The biosynthesis of the thiazole phosphate moiety of thiamin (vitamin B1): the early steps catalyzed by thiazole synthase

Thiazole synthase (ThiG) catalyzes an Amadori-type rearrangement of 1-deoxy-d-xylulose-5-phosphate (DXP) via an imine intermediate. In support of this, we have demonstrated enzyme-catalyzed exchange of the C2 carbonyl of DXP. Borohydride reduction of the enzyme DXP imine followed by top-down mass spectrometric analysis localized the imine to lysine 96. On the basis of these observations, a new mechanism for the biosynthesis of the thiazole phosphate moiety of thiamin pyrophosphate in Bacillus subtilis is proposed. This mechanism involves the generation of a ketone at C3 of DXP by an Amadori-type rearrangement of the imine followed by nucleophillic addition of the sulfur carrier protein (ThiS-thiocarboxylate) to this carbonyl group.     

Hypersurfaces which are equiaffine extremal and centroaffine extremal

For a centroaffine hypersurface, we use the relations between equiaffine and centroaffine invariants to find many newexamples of equiaffine extremal hypersurfaces. *Supported by NSFC No. 10371013; Joint Research of NSFC and KOSEF; NEU. **Supported by ABRL No. R14-2002-003-01002-0 from KRF; No. F01-2005-000-100024-0 from KOSEF.     

Third-order nonlinear optical response of silicon nanostructures dispersed in organic solvent under 1064 nm and 532 nm laser excitations

Silicon nanostructures are dispersed into an organic solvent and the third order optical properties of the system are studied by z-scan technique under 1064 nm and 532 nm excitations. The experiment results show that the silicon nanostructures exhibit self-focus and saturable absorption with both excitation wavelengths. Nonlinear absorption results suggest that a new optical bleaching band exists under 532 nm excitation, and a two-step mechanism is tentatively put forward to explain the saturable absorption under 1064 nm excitation.     

Analysis of sunlight loss for femtosecond laser microstructed silicon and its solar cell efficiency

Black silicon, which is obtained by irradiating the surface of a Si wafer with femtosecond laser pulses in the presence of a sulfur-bearing gas, holds great promise in the preparation of high-performance intermediate band silicon solar cells. Using a three-level model, the enhanced usefulness of sunlight of the microstructured silicon was firstly analyzed. A detailed study on the relationship between the light loss, the ionization energy of doped impurities in silicon and the impurity band width were given. Then the effect of the position of intermediate band within the forbidden gap of silicon on the theoretical conversion efficiency for the corresponding solar cell is discussed using the Detailed Balance Theory. Finally problems need to be resolved in making intermediate band solar cells based on femtosecond laser microstructured silicon are pointed out with great emphasis.     

变量代换法在解微分方程中的应用

归纳总结利用变量代换求解不同类型常微分方程的方法与技巧,并借助两个实例的多种解法加以说明.     

Research on the multi‐crystalline structure in sapphire grown by Kyropoulos technique

The macroscopic distribution of multi‐crystalline structure (MCS) in a sapphire boule grown by Kyropoulos technique was characterized using polariscope. The morphology and texture of subgrains from , and orientations were investigated using Synchrotron radiation white‐beam topography technique. The morphology of subgrains was found to be diverse from crystallographic orientation. The relative angles between subgrains are smaller than 1°. The defect structure on a basal‐plane specimen from the lower part of the “shoulder” was examined by X‐ray rocking curve and chemical etching technique. It was found that the defect concentration changes drastically near MCS center. The nature of spatial distribution of MCS and the defect distribution near MCS in a grown boule was discussed in detail.     

Crystal‐Facet Engineering of Ferric Giniite by Using Ionic‐Liquid Precursors and Their Enhanced Photocatalytic Performances under Visible‐Light Irradiation

In the work presented here, well‐dispersed ferric giniite microcrystals with controlled sizes and shapes are solvothermally synthesized from ionic‐liquid precursors by using 1‐n‐butyl‐3‐methylimidazolium dihydrogenphosphate ([Bmim][H₂PO₄]) as phosphate source. The success of this synthesis relies on the concentration and composition of the ionic‐liquid precursors. By adjusting the molar ratios of Fe(NO₃)₃ ? 9H₂O to [Bmim][H₂PO₄] as well as the composition of ionic‐liquid precursors, we obtained uniform microstructures such as bipyramids exposing {111} facets, plates exposing {001} facets, hollow spheres, tetragonal hexadecahedron exposing {441} and {111} facets, and truncated bipyamids with carved {001} facets. The crystalline structure of the ferric giniite microcrystals is disclosed by various characterization techniques. It was revealed that [Bmim][H₂PO₄] played an important role in stabilizing the {111} facets of ferric giniite crystals, leading to the different morphologies in the presence of ionic‐liquid precursors with different compositions. Furthermore, since these ferric giniite crystals were characterized by different facets, they could serve as model Fenton‐like catalysts to uncover the correlation between the surface and the catalytic performance for the photodegradation of organic dyes under visible‐light irradiation. Our measurements indicate that the photocatalytic activity of as‐prepared Fenton‐like catalysts is highly dependent on the exposed facets, and the surface area has essentially no obvious effect on the photocatalytic degradation of organic dyes in the present study. It is highly expected that these findings are useful in understanding the photocatalytic activity of Fenton‐like catalysts with different morphologies, and suggest a promising new strategy for crystal‐facet engineering of photocatalysts for wastewater treatment based on heterogeneous Fenton‐like process.