Efficient excitation energy transfer in long meso-meso linked Zn(II) porphyrin arrays bearing a 5,15-bisphenylethynylated Zn(II) porphyrin acceptor

Electronically coupled porphyrin arrays are suitable for artificial light harvesting antenna in light of a large absorption cross-section and fast excitation energy transfer (EET). Along this line, an artificial energy transfer model system has been synthesized, comprising of an energy donating meso-meso linked Zn(II) porphyrin array and an energy accepting 5,15-bisphenylethynylated Zn(II) porphyrin linked via a 1,4-phenylene spacer. This includes an increasing number of porphyrins in the meso-meso linked Zn(II) porphyrin array, 1, 2, 3, 6, 12, and 24 (Z1A, Z2A, Z3A, Z6A, Z12A, and Z24A). The intramolecular singlet-singlet EET processes have been examined by means of the steady-state and time-resolved spectroscopic techniques. The steady-state fluorescence comes only from the acceptor moiety in Z1A-Z12A, indicating nearly the quantitative EET. In Z24A that has a molecular length of ca. 217 A, the fluorescence comes largely from the acceptor moiety but partly from the long donor array, indicating that the intramolecular EET is not quantitative. The transient absorption spectroscopy has provided the EET rates in real time scale: (2.5 ps)(-1) for Z1A, (3.3 ps)(-1) for Z2A, (5.5 ps)(-1) for Z3A, (21 ps)(-1) for Z6A, (63 ps)(-1) for Z12A, and (108 ps)(-1) for Z24A. These results have been well explained by a revised F?rster equation (Sumi formula), which takes into account an exciton extending coherently over several porphyrin pigments in the donor array, whose length is not much shorter than the average donor-acceptor distance. Advantages of such strongly coupled porphyrin arrays in light harvesting and transmission are emphasized in terms of fast EET and a large absorption cross-section for incident light.     

Duplex dissociation of telomere DNAs induced by molecular crowding

Because of the importance of telomere DNAs, the structures of these DNAs in vivo are currently of great research interest in the medical, pharmaceutical, chemical, and industrial fields. To understand the structure of biomolecules in vivo, their properties studied in vitro are extrapolated to the in vivo condition, while the condition in a living cell is inherently molecularly crowded and a nonideal solution contains various biomolecules. We investigated the effect of molecular crowding, which is one of the most important cellular environmental conditions, on the structure and stability of the telomere and G-rich and C-rich DNAs using circular dichroism (CD) spectra, CD melting curves, and isothermal titration calorimetry (ITC). The CD spectra and CD melting curves of G-rich DNA, C-rich DNA, and the 1:1 mixture of G-rich and C-rich DNAs showed that each G-rich DNA, C-rich DNA, and the 1:1 mixture form the antiparallel G-quadruplex, I-motif, and duplex, respectively, in the noncrowding condition as previously considered. On the contrary, the G-rich and C-rich DNAs individually form the parallel G-quadruplex and I-motif, respectively, in the molecular crowding condition, and the 1:1 mixture folds into the parallel G-quadruplex and I-motif but does not form a duplex. The ITC measurements indicated that the thermodynamic stability (DeltaG degrees (20)) of the duplex formation between the G-rich and C-rich DNAs in the noncrowding condition was -10.2 kcal mol(-)(1), while only a small heat change was observed in the ITC measurements in the molecular crowding condition. These ITC results also demonstrated that the molecular crowding condition prevents any duplex formation between G-rich and C-rich DNAs. These results indicate that a structural polymorphism of the telomere DNAs is induced by molecular crowding in vivo.     

New photoswitching unit for magnetic interaction: diarylethene with 2,5-bis(arylethynyl)-3-thienyl group

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene, and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have an extended pi-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity. Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum originating from the change in the magnetic interaction between two unpaired electrons. The open-ring isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic interaction between two radicals via the pi-conjugated chain was altered by photocyclization due to the change of the hybrid orbital at the 2-position of the thiophene ring from sp(2) to sp(3).     

Methyltriethoxysilane-derived sol-gel coatings doped with silver metal particles

Silica sol-gel films were prepared by dipping, starting from an acid catalyzed solution of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS). Silver metal nanoparticles were produced in the silica layer by introducing in the sol-gel precursor solution AgNO₃ or AgClO₄·H₂O. The silver ions were thermally reduced in air at 800°C, giving an intense yellow coating film. The silver metal particles were observed by transmission electron microscopy and X-ray diffraction. The diameter of the silver particles was found to be about 10 nm. Absorption measurements in the UV-Vis were used to evaluate the volume fraction of silver colloids embedded in the silica layer.     

Phenomenological analysis of elongational flow birefringence in semidilute solutions of hydroxypropylcellulose

The relaxation time of a polymer chain in an elongational flow field was investigated for hydroxypropylcellulose (HPC) semidilute solution systems by two methods: phenomenological analysis of elongational flow-induced birefringence, and dynamic light scattering (DLS) and rheological measurements. To understand the relaxation time of an entangled semiflexible polymer solution in an elongational flow field, scaling analysis of the elongational flow-induced birefringence curve was performed. The results of both temperature and concentration scaling analyses showed that birefringence curves at different temperatures and at several HPC concentrations were described well by a universal birefringence–strain rate curve. This scaling behavior was compared with the "fuzzy cylinder" model. The critical strain rate corresponded to the correlation time of the slow relaxation mode determined by DLS measurement and the relaxation spectrum obtained by dynamic viscoelasticity measurement. The elongational flow-induced birefringence observed in an HPC semidilute solution was concluded to be attributed to the orientation of the HPC segment in the entangled molecular system, because the dominant relaxation mode is found to be the concentration fluctuation of an entangled molecular cluster in a quiescent state.     

