Asymmetric synthesis of alpha,alpha-difluoro-beta-amino acid derivatives from enantiomerically pure N-tert-butylsulfinimines

Addition of the Reformatsky reagent derived from ethyl bromodifluoroacetate to alkyl- and aryl-substituted N-tert-butylsulfinimines furnishes beta-tert-butylsulfinamyl-beta-substituted alpha,alpha-difluoroproponiates in diastereomeric ratios ranging from 80:20 to 95:5. The diastereomers are easily separated and the enantiomerically pure, protected beta-amino esters are readily transformed to the corresponding acid, amide, and amine derivatives as useful synthons for medicinal chemistry targets.     

Simple cation-pi interaction between a phenyl ring and a protonated amine stabilizes an alpha-helix in water

Cation-pi interactions have been proposed to be important contributors to protein structure and function. In particular, these interactions have been suggested to provide significant stability at the solvent-exposed surface of a protein. We have investigated the magnitude of cation-pi interactions between phenylalanine (Phe) and lysine (Lys), ornithine (Orn), and diaminobutanoic acid (Dab) in the context of an alpha-helix and have found that only the Phe...Orn interaction provides significant stability to the helix, stabilizing it by -0.4 kcal/mol. This interaction energy is in the same range as a salt bridge in an alpha-helix, and equivalent to the recently reported Trp...Arg interaction in an alpha-helix, despite the fact that Trp...guanidinium interactions have been proposed to be stronger than Phe...ammonium interactions. These results indicate that even the simplest cation-pi interaction can provide significant stability to protein structure and demonstrate the subtle factors that can influence the observed interaction energies in designed systems.     

Temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO3

The temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO₃ is reported. We show that the competition between deep and shallow traps depends on temperature and writing intensity, and influences two-beam coupling and dark decay. The dynamics of dark decay, characterized by a fast decay of partial erasure and a subsequent slow decay, is influenced by the presence of deep and shallow traps. Partial erasure, due to thermal excitation of charges from the shallow traps, decreases with temperature and increases with writing intensity. The time constant of the slow decay, due to thermal excitation of charges from the deep traps, depends strongly on temperature, but not on the writing intensity. At room temperature, the existence of deep and shallow trap leads to intensity-dependent photorefractive gains. As temperature increases, the influence from the shallow trap decreases, and the photorefractive gain becomes independent of the intensity. However, at much higher temperatures (100°C), the photorefractive gain resumes its dependence on intensity due to an increase in dark conductivity at elevated temperature.     

The synthesis and conformational analysis of amino acid-tetrahydroanthranilic acid hybrids

The optimization of a palladium-catalyzed amidation reaction providing a new class of amino acid-tetrahydroanthranilic acid derivatives has been achieved. The scope of the reaction and preliminary conformational analysis of the resulting series of molecules is discussed.     

The bending recovery of polymer films. I. A phenomenological model

When a flat film is bent to some fixed curvature, held at this state for some time, and then released, its curvature is usually observed to drop instantaneously to some finite value and then gradually decrease with time. This phenomenon, generally referred to as bending recovery, has been modeled by simple extension of the classical plate-bending theory to linear-viscoelastic materials. The model assumes that the relaxation spectra in tension and compression are dissimilar but are interrelated through a simple temporal shift. Experimental data, including instantaneous recovery at ambient and elevated temperatures as well as recovery over extended times, have been generated for four films with widely different relaxation characteristics. The data are in reasonable agreement with predictions of the model if a constant value of the compressive shift parameter (see text) is used for all materials. The model indicates that the bending recovery of a polymer film is intimately related to its relaxation spectrum and can be effectively manipulated by changes in temperature and winding time.     

Synthesis of 5-lodo- and 5-nitro-3-indolyl phosphates as cytochemical substrates for acid phosphatase

The synthesis of 5-iodo-3-indolyl and 5-nitro-3-indolyl phosphate for cytochemical demonstration of acid phosphatase is reported. Enzymatic hydrolysis of these compounds liberates the corresponding indoxyl which is oxidized to the substituted indigo at an acid pH. In contrast, the 5-bromo and the unsubstituted indoxyl are not oxidized readily at an acid pH. Comparison of the UV and visible, spectra of the disubstituted indigo has offered a partial explanation for this difference. E.S.R. measurements showed that 5,5′-diiodoindigo, unlike 5,5′-dibromoindigo, gives a detectable radical ion at an acid pH.     

Nd: YAG laser annealing of gallium-implanted silicon

A Q-switched Nd: YAG laser with a pulse duration of 20 ns was used to investigate effects of laser annealing in gallium implanted silicon. Rutherford backscattering and Hall-effect measurements were performed to evaluate the annealed layer. Differential Hall-effect measurements were carried out to obtain carrier concentration profiles after annealing. It was found that a maximum sheet carrier concentration of 8×10¹⁵ cm⁻² can be obtained for a gallium implantation of 10¹⁶ cm⁻² by laser annealing with an energy density of more than 1.0 J cm⁻². Although the peak carrier concentration was found to be 8.0×10²⁰ cm⁻³, the annealed layer showed polycrystalline structures even after annealing with an energy density up to 4J cm⁻². The annealing took place in the solid phase in this energy density range.     

Likelihood inferences for the link function without knowing the true underlying distributions

This article is concerned with inference about link function in generalized linear models. A parametric and yet robust likelihood approach is introduced to accomplish the intended goal. More specifically, it is demonstrated that one can convert normal and gamma likelihoods into robust likelihood functions for the link function. The asymptotic validity of the robust likelihood requires only the existence of the second moments of the underlying distributions. The application of this novel robust likelihood method is demonstrated on the Box–Cox transformation. Simulation studies and real data analysis are provided to demonstrate the efficacy of the new parametric robust procedures.