Identification and stereochemical assignment of the beta-hydroxytryptophan intermediate in the echinomycin biosynthetic pathway

[reaction: see text] Little is known about how quinoxaline-2-carboxylic acid (QC) is synthesized in nature. On the basis of analysis of echinomycin biosynthetic gene clusters as well as feeding experiments with labeled precursors, we have proposed a biosynthetic pathway to QC and identified the (2S,3S)-beta-hydroxytryptophan as a key intermediate.     

Thin and defect-free Pd-based composite membrane without any interlayer and substrate penetration by a combined organic and inorganic process

A novel combined organic and inorganic process for preparing thin supported membrane was developed, using which a thin and defect-free Pd membrane with uniform thickness of 5 microm was directly coated onto porous alpha-Al2O3 hollow fiber without any interlayer and substrate penetration; at the same time, there existed a small interstice between membrane and substrate, which led to higher hydrogen permeance, infinite selectivity, and better membrane stability.     

Second hyperpolarizabilities (gamma) of bisimidazole and bistriazole benzenes: diradical character, charged state, and spin state dependences

The second hyperpolarizabilities of bisimidazole- and bistriazole-benzene compounds have been calculated at different levels of approximation to unravel the effects of diradical character as well as of charge and spin multiplicity. The largest second hyperpolarizabilities are associated with intermediate diradical character, provided positive charging does not compensate for this effect. For the neutral diradical bisimidazole compound, the singlet diradical species possesses a second hyperpolarizability two to three times larger than the corresponding triplet, demonstrating the possibility of spin state control of the third-order NLO responses for diradical species.     

Effect of pressure on the vibrational structure of insensitive energetic material 5-nitro-2,4-dihydro-1,2,4-triazole-3-one

The Raman spectra of alpha form 5-nitro-2,4-dihydro-1,2,4-triazole-3-one (alpha-NTO, space group P) were measured in a high-pressure vessel diamond anvil cell (DAC). The pressure was increased to 27.6 GPa. In general, Raman bands show a blue shift because of the nature of the molecule packing as a high-pressure effect, but some particular bands exhibited a red shift, disappearance, split, or slight shifting in our experiments. Those red-shifting bands concerning hydrogen bonds, i.e., carbonyl and amino groups, are likely to work as a stabilizer against stimuli to the molecule or crystal. This stabilizing nature might characterize the insensitivity of NTO. Molecular dynamic (MD) calculations were performed to reveal the high-pressure effect of the alpha-NTO crystal. The coordinates of individual atoms in the crystal structure were obtained using X-ray diffraction analysis. The pressure dependence of the power spectra of the correlation functions of the C=O bond length in NTO was calculated. A unique high-pressure effect of the alpha-NTO crystal was found on the power spectra. The peak frequency in the power spectrum of the C=O stretching vibration exhibited a red shift with an increase in pressure to 10.0 GPa, while the peak intensity considerably decreased under the same pressure process, because this bond length increased with an increase in pressure to 10.0 GPa. At a pressure of >20.0 GPa, a blue shift appeared. These results of the MD calculations are in good agreement with our experimental data.