the PHHC thin film, which is not easily obtained using conventional techniques. Moreover, we used the in situ measurements carried out at various temperatures and the data analyses to establish mechanisms for the evolution of the self-assembled lamellar structures in the film and for their melting. In addition, we propose molecular structure models of the PHHC polymer molecules in the thin film at various temperatures.

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.

X-ray scattering study of thermal nanopore templating in hybrid films of organosilicate precursor and reactive four-armed porogen

The miscibility and the mechanism for thermal nanopore templating in films prepared from spin-coating and subsequent drying of homogenous solutions of curable polymethylsilsesquioxane dielectric precursor and thermally labile, reactive triethoxysilyl-terminated four-armed poly(epsilon-caprolactone) porogen were investigated in detail by in situ two-dimensional grazing incidence small-angle x-ray scattering analysis. The dielectric precursor and porogen components in the film were fully miscible. On heating, limited aggregations of the porogen, however, took place in only a small temperature range of 100-140 degrees C as a result of phase separation induced by the competition of the curing and hybridization reactions of the dielectric precursor and porogen; higher porogen loading resulted in relatively large porogen aggregates and a greater size distribution. The developed porogen aggregates underwent thermal firing above 300 degrees C without further growth and movement, and ultimately left their individual footprints in the film as spherical nanopores.

X-Ray Scattering Studies on Molecular Structures of Star and Dendritic Polymers

We studied the molecular shapes and structural characteristics of a 33-armed, star polystyrene (PS-33A) and two 3rd-generation, dendrimer-like, star-branched poly(methyl methacrylate)s with different architectures (PMMA-G3a and PMMA-3Gb) and 32 end-branches under good solvent and theta (Θ) solvent conditions by using synchrotron small angle X-ray scattering (SAXS). The SAXS analyses were used to determine the structural details of the star PS and dendrimer-like, star-branched PMMA polymers. PS-33A had a fuzzy-spherical shape, whereas PMMA-G3a and PMMA-G3b had fuzzy-ellipsoidal shapes of similar size, despite their different chemical architectures. The star PS polymer’s arms were more extended than those of linear polystyrene. Furthermore, the branches of the dendrimer-like, star-branched polymers were more extended than those of the star PS polymer, despite having almost the same number of branches as PS-33A. The differences between the internal chain structures of these materials was attributed to their different chemical architectures.

Detection of an intermediate during the unfolding process of the dimeric ketosteroid isomerase.

Failure to detect the intermediate in spite of its existence often leads to the conclusion that two‐state transition in the unfolding process of the protein can be justified. In contrast to the previous equilibrium unfolding experiment fitted to a two‐state model by circular dichroism and fluorescence spectroscopies, an equilibrium unfolding intermediate of a dimeric ketosteroid isomerase (KSI) could be detected by small angle X‐ray scattering (SAXS) and analytical ultracentrifugation. The sizes of KSI were determined to be 18.7 Å in 0 M urea, 17.3 Å in 5.2 M urea, and 25.1 Å in 7 M urea by SAXS. The size of KSI in 5.2 M urea was significantly decreased compared with those in 0 M and 7 M urea, suggesting the existence of a compact intermediate. Sedimentation velocity as obtained by ultracentrifugation confirmed that KSI in 5.2 M urea is distinctly different from native and fully‐unfolded forms. The sizes measured by pulse field gradient nuclear magnetic resonance (NMR) spectroscopy were consistent with those obtained by SAXS. Discrepancy of equilibrium unfolding studies between size measurement methods and optical spectroscopies might be due to the failure in detecting the intermediate by optical spectroscopic methods. Further characterization of the intermediate using 1H NMR spectroscopy and Kratky plot supported the existence of a partially‐folded form of KSI which is distinct from those of native and fully‐unfolded KSIs. Taken together, our results suggest that the formation of a compact intermediate should precede the association of monomers prior to the dimerization process during the folding of KSI.