Measurement and control of reagent concentrations in MOCVD reactor using ultrasonics

An ultrasonic cell for monitoring gas mixtures on MOCVD reactors has been developed from being a laboratory instrument to a standard, rack-mounted, system which interacts with the reactor's computer, and is marketed under the trade name of EPISON. A feedback circuit has been implemented to stabilize the TMI mole fraction in the reactor. The effectiveness of the feedback circuit has been tested by growing InGaAs layers, during which the TMI concentration was deliberately perturbed by changing the pressure of the TMI line. Using feedback correction an X-ray rocking-curve peak with a FWHM of 25 arc sec was obtained for a layer grown with the same perturbations which had previously led to a 400 arc sec FWHM without feedback.     

Nonadiabatic alignment of asymmetric top molecules: field-free alignment of iodobenzene

Nonadiabatic laser alignment of an asymmetric top molecule is studied using the combination of a quantum dynamical theory and time-resolved photofragment imaging experiments. In particular, the degree of alignment of iodobenzene, induced by an intense, linearly polarized picosecond laser pulse, is calculated and measured. Pronounced alignment is obtained under field-free conditions.     

Selection of a cyclic nonapeptide inhibitor to α-chymotrypsin using a phage display peptide library

A cyclic nonapeptide library displayed on filamentous bacteriophages was selected 6 times against α-chymotrypsin (EC 3.4.21.1) at three different pH conditions (6.5, 7.0, and 7.5). Phage peptide clones from the sixth selection, at all three pH conditions, interacted more strongly with α-chymotrypsin than the original library and a wild-type phage did. DNA sequencing of the selected phage peptide clones showed that different cyclic nonapeptide sequences had been selected at the different pH conditions. The oxidized form of the synthetic peptide, Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys, selected at pH 7.5, could completely inhibit the enzymatic activity of α-chymotrypsin. The structurally related enzymes trypsin (bovine) and elastase (porcine) were only marginally inhibited by the same peptide under the same conditions. The inhibition constant for α-chymotrypsin was estimated to be 10^-6 M. Phage clones expressing this peptide had a lower affinity for phenylmethylsulfonylfluoride-modified α-chymotrypsin than for natural α-chymotrypsin as determined by an enzyme immunosorbent assay. This peptide phage clone was also competitively prevented from binding to α-chymotrypsin by the corresponding synthetic oxidized peptide. Collectively, the results suggest that the oxidized form of the selected peptide Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys interacts with the active site of α-chymotrypsin and acts as a specific inhibitor to the enzyme. To our knowledge, the selected sequence Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys has not been found in nature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Delay Time Models of Industrial Inspection Maintenance Problems

The technique of delay time analysis is here extended to problems of industrial plant maintenance. A basic model of inspection maintenance is presented, along with models of some of the variations which have been observed in practice. Numerical examples indicate both the nature and sensitivity of the output from the models.     

Constructing, deconstructing, and reconstructing ternary supermolecules

A hypothesis-driven protocol comprising precise and predictable molecular recognition events based upon an electrostatic view of competing hydrogen-bond interactions is proposed and subsequently employed in the construction of ternary co-crystals.     

Combining halogen bonds and hydrogen bonds in the modular assembly of heteromeric infinite 1-D chains

Hydrogen bonds and halogen bonds operate in concert in the directed assembly of infinite 1-D chains in binary co-crystals of iodine iso-nicotinamide (2 : 2), 1 and tetrafluorodiiodobenzene iso-nicotinamide (1 : 2) 2.     

Oxime decorated cavitands functionalized through solvent-assisted grinding

In order to assemble supramolecular capsules, there is a need for reliable and effective synthetic methods for decorating cavitand-based host structures with appropriate functional groups. The synthesis of four different cavitands of significantly different depth and interior volume functionalized with four aldoxime groups capable of forming homomeric or heteromeric capsules through hydrogen bonding is reported. The final step in each synthesis, the aldehyde to oxime transformation, has been achieved in excellent yields through 'solvent assisted grinding'.     

Excess thermodynamic properties of thin water films confined between hydrophobized gold surfaces

Surface forces between gold surfaces were measured in pure water at temperatures in the range of 10-40 °C using an atomic force microscope (AFM). The surfaces were hydrophobized by self-assembly of alkanethiols (C(n)SH) with n=2 and 16 in ethanol solutions. The data were used to determine the changes in excess free energies (ΔG(f)) of the thin water films per unit area by using the Derjaguin approximation [1]. The free energy data were then used to determine the changes in excess film entropy (ΔS(f)) and the excess film enthalpy (ΔH(f)) per unit area. The results show that both ΔS(f) and ΔH(f) decrease with decreasing film thickness, suggesting that the macroscopic hydrophobic interaction involves building some kind of structures in the intervening thin films of water. It was found that |ΔH(f)|>|TΔS(f)|, which is a necessary condition for an attractive force to appear when the enthalpy and entropy changes are both negative. That macroscopic hydrophobic interaction is enthalpically driven is contrary to the hydrophobic interactions at molecular scale. The results obtained in the present work are used to discuss possible origins for the long-range attractions observed between hydrophobic surfaces.