Interaction of two parallel plane jets of different velocities

The interaction of two parallel plane jets of different velocities is studied by flow visualization and PIV measurement to examine the influence of velocity ratio on the development ofjets in the initial region. It is found that the parallel plane jets develop toward the high velocity side and the jet width is reduced with a decrease in the jet velocity ratio. Corresponding to the variation of mean velocity field to the velocity ratio, the magnitudes of turbulence intensities, Reynolds stress and static pressure are weakened in the merging region of the jets and their peak locations of the properties are shifted to the high velocity side. These results indicate that the interaction of two parallel jets is weakened with a decrease in the velocity ratio of the jets.     

Excess molar heat capacities of ((<Emphasis Type="Italic">R</Emphasis>)-(+)-α-pinene+(<Emphasis Type="Italic">S</Emphasis>)-(-)- α-pinene) at temperatures between 293.15-308.15 K

Summary {\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs20 New explanations are given for two types of irregular thermogravimetric (TG) data. A TG relationship between mass and temperature is derived on the basis of migration behavior of bubbles generated in bulk of sample system, and superposed on that derived on the basis of kinetics of the 4\super th\nosupersub order event, which is superposed on the experimental TG data obtained from three reference papers. This suggests that these TG data are reflecting migration of bubbles. A dependence of TG behavior on heating rate, which is contrary to usual that, is shown and is explained in terms of event-rate determination by boiling. \par }     

Synthesis and Conformational Switching of Partially and Differentially Bridged Resorcin[4]arenes Bearing Fluorescent Dye Labels. Preliminary Communication

We report the synthesis of modified Cram‐type cavitands bearing one or two fluorescent labels for single‐molecule spectroscopic studies of vase? kite conformational switching (Scheme 3). Syntheses were performed by stepwise bridging of the four couples of neighboring H‐bonded OH groups of resorcin[4]arene bowls (Schemes 2 and 3). The new substitution patterns enable the construction of a large variety of future functional architectures. ¹H‐NMR Investigations showed that the new partially and differentially bridged cavitands feature temperature‐ and pH‐triggered vase? kite conformational isomerism similar to symmetrical cavitands with four identical quinoxaline bridges (Table). It was discovered that vase? kite switching of cavitands is strongly solvent‐dependent.     

Thermody namic functions of 1,2-alkanediols in dilute aqueous solutions

The heat capacities of binary aqueous solutions of 1,2-ethanediol, 1,2-propanediol and 1,2-butanediol were measured at temperatures ranging from 283.15 to 338.15 K by differential scanning calorimetry. The partial molar heat capacities at the infinite dilution were then calculated for the respective alkanediols. For 1,2-ethanediol or 1,2-propanediol, the partial molar heat capacities at the infinite dilution of increased with increasing temperature. In contrast, the partial molar heat capacities of 1,2-butanediol at the infinite dilution decreased with increasing temperature. Heat capacity changes by dissolution of the alkanediols were also determined. Heat capacity changes caused by the dissolution of 1,2-ethanediol or 1,2-propanediol were increase with increasing temperature. On the other hand, heat capacity changes caused by the dissolution of 1,2-butanediol are decrease with increasing temperature. Thus our results indicated that the structural changes of water caused by the dissolution of 1,2-butanediol differed from that of the two other alkanediols. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enthalpies of mixing of <Emphasis Type="Italic">o</Emphasis>- and <Emphasis Type="Italic">m</Emphasis>-isomers at 298.15 K

Excess enthalpies (H ^E) of 17 binary mixtures of o- and m-isomers of dichlorobenzene, difluorobenzene, methoxymethylbenzene, dimethylbenzene, dimethoxybenzene, aminofluorobenzene, fluoronitrobenzene, diethylbenzene, chlorofluorobenzene, fluoroiodobenzene, bromofluorobenzene, chloromethylbenzene, fluoromethylbenzene, bromomethylbenzene, iodomethylbenzene, fluoromethoxybenzene, dibromobenzene at 298.15 K were measured. All excess enthalpies measured were very small, and those of o-+m-isomers of aminofluorobenzene, dibromobenzene and iodomethylbenzene were negative but 14 other binary mixtures of isomers were positive over the whole range of mole fractions. H ^E of o-+m-isomers of dimethoxybenzene showed the largest enthalpic instability and those of aminofluorobenzene showed the largest enthalpic stability. There was a correlation between dipole–dipole interaction, dipole–induced dipole interaction or entropies of vaporization and excess partial molar enthalpies at infinite dilution.     

