7,7,8,8‐Tetraaryl‐o‐quinodimethane Stabilized by Dibenzo Annulation: A Helical π‐Electron System That Exhibits Electrochromic and Unique Chiroptical Properties

When two benzene rings are fused to a tetraaryl‐o‐quinodimethane skeleton, sterically hindered helical molecules 1 acquire a high thermodynamic stability. Because the tetraarylbutadiene subunit contains electron‐donating alkoxy groups, 1 undergo reversible two‐electron oxidation to 2 ²⁺, which can be isolated as deeply colored stable salts. Intramolecular transfer of the point chirality (e.g., sec‐butyl) on the aryl groups to helicity induces a diastereomeric preference in dications 2 b ²⁺ and 2 c ²⁺, which represents an efficient method for enhancing circular‐dichroism signals. Thus, those redox pairs can serve as new electrochiroptical response systems. X‐ray analysis of dication 2 ²⁺ revealed π–π stacking interaction of the diarylmethylium moieties, which is also present in solution. The stacking geometry is the key contributor to the chirosolvatochromic response.     

Ethylene and propylene polymerization behavior of a series of bis(phenoxy–imine)titanium complexes

This contribution reports ethylene and propylene polymerization behavior of a series of Ti complexes bearing a pair of phenoxy–imine chelate ligands. The bis(phenoxy–imine)Ti complexes in conjunction with methylalumoxane (MAO) can be active catalysts for the polymerization of ethylene. Unexpectedly, this C₂ symmetric catalyst produces syndiotactic polypropylene. ¹³C NMR spectroscopy has revealed that the syndiotacticity arises from a chain-end control mechanism. Substitutions on the phenoxy–imine ligands have substantial effects on both ethylene and propylene polymerization behavior of the complexes. In particular, the steric bulk of the substituent ortho to the phenoxy–oxygen is fundamental to obtaining high activity and high molecular weight for ethylene polymerization and high syndioselectivity for the chain-end controlled propylene polymerization. The highest ethylene polymerization activity, 3240 kg/mol-cat h, exhibited by a complex having a t-butyl group ortho to the phenoxy–oxygen, represents one of the highest reported to date for Ti-based non-metallocene catalysts. Additionally, the polypropylene produced exhibits a T_m, 140 °C, and syndioselectivity, rrrr 83.7% (achieved by a complex bearing a trimethylsilyl group ortho to the phenoxy–oxygen) that are among the highest for polypropylenes produced via a chain-end control mechanism. Hence, the bis(phenoxy–imine)Ti complexes are rare examples of non-metallocene catalysts that are useful for the polymerization of not only ethylene but also propylene.     

Prenylated xanthones from Garcinia xanthochymus

A new prenylated xanthone, 1,3,5,6-tetrahydroxy-4,7,8-tri(3-methyl-2-butenyl)xanthone (1), was isolated from the wood of Garcinia xanthochymus together with a known xanthone, garciniaxanthone E (2). Their structures were determined by spectroscopic analysis. Compounds 1 (3 microM) and 2 (10 microM) elicited marked enhancement of nerve growth factor-mediated neurite outgrowth in PC12D cells.     

Synthesis of directly linked zinc(II) porphyrin-imide dyads and energy gap dependence of intramolecular electron transfer reactions

A series of zinc(II) porphyrin-imide dyads (ZP-Im), in which an electron donating ZP moiety is directly connected to an electron accepting imide moiety in the meso position, have been prepared for the examination of energy gap dependence of intramolecular electron transfer reactions with large electronic coupling. The nearly perpendicular conformation of the imide moiety towards the porphyrin plane has been revealed by Xray crystal structures. The energy gap for charge separation, 1ZP* - Im --> ZP+ - Im-, is varied by changing the electron accepting imide moiety to cover a range of about 0.8 eV in DMF. Definitive evidence for electron transfer has been obtained in three solvents (toluene, THF, and DMF) through picosecond-femtosecond transient absorption studies, which have allowed us to determine the rates of photoinduced charge separation, 1ZP* - Im --> ZP+ - Im-, and subsequent thermal charge recombination ZP+ - Im- --> ZP - Im. The free-energy gap dependence (energy gap law) has been probed from the normal to the nearly top region for the charge separation rate alone, and only the inverted region for the charge recombination rate. Although both of the energy gap dependencies can be approximately reproduced by means of the simplified semiclassical equation, when we take into consideration the effect of the high frequency vibrations replaced by one mode of averaged frequency, many features, including the effects of solvent polarity and the electron tunneling matrix element on the energy gap law, differ considerably from those of the previously studied porphyrin-quinone systems, which have weaker interchromophore electronic interactions.     