Lewis Acid-Catalyzed Hydrostannation of Acetylenes. Regio- and Stereoselective Trans-Addition of Tributyltin Hydride and Dibutyltin Dihydride

Lewis acids such as ZrCl(4) or HfCl(4) catalyze the hydrostannation of acetylenes 1 by tributyltin hydride to produce the cis vinylstannanes 2 by regio- and stereoselective anti-hydrostannation. The hydrostannation of acetylenes using dibutyltin dihydride was also catalyzed by ZrCl(4) to give the stereodefined Z-Z divinyltin derivatives 4 by an anti-hydrostannation pathway. The use of nonpolar solvents such as toluene or hexane was essential for obtaining high stereoselectivity and chemical yield. Since ZrCl(4) and HfCl(4) are not soluble in such solvents, the hydrostannations were carried out in a heterogeneous system. The reactions of internal acetylenes with Bu(3)SnH proceeded smoothly, although the use of stoichiometric amounts of ZrCl(4) gave better results. The ZrCl(4)-catalyzed hydrostannation at 0 degrees C gave better yields and stereoselectivities than the reaction at room temperature. To help clalify the reason, the reaction of Bu(3)SnH with ZrCl(4) was monitored by (1)H and (119)Sn NMR spectroscopy, and it was found that Bu(3)SnH reacted with ZrCl(4) at room temperature to afford a mixture of tributyltin hydride, dibutyltin dihydride, and tetrabutyltin.     

Stereoselective syntheses of highly functionalized bicyclo[3.1.0]hexanes: a general methodology for the synthesis of potent and selective mGluR2/3 agonists

[Chemical reaction: See text] A Et3Al mediated intramolecular epoxide opening, cyclopropanation reaction is described. The transformation provided highly functionalized bicyclo[3.1.0]hexane systems in high efficiency and with perfect H or F endo selectivity. Application of this reaction to the synthesis of mGluR2/3 agonist 1 (43% overall yield) and a few intermediates suitable for the synthesis of other bicyclo[3.1.0]hexane mGluR2/3 agonists is discussed.     

Influence of amino acid side chain packing on Fe-methionine coordination in thermostable cytochrome C

Paramagnetic NMR and optical studies of the oxidized forms of mesophile Pseudomonas aeruginosa cytochrome c(551) and its quintuple mutant (F7A/V13M/F34Y/E43Y/V78I), and thermophile Hydrogenobacter thermophilus cytochrome c(552) demonstrated that the amino acid side chain packings in the protein interior influence the coordination bond between the heme iron and the axial methionine in the proteins. The strength of heme axial coordinations was found to correlate with the overall protein thermostability.     

Origin of visible-light-driven photocatalysis: A comparative study on N/F-doped and N-F-codoped TiO2 powders by means of experimental characterizations and theoretical calculations

An overall comparative study was carried out on N-doped, F-doped, and N-F-codoped TiO₂ powders (NTO, FTO, NFTO) synthesized by spray pyrolysis in order to elucidate the origin of their visible-light-driven photocatalysis. The comparisons in their experimentally obtained characteristics were based on the analysis of XPS, UV-Vis, PL, NH₃-TPD and ESR spectra. The comparisons in their theoretically predicted properties were based on the analysis of the calculated electronic structures. As the results, N-doping into TiO₂ resulted in not only the improvement in visible-light absorption but also the creation of surface oxygen vacancies. F-doping produced several beneficial effects including the creation of surface oxygen vacancies, the enhancement of surface acidity and the increase of Ti³⁺ ions. Doped N atoms formed a localized energy state above the valence band of TiO₂, whereas doped F atoms themselves had no influence on the band structure. The photocatalytic tests indicated that the NFTO demonstrated the highest visible-light activity for decompositions of both acetaldehyde and trichloroethylene. This high activity was ascribed to a synergetic consequence of several beneficial effects induced by the N-F-codoping.     

Fluorine-doped TiO2 powders prepared by spray pyrolysis and their improved photocatalytic activity for decomposition of gas-phase acetaldehyde

F-doped TiO₂ (FTO) powders were synthesized by spray pyrolysis (SP) from an aqueous solution of H₂TiF₆. The resulting FTO powders possessed spherical particles with a rough surface morphology and a strong surface acidity. The fluorine concentrations in the FTO powders calculated from XPS spectra significantly depended on SP temperature and ranged from 2.76 to 9.40 at.%. The FTO powder prepared at SP temperature of 1173 K demonstrated the highest photocatalytic activity for the decomposition of gas-phase acetaldehyde under both ultraviolet (UV) and visible light (vis) irradiations, and it was higher than that of commercial P 25. This high photocatalytic activity was ascribed to several beneficial effects produced by F-doping: enhancement of surface acidity, creation of oxygen vacancies, and increase of active sites. It was interesting to point out that the vis photocatalytic activity of FTO powders was achieved by the creation of surface oxygen vacancies rather than the improvement of optical absorption property of bulk TiO₂ in vis region.