Dithienylethenes with a novel photochromic performance

Dithienylethenes with low decoloration quantum yields and thermal reversibility at high temperature above 100 degrees C were prepared. Introduction of bulky alkoxy substituents at 2- and 2'-positions of the thiophene rings strongly suppressed the cycloreversion quantum yields. The quantum yields were lower than 10(-3), and the photogenerated color remained stable enough under room light. On the other hand, the bulky alkoxy substituent decreased the thermal stability of the colored closed-ring isomers at high temperature. The color of the dithienylethene with cyclohexyloxy substituents faded out in less than 1 min at 160 degrees C.     

Amphotericin B dimers with bisamide linkage bearing powerful membrane-permeabilizing activity

[structure: see text] Covalently linked dimers of amphotericin B were prepared by cross-linking its carboxylic acid. Among these, a dimer with a linkage of 1,6-hexanediamine revealed potent hemolytic activity (EC50, 0.25 microM) while its N-acetyl derivative gave rise to large K+ ion flux in phosphatidylcholine liposomes, regardless of the presence or absence of sterols, suggesting that the dimers may serve as a tool for elucidating the structure of the ion channel assemblage formed by amphotericin B.     

Three-directional spectral dispersion for smoothing of a laser irradiance profile

In inertial confinement fusion research, uniform laser irradiation on a fusion target is a key issue. We propose a new method of beam smoothing in which we use three-directional spectral dispersion to reduce the coherent speckle that is unavoidable in the usual two-directional spectral-dispersion scheme. We have used this smoothing technique in a Nd:glass laser system and have demonstrated that the coherent speckle is reduced by a factor of 2.9 from that in two-directional spectral dispersion.     

Reaction of 2,3-dichloro-5,6-dicyanopyrazine with enamines and some tertiary amines

The reaction of 2,3-dichloro-5,6-dicyanopyrazine ( 1 ) with enamines as well as a few tertiary amines as enamine precursors was investigated. Both reactions gave aminovinyl-substituted pyrazine derivatives. During the attempted purification of 3c or 3d by column chromatography on silica gel, 2-chloro-5,6-dicyano-3-(1′-oxocyclopent-2′-yl)pyrazine ( 4 ) was obtained, apparently by hydrolytic cleavage. The products prepared are all of interest as potential pesticides.     

A realistic structural model of glassy iron

The Ichikawa model with k = 2.0 (the Bennett model) for iron was relaxed using Johnson's potential and the potential by Pak and Doyama. The relaxed models reproduce the experimental pair-distribution function and structure sensitive intensity and have high packing fractions.     

Grid points of Gauss-Legendre quadrature as exponent parameters of basis functions of atomic hartree-fock wavefunctions

Use of the grid points of the Gauss-Legendre numerical quadrature is examined as the exponent parameters of the basis functions of atomic Hartree-Fock wavefunctions. Calculations on He, Be, and Ne have been performed and reasonably accurate wavefunctions are obtained using 20 grid points or basis functions per symmetry species without any optimization of non-linear parameters.     

Spontaneous racemization and epimerization behavior in solution of chiral nitroxides

[reaction: see text] Enantiomerically enriched samples of chiral cyclic nitroxides with a 4-hydroxyphenyl group on the stereogenic center bearing the NO radical group undergo unprecedented spontaneous racemization and/or epimerization in aprotic solvents, which can be well accounted for by the multistep equilibrations involving planar quinoid intermediates.     

Asymmetric total synthesis of (-)-spirofungin A and (+)-spirofungin B

[chemical reaction: see text]. The stereocontrolled total synthesis of (-)-spirofungin A (1) and (+)-spirofungin B (2a), polyketide-type antibiotics having various antifungal activities, has been achieved employing the Weinreb amide 8, the alkyne 9, and the vinyl boronate 5 readily available from the common intermediate 10. The first synthesis proceeded with a longest linear sequence of 31 steps, affording (-)-1 and (+)-2a in 7.9% and 5.2% overall yields, respectively.     

Beam-polarization asymmetries for the p(gamma-->, K+)Lambda and p(gamma-->, K+)Sigma(0) reactions for E(gamma)=1.5-2.4 GeV

Beam polarization asymmetries for the p(gamma-->,K+)Lambda and p(gamma-->,K+)Sigma(0) reactions are measured for the first time for E(gamma)=1.5-2.4 GeV and 0.6相似文献   

A new strategy for the design of water-soluble synthetic receptors: specific recognition of DNA intercalators and diamines

Water-soluble zinc bisporphyrin receptors 1 and 2 having two Lewis acidic sites (zinc) in the hydrophobic environment consisting of alkyl chains and a bisporphyrin framework, and covered with hydrophilic exterior (twelve or eighteen carboxyl groups) were prepared. The receptors show high affinity for diamines and DNA intercalators in water where the binding constants K(a) are of the order of 10(7) and 10(8) M(-1), respectively. Diamines and DNA intercalators are bound to the receptor through different mechanisms. Diamines are bound through hydrophobic interactions and zinc-nitrogen interactions, while DNA intercalators are bound through hydrophobic interactions and charge-transfer interactions. Flexible alkyl chains can make van der Waals contact with guests and create a hydrophobic environment around the bound guest by an induced-fit-type mechanism. For the binding of DNA intercalators, the following features are noteworthy: 1). Binding constants are similar between the zinc porphyrins and zinc-free porphyrins; 2). the binding constant is larger for the guest having the lower LUMO; this indicates the important contribution of charge-transfer interactions to binding; 3). the hydrophobic and cationic nature of DNA intercalators is substantially important, and 4). higher ionic strength reduced the binding affinities; this shows a moderate contribution of electrostatic interactions. The conformational instability of the receptors also contributes to the tight binding: hydrophobic and electrostatic interactions cannot both be favorable at the same time in the guest-free receptor. Enthalpy-entropy compensation observed for the binding of diamines and DNA intercalators is characterized by a relatively small slope (alpha=0.74) and a large intercept (beta=7.75 kcal mol(-1)) in the DeltaH degrees versus TDeltaS degrees plot; this shows that a conformational change of receptors and a significant desolvation occur upon binding. The receptor can competitively bind to propidium iodide to deprive DNA of the intercalated propidium iodide. These features of water-soluble receptors consisting of a rigid framework and flexible side chains with a large solvent-accessible area are in contrast to highly preorganized rigid receptors, and they can provide useful guidelines for rational design of induced-fit artificial receptors in water.