Towards an identification of chemically different flavin radicals by means of theirg-tensor

Theg-tensors of two chemically different flavin mononucleotide (FMN) radicals, one of which is covalently bound via N(5) of its 7,8-dimethyl isoalloxazine moiety, and the other one non-covalently bound to mutant LOV domains of the blue-light receptor phototropin, LOV1 C57M and LOV2 C450A, respectively, have been determined by very high microwave frequency and high magnetic field electron paramagnetic resonance (EPR) performed at 360 GHz and 12.8 T. Due to the high spectral resolution of the frozen-solution continuous-wave EPR spectra, the anisotropy of theg-tensors could be fully resolved. By least-squares fittings of spectral simulations to expermental data, the principal values ofg have been established:g _X=2.00554(5),g _Y=2.00391(5), andg _Z=2.00247(7) for the N(5)-alkyl-chain-linked FMN radical in LOV1 C57M-675, andg _X=2.00427(5),g _Y=2.00360(5), andg _Z=2.00220(7) for the noncovalently bound FMN radical in LOV2 C450A-605. By a comparison of these values to the ones from the flavin adenine dinucleotide radicals in two photolyases, the radical in LOV2 C450A-605 could be clearly identified as a neutral FMN radical, FMNH. In contrast, LOV1 C57M-675 exhibits significantly shifted principal components ofg, the differences being caused by spin-orbit coupling of the nearby sulfur from the reactive methionine residue, and the modified chemical structure due to the covalent attachment at N(5) of the radical to the apoprotein. The results clearly show the potential of using theg-tensor as probe of the global electronic and chemical structure of protein-bound flavin radicals.     

A Versatile and Practical Synthesis of 1,1‐Diacetates from Aldehydes Catalyzed by Cyanuric Chloride

Structurally diverse aldehydes are successfully converted into 1,1‐diacetates with acetic anhydride using cyanuric chloride as a mild, convenient and inexpensive catalyst under solvent‐free conditions. The noteworthy features of the present system are shorter reaction times, and mild and solvent‐free conditions. Furthermore, it offers chemoselective protection of aldehydes.     

Photoelectron spectroscopic investigation of transformation of trifluoroacetate precursors into superconducting YBa2Cu3O7−δ films

X-ray photoelectron spectroscopy (XPS) has been used to investigate the evolution of surface chemistry of YBa₂Cu₃O_7−δ (Y123) films prepared by the metalorganic deposition (MOD) process using trifluoroacetate (TFA) precursors. Detailed XPS core-level spectra obtained from the samples quenched from various points during the calcining and firing stages have been reported for the first time and are used to identify surface species. The XPS data show evidence of formation of intermediate phases such as Y-O-F, BaF₂, and CuO during the calcining process, which are the decomposition products of yttrium, barium, and copper trifluoroacetates, respectively. The TFA precursors are completely decomposed at the end of calcination. The change of binding energies for Y 3d_5/2, Ba 3d_5/2, and O 1s during the firing process indicates that Y123 starts to form at 800 °C after 0.5 h firing. Based on the experimental results, an alternative mechanism of the chemical evolution from precursor to final film in the TFA-MOD process is proposed.     

Intermolecular Forces of Sugars in Water

Apparent molar volume and viscosities of fructose, glucose, mannose, and sucrose have been measured in dilute aqueous solution, concentration range 0.028–0.336 M at 293 K. The viscosity coefficient B and A were calculated from the viscosity data using the Jones-Dole equation for all the studied sugars. The data were also analysed for Stauarding equation. The structure making behavior was obtained for all the sugars. A modified Jone-Dole equation was proposed by using ratio of mole fractions of solute and solvent in place of concentrations of solute.     

Role of host liquid in optical limiting in ink suspensions

Optical limiting of 532 nm, 15 ns duration laser pulses in suspensions made by diluting black ink in water, ethanol, and isopropanol was studied and the strength of limiting was found to be quite different for the three suspensions. Microscopic investigations showed that the sizes of suspended aggregates were different for different host liquids. The suspension with largest aggregate was found to exhibit strongest optical limiting. Our work thus shows that host liquid influences optical limiting in suspensions not only through its thermodynamic properties but also by determining the particle size which has a strong effect on limiting strength.     

Synthetic Studies of a Didemnin B Analog Based on a 2,3‐Diamino Sugar Scaffolding

Reductive amination, amide formation using BOP [(1H‐1,2,3‐benzotriazol‐1‐yloxy)tris(dimethylamino)‐phosphonium hexafluorophosphate] and esterification via DCC (dicyclohexylcarbodiimide) were the key synthetic steps to generate an advanced intermediate in the preparation of a didemnin B analog based on a sugar scaffolding ( 2 ). This analog should provide insight into the bioactive conformation of didemnin B.     

2,4,6-Trichloro-1,3,5-triazine Catalyzed Chemoselective Transthioacetalization of Aldehyde Acetals and Oxathioacetals

A mild and efficient transthioacetalization of aldehyde acetals and oxathioacetals was carried out using 2,4,6-trichloro-1,3,5-triazine as a mild and inexpensive catalyst. Chemoselective transacetalization is impressive as aldehyde O,O- and O,S-acetals are converted into the corresponding S,S-acetals in the presence of ketones or their acetals and oxathiocetals in nearly quantitative yields.     

Axisymmetric buckling of laminated, moderately thick shallow conical cap

Summary  Axisymmetric buckling and postbuckling analysis is presented for a moderately thick, laminated shallow conical cap under static transverse load. Marguerre-type, first-order shear deformation shallow-shell theory is formulated in terms of transverse deflection w, the rotation ψ of the normal to the midsurface and the stress function Φ. The governing equations are solved by the orthogonal point-collocation method. Clamped conical caps and simple supports with movable and immovable edge conditions are considered. Typical numerical results are presented, illustrating the effect of various parameters. The dependence of the effect of the shear deformation on the thickness parameter, boundary conditions, ratio of Young's moduli and cap height is investigated. Received 10 February 2000; accepted for publication 3 May 2000     

Quaternionic quantum mechanics is consistent with complex quantum mechanics

Quaternionic quantum mechanics is investigated in the light of the great success of complex quantum mechanics. It is shown that to reproduce the results of complex quantum mechanics, quaternionic quantum mechanics must contain complex quantum mechanics.