Engineered biosynthesis of plant polyketides: chain length control in an octaketide-producing plant type III polyketide synthase

The chalcone synthase (CHS) superfamily of type III polyketide synthases (PKSs) produces a variety of plant secondary metabolites with remarkable structural diversity and biological activities (e.g., chalcones, stilbenes, benzophenones, acrydones, phloroglucinols, resorcinols, pyrones, and chromones). Here we describe an octaketide-producing novel plant-specific type III PKS from aloe (Aloe arborescens) sharing 50-60% amino acid sequence identity with other plant CHS-superfamily enzymes. A recombinant enzyme expressed in Escherichia coli catalyzed seven successive decarboxylative condensations of malonyl-CoA to yield aromatic octaketides SEK4 and SEK4b, the longest polyketides known to be synthesized by the structurally simple type III PKS. Surprisingly, site-directed mutagenesis revealed that a single residue Gly207 (corresponding to the CHS's active site Thr197) determines the polyketide chain length and product specificity. Small-to-large substitutions (G207A, G207T, G207M, G207L, G207F, and G207W) resulted in loss of the octaketide-forming activity and concomitant formation of shorter chain length polyketides (from triketide to heptaketide) including a pentaketide chromone, 2,7-dihydroxy-5-methylchromone, and a hexaketide pyrone, 6-(2,4-dihydroxy-6-methylphenyl)-4-hydroxy-2-pyrone, depending on the size of the side chain. Notably, the functional diversity of the type III PKS was shown to evolve from simple steric modulation of the chemically inert single residue lining the active-site cavity accompanied by conservation of the Cys-His-Asn catalytic triad. This provided novel strategies for the engineered biosynthesis of pharmaceutically important plant polyketides.     

Electronic structure and reactivity of high-spin iron--alkyl- and--pterinperoxo complexes

The spectroscopic properties and electronic structure of the four-coordinate high-spin [FeIII(L3)(OOtBu)]+ complex (1; L3 = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate; tBu = tert-butyl) are investigated and compared to the six-coordinated high-spin [Fe(6-Me3TPA)(OHx)(OOtBu)]x+ system (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2) studied earlier [Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 12802-12816]. Complex 1 is characterized by Raman features at 889 and 830 cm-1 which are assigned to the O-O stretch (mixed with the symmetric C-C stretch) and a band at 625 cm-1 that corresponds to nu(Fe-O). The UV-vis spectrum shows a charge-transfer (CT) transition at 510 nm from the alkylperoxo pi v* (v = vertical to C-O-O plane) to a d orbital of Fe(III). A second CT is identified from MCD at 370 nm that is assigned to a transition from pi h* (h = horizontal to C-O-O plane) to an Fe(III) d orbital. For the TPA complex the pi v* CT is at 560 nm while the pi h* CT is to higher energy than 250 nm. These spectroscopic differences between four- and six-coordinate Fe(III)-OOR complexes are interpreted on the basis of their different ligand fields. In addition, the electronic structure of Fe-OOPtn complexes with the biologically relevant pterinperoxo ligand are investigated. Substitution of the tert-butyl group in 1 by pterin leads to the corresponding Fe(III)-OOPtn species (2), which shows a stronger electron donation from the peroxide to Fe(III) than 1. This is related to the lower ionization potential of pterin. Reduction of 2 by one electron leads to the Fe(II)-OOPtn complex (3), which is relevant as a model for potential intermediates in pterin-dependent hydroxylases. However, in the four-coordinate ligand field of 3, the additional electron is located in a nonbonding d orbital of iron. Hence, the pterinperoxo ligand is not activated for heterolytic cleavage of the O-O bond in this system. This is also evident from the calculated reaction energies that are endothermic by at least 20 kcal/mol.