Structural insight into chain-length control and product specificity of pentaketide chromone synthase from Aloe arborescens

The crystal structures of a wild-type and a mutant PCS, a novel plant type III polyketide synthase from a medicinal plant, Aloe arborescens, were solved at 1.6 A resolution. The crystal structures revealed that the pentaketide-producing wild-type and the octaketide-producing M207G mutant shared almost the same overall folding, and that the large-to-small substitution dramatically increases the volume of the polyketide-elongation tunnel by opening a gate to two hidden pockets behind the active site of the enzyme. The chemically inert active site residue 207 thus controls the number of condensations of malonyl-CoA, solely depending on the steric bulk of the side chain. These findings not only provided insight into the polyketide formation reaction, but they also suggested strategies for the engineered biosynthesis of polyketides.     

Malycorins A--C, New lycopodium alkaloids from Lycopodium phlegmaria

A novel C(19)N-type Lycopodium alkaloid, malycorin A (1) consisting of a serratinane skeleton with 2-propanol unit has been isolated from the club moss Lycopodium phlegmaria, together with two new C(16)N-type alkaloids, malycorins B (2) and C (3), and the structures and relative stereochemistry were elucidated on the basis of spectroscopic data.     

Mechanical characteristics of hydrogenated natural rubber vulcanizates

Mechanical properties of partially hydrogenated natural rubber (HNR) vulcanizates were evaluated regarding their chemical structure and crystallizable nature of HNR, and are reported here, to the best of our knowledge, for the first time. HNRs of three levels of hydrogenation (20.6, 29.0, and 40.6 mol%) were successfully prepared by the chemical modification of natural rubber (NR) latex using N₂H₄ and H₂O₂ as reagents, in a sufficient amount for preparing sulfur‐crosslinked samples to be subjected to mechanical and structural measurements. The three HNR vulcanizates were found to be crystallizable upon stretching; it is noted that even 40.6 mol% hydrogenation did not prevent HNR vulcanizates from crystallization upon stretching, while their onset strain of crystallization was higher than that of NR vulcanizate. The hysteresis loss and residual strain up to a stretching ratio of 2 for the HNR vulcanizates tended to become larger with the increase in the degree of the hydrogenation. Tensile and dynamic mechanical properties of 20.6 mol% hydrogenated HNR vulcanizate were comparable to those of NR vulcanizate. From differential scanning calorimetry and temperature dispersion of dynamic modulus or loss, the glass transition temperatures of HNR vulcanizates were found to be almost the same as that of NR vulcanizate, which is also notable. The thermal stability of HNR vulcanizates was better than that of NR vulcanizate. Thus, this chemical modification seems to give a promising NR derivative whose properties can be equivalent or even better than the mother polymer. Copyright © 2008 John Wiley & Sons, Ltd.     

Reconstruction of the standard model in a generalized differential geometry based on the real structure

The standard model is reconstructed in a generalized differential geometry (GDG) based on the idea of a real structure as proposed by Coquereaux et al. and Connes. The GDG considered in this article is a kind of non-commutative geometry (NCG) on the discrete space that successfully reproduces the Higgs mechanism of the spontaneously broken gauge theory. Here, a GDG is a direct generalization of the differential geometry on an ordinary continuous manifold to the product space of this manifold with a discrete manifold. In a GDG, a one-form basis on the discrete space is incorporated in addition to the one-form basis on Minkowski space, rather than as in Connes's original work. Although the Lagrangians obtained in this way are the same as those obtained in our previous formulation of GDG, the basic formalism becomes very simply and clear. Received: 31 January 2000 / Revised version: 28 July 2000 / Published online: 25 April 2001     

Ultraviolet spectra of Mg in liquid helium

Emission and absorption spectra of Mg atoms implanted in liquid helium have been observed in the ultraviolet region. We have presented a model of exciplex formation of Mg-He₁₀ and found that this model is more suitable for understanding the dynamics in the 3s3p ¹P? 3s^2 1S3s3p^{\:1}P\rightarrow 3s^{2\:1}S transition than the bubble model.     

Long time evolution of collisionless shocks in laser produced counterstreaming plasmas

Long time evolution of collisionless shocks in laser-produced plasmas is discussed. By irradiating a double plane target a high Mach number collisionless shock has been observed in laser produced counterstreaming plasmas [Kuramitsu et al., Phys. Rev. Lett., 106, 175002 (2011)]. While in early time we observe the shock in front of one plane, which is irradiated with the laser, we observe another shock in front of the other plane in much later time than the first shock formation. These two shocks coexist and collide or merge with each other as time passes. This means that the upstream plasmas for the first and second shocks have to be provided from the second and first shock sides, respectively, i.e., both the first and second shock have to be collisionless. There are two major candidates to account for the long time evolution of the collisionless shocks. One is that the secondary plasmas at the planes can be continuously created by the plasmas from the other planes. Another is that the actual shock thickness is much thiner than the detection limit, as indicated by numerical simulations.     

208Hg β－ 衰变γ纲图

以¹⁸O重离子束轰击厚天然铅靶,通过多核子转移反应生成²⁰⁸Hg核.辐照铅片放入离线熔化铅靶气相热色谱装置中处理,分离并收集汞元素产物.完成了4πΔE_βγ-γ,γX(γ)关联事件谱及γ射线时序单谱测量.建议了21条γ射线,以及能量为1365,1727keV的两条²⁰⁸Tl新能级的²⁰⁸Hg β^－ 衰变部分γ纲图.     

Nano‐Structural Observation of in situ Silica in Natural Rubber Matrix by Three Dimensional Transmission Electron Microscopy

Summary: Three dimensional (3D) nanostructures of particulate silicas in natural rubber (NR) were observed for the first time by use of 3D transmission electron microscopy (3D‐TEM) combined with electron tomography. The method enabled us to visualize and evaluate structural characteristics in 3D space, such as the size and the volume of in situ silica generated in the NR matrix by the sol‐gel reaction of tetraethoxysilane, at nanometer scale resolution.

The reconstructed mass density view of the silica in an in situ silica‐filled natural rubber vulcanizate, as determined by 3D‐TEM.