Optically Active Polysilylenes: State‐of‐the‐Art Chiroptical Polymers

Controlled synthesis, chiroptical characterization, and manipulation of artificial helical polymers are challenging issues in modern polymer stereochemistry. Although many artificial polymers adopting a preferential screw‐sense helical structure have been investigated, optically active polysilylenes bearing chiral side chains may be among the most suitable to elucidate the inherent nature of the helical structure, since these polymers offer powerful spectroscopic probes as a result of their ideal chromophoric and fluorophoric main chain properties around 300–330 nm. The present paper will review comprehensively the helix‐property‐functionality relationship between side chain structure, global and local main chain conformation, (chir)optical properties, electronic properties, several helical cooperative phenomena, the effects of temperature and solvent polarity, and molecular imaging. This knowledge and understanding of the nature of the polysilylene helix might constitute a bridge between artificial polymers and biopolymers and will assist in designing and controlling new types of helical polymers directed to diverse screw‐sense‐related properties and applications in the future.     

Cytotoxic triterpenes from a Chinese medicine, Goreishi

Bioactivity-guided fractionation of the methanol extract of Goreishi (the feces of Trogopterus xanthipes Milne-Edwards) afforded one new and three known cytotoxic triterpenes, namely, 3-O-cis-p-coumaroyltormentic acid, pomolic acid, 2 alpha-hydroxyursolic acid, and jacoumaric acid. In the course of this investigation, six additional compounds having no cytotoxic activity were isolated, namely, maslinic acid, 3-O-trans-p-coumaroylmaslinic acid, ursolic acid, tormentic acid, euscaphic acid, and a new triterpene, 3-O-trans-p-coumaroyltormentic acid. The structures of the new compounds were established on the basis of X-nucleus-proton correlation with fixed evolution time (XCORFE) and other spectroscopic evidence.     

Preparation of the PdCl2 Complex of TADDOP,the Bis(diphenylphosphinite) of TADDOL: Use in enantioselective 1,3-diphenylallylations of nucleophiles and discussion of the mechanism

Reaction of the tartrate-derived diol (R,R)-α,α,α′,α′-tetraphenyl-2,2-dimethyl-1,3-dioxolane-4,5-dimethanol (TADDOL) with chlorodiphenylphosphane gives a new bis(diphenylphosphanyl) ligand (TADDOP). The complex 4 formed with PdCl₂ has been crystallized and its structure determined by X-ray diffraction (Fig.1). The complex is used for Pd-catalyzed enantioselective 1,3-diphenylallylations of various nucleophiles which give products with enantiomer ratios of up to 88:12 (Scheme 2). Crystallization procedures lead to the enantiomerically pure (> 99:1) product 11 derived from dimethyl malonate. The structure of the TADDOP complex 4 is compared with those of other transition-metal complexes containing chelating bis(diphenylphosphanyl) ligands (Fig.2). A crystallographic data base search reveals that the structures of transition-metal complexes containing two Ph₂P groups (superpositions in Fig.3) fall into one of two categories: one with approximate C₂ symmetry and the other with C₁ symmetry (20 and 19 examples, resp.). A mechanistic model is proposed which correlates the conformational chirality (δ or λ) of the four Ph groups' arrangement in such complexes with the topicity of nucleophile approach on Pd-bound trans,trans-1,3-diphenylallyl groups (Scheme 3 and Table).     

Grafting of polymers onto a carbon‐fiber surface by ligand‐exchange reaction of poly(vinyl ferrocene‐co‐vinyl monomer) with polycondensed aromatic rings of the surface

To improve the surface of carbon fiber, the grafting reaction of copolymer containing vinyl ferrocene (VFE) onto a carbon‐fiber surface by a ligand‐exchange reaction between ferrocene moieties of the copolymer and polycondensed aromatic rings of carbon fiber was investigated. The copolymer containing VFE was prepared by the radical copolymerization of VFE with vinyl monomers, such as methyl methacrylate (MMA) and styrene, using 2,2′‐azobisisobutyronitrile as an initiator. By heating the carbon fiber with poly(VFE‐co‐MMA) (number‐average molecular weight: 2.1 × 10⁴) in the presence of aluminum chloride and aluminum powder, the copolymer was grafted onto the surface. The percentage of grafting reached 46.1%. On the contrary, in the absence of aluminum chloride, no grafting of the copolymer was observed. Therefore, it is considered that the copolymer was grafted onto the carbon‐fiber surface by a ligand‐exchange reaction between ferrocene moieties of the copolymer and polycondensed aromatic rings of carbon fiber. The molar number of grafted polymer chain on the carbon‐fiber surface decreased with increasing molecular weight of poly(VFE‐co‐MMA) because the steric hindrance of grafted copolymer on the carbon‐fiber surface increases with increasing molecular weight of poly(VFE‐co‐MMA). © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1868–1875, 2002     

Ultrafast excitation relaxation dynamics of lutein in solution and in the light-harvesting complexes II isolated from Arabidopsis thaliana

Ultrafast excitation relaxation dynamics and energy-transfer processes in the light-harvesting complex II (LHC II) of Arabidopsis thaliana were examined at physiological temperature using femtosecond time-resolved fluorescence spectroscopy. Energy transfer from lutein to Chl a proceeded with a rate constant of k(ET) = 1.8-1.9 x 10(13) s(-1) and a yield of approximately Phi(ET) = 0.70, whereas that from neoxanthin to Chl a had a rate constant of k(ET) = 6.5 x 10(11) s(-1) and a yield at the most of Phi(ET) = 0.09. Fluorescence anisotropic decay of lutein in LHC II showed a value larger than 0.4 at the initial state and decayed to approximately 0.1 in 0.3 ps, indicating that two lutein molecules interact with each other in LHC II. In solution, anisotropy of lutein remained constant (0.38) independent of time, and thus a new excited state inferred between the S(2) (1B(u)) state and the S(1) (2A(g)) state was not applicable for lutein in solution. Energy migration processes among Chl a or Chl b molecules were clearly resolved by kinetic analysis. On the basis of these results, relaxation processes and energy-transfer kinetics in LHC II of A. thaliana are discussed.     

Translocation of Ca2+ across lipid bilayer membrane due to defects induced by teleocidin

The effect of defects in a dipalmitoylphosphatidylcholine (DPPC) membrane on Ca²⁺ permeability across the membrane was studied. Addition of teleocidin to a suspension of DPPC vesicles encapsulating Quin 2 increased the fluorescence intensity of Quin 2. Change of fluorescence intensity was significant below the phase-transition temperature of the membrane, and increased according to the kind of divalent metal ions in the medium in the order of Mg²⁺2+2+. It was confirmed that DPPC vesicles did not change the vesicular structure upon binding teleocidin to the membrane. Therefore, the fluorescence increase below the phase-transition temperature was ascribed to the influx of divalent cations into DPPC vesicles through cracks formed in the membrane upon distribution of teleocidin. By contrast, 12-0-tetradecanoylphorbol-13-acetate (TPA) did not change the fluorescence intensity of Quin 2 significantly. It should be noted that teleocidin, which located at the membrane surface, yielded more significant defects across the lipid membrane than TPA, which was incorporated into the hydrophobic core of the membrane.