Tetra‐n‐butyl­ammonium tetrakis(penta­fluoro­benzenethiol­ato‐κS)­aurate(III)

The title compound, (C₁₆H₃₆N)[Au(C₆F₅S)₄], is the first example of a structurally characterized gold(III) complex with monodentate benzene­thiol­ate ligands. The Au atom lies on a fourfold axis and the AuS₄ group has square‐planar geometry. The anion shows a two‐dimensional linkage through π–π and C—F⋯π intermolecular interactions.     

Stereospecific synthesis of new 2,3,4,5-piperidinetetracarboxylic acids and 2,3,5-piperidinetricarboxylic acids

Diels-Alder adducts of 1,2-dihydropyridine with maleic and acrylic acid derivatives were stereospecifically converted by way of RuO₄ oxidation into new r-2,c-3,c-4,c-5-piperidinetetracarboxylic acid, r-2,t-3,t-4,c-5-piperidinetetracarboxylic acid, r-2,c-3,c-5-piperidinetricarboxylic acid, and r-2,t-3,c-5-piperidinetricarboxylic acid.     

Array of aromatic amino acid side chains located near the chromophore of photoactive yellow protein

The role of the array of aromatic amino acid side chains located close to the chromophore binding loop of photoactive yellow protein (PYP) was studied using the alanine-substitution mutagenesis. Phe92, Tyr94, Phe96 and Tyr98 were replaced with alanine (F92A, Y94A, F96A and Y98A, respectively), then these mutants were characterized by UV-visible absorption spectra, circular dichroism (CD) spectra, thermal stability and photocycle kinetics. Absorption maxima of F92A, Y94A, F96A and Y98A were 444, 442, 439 and 447 nm, respectively, different to wild type (WT) at 446 nm. Far-UV CD spectra of mutants other than F92A were different from WT, indicating that Tyr94, Phe96 and Tyr98 maintain the native secondary structure of PYP. Mid-point temperatures of thermal denaturation of F92A, Y94A and F96A, estimated by the CD signal at 222 nm, were 5-10 degrees C lower than WT. Time constants of the photocycle estimated by flash-induced absorbance change were 0.36 s for WT and 1.4 s for Y98A, however, 100, 30 and 3000 times slower than WT for F92A, Y94A and F96A, respectively. Tyr98 is located in the loop region, whereas Phe92, Tyr94 and Phe96 are incorporated in the beta4 strand, showing that aromatic amino acid residues in the beta-sheet regulate the absorption spectrum, thermal stability and photocycle of PYP. Aromatic rings of Phe92, Tyr94 and Phe96 lie nearly perpendicular to the aromatic ring of Phe75 or chromophore. Possible weak hydrogen bonds between the aromatic ring hydrogen and pi-electrons of these residues are discussed.     

Proteomic LC-MS systems using nanoscale liquid chromatography with tandem mass spectrometry

Current nano-scale liquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) approaches in proteome research are reviewed from an analytical perspective. For comprehensive analysis of cellular proteins, analytical methods with higher resolution, sensitivity, and wider dynamic range are required. Miniaturized LC coupled with tandem mass spectrometry is currently one of the most versatile techniques. In this review, the current status of nanoLC-MS/MS systems as well as data management systems is addressed. In addition, the future prospects for complete proteomics are discussed.