Dissections: self-assembled aggregates that spontaneously reconfigure their structures when their environment changes

This Communication describes a new strategy for the design of adaptive structures based on reconfigurable mesoscale self-assembly. Several sets of millimeter-scale objects have been designed that can self-assemble into two different, regular aggregates at the interface between an aqueous solution and perfluorodecalin; the choice between the two aggregates is determined by the density of the aqueous phase.     

Synthesis of polycyclic guanidines by cyclocondensation reactions of N-amidinyliminium ions

A new method for the synthesis of polycyclic guanidines is described. The N-amidinyliminium ion generated from alpha-(phenylthio)amidine precursor 16 by reaction with Cu(OTf)(2) undergoes cyclocondensation with 1,3-dienes, styrenes, and beta-dicarbonyl compounds to give 1-iminohexahydropyrrolo[1,2-c]pyrimidines having side chains at C3 and C7. In all cases, major products have a cis relationship of the C7 side chain and angular C4a hydrogen, whereas C3 side chains are incorporated with lower stereoselectivity (dr = 2--5:1) in cyclocondensations with dienes and styrenes to give stereoisomer 39 as the major product. In contrast to most cycloadditions of alkenes with N-acyliminium ions, cyclocondensations of alkenes with N-amidinyliminium ions proceed by a stepwise pathway. Cyclocondensation of the cognate ureido aminal 31 with styrene provides the rare 2-imino-5,6-dihydro-4H-1,3-oxazine derivative 32, rather than a pyrimidine as the major product. The high stereoselectivity observed in condensations of 16 with benzyl acetoacetate to afford Biginelli adduct 29 supports the intermediacy of N-amidinyliminium ions in related tethered Biginelli condensations of guanidines reported earlier from our laboratories.     

Recent progress in lysosomal alpha-mannosidase and its deficiency

Lysosomal alpha-mannosidase (EC 3.2.1.24) is a major exoglycosidase in the glycoprotein degradation pathway. A deficiency of this enzyme causes the lysosomal storage disease, alpha-mannosidosis, which has been described in humans, cattle, domestic cats and guinea pigs. Recently, great progress has been made in studying the enzyme and its deficiency. This includes cloning of the gene encoding the enzyme, characterization of mutations related to the disease, establishment of valuable animal models, and encouraging results from bone marrow transplantation experiments.     

Simulation of jet-noise excitation in an acoustic progressive wave tube facility

Acoustic excitation produced by jet-engine effluxes was simulated in a progressive wave tube (APWT) facility with a computer-based control system. The APWT siren is driven by a signal generated numerically in a PC and then converted into analog form. Characteristics of the acoustic pressure measured by a microphone are analyzed in digital form and compared with those prescribed for simulation. Divergence is compensated by immediate modification of the driving signal and this action is repeated in the form of iterative process until the test specification is attained. Typical power spectral density (PSD) shapes with maxima at low and high frequencies were simulated. A "tailoring" approach has been also achieved when a test specification was determined directly from field measurements for the particular aircraft under consideration. Since acoustic pressure signals of high level differ from the Gaussian random process model, particularly in terms of asymmetric probability density function, a method has been developed to make the driving signal also non-Gaussian by simulating skewness and kurtosis parameters of the APWT acoustic excitation simultaneously with PSD control. Experimental results with Gaussian and non-Gaussian characteristics obtained for various PSD specifications including sharp and narrow peaks are presented in the paper.     

Asymmetric Palladium‐Catalyzed Alkene Carboamination Reactions for the Synthesis of Cyclic Sulfamides

The synthesis of cyclic sulfamides by enantioselective Pd‐catalyzed alkene carboamination reactions between N‐allylsulfamides and aryl or alkenyl bromides is described. High levels of asymmetric induction (up to 95:5 e.r.) are achieved using a catalyst composed of [Pd₂(dba)₃] and (S)‐Siphos‐PE. Deuterium‐labelling studies indicate the reactions proceed through syn‐aminopalladation of the alkene and suggest that the control of syn‐ versus anti‐aminopalladation pathways is important for asymmetric induction.     

Squeeze film pressures

Summary As parallel faces separated by a fluid film approach one another, a pressure is developed in the film. The reduced Navier-Stokes equations are solved by transform methods for both rectangular and cylindrical geometries. Relatively high pressures can exist at small asperities on the surfaces as they approach one another.     

Measurement of the positive muon lifetime and determination of the Fermi constant to part-per-million precision

We report a measurement of the positive muon lifetime to a precision of 1.0 ppm; it is the most precise particle lifetime ever measured. The experiment used a time-structured, low-energy muon beam and a segmented plastic scintillator array to record more than 2×10(12) decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give τ(μ(+)) (MuLan)=2?196?980.3(2.2) ps, more than 15 times as precise as any previous experiment. The muon lifetime gives the most precise value for the Fermi constant: G(F) (MuLan)=1.166?378?8(7)×10(-5) GeV(-2) (0.6 ppm). It is also used to extract the μ(-)p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling g(P).     

Phonon focusing of elastically scattered phonons in GaAs

The propagation of high-frequency phonons through crystals at low temperatures is characterized by both ballistic and diffusive processes. Ballistic propagation of heat pulses is highly anisotropic due to phonon focusing, while diffusive propagation is expected to be nearly isotropic in cubic crystals. By using phonon imaging techniques, we have attempted to identify the heat flux from ballistic and scattered phonons in GaAs. Comparison of this data to Monte Carlo calculations which incorporate elastic scattering shows that the flux from phonons scattered a few times in the bulk retains a significant degree of anisotropy. In particular, a sharp feature discovered by Stock, Ulbrich, and Fieseler and attributed to ballistic propagation of phonons with frequencies up to 1.5 THz is now identified with the scattering of sub-THz phonons. Our analysis provides insights into the evolution of heat propagation from the ballistic to diffusive regimes.     

Transport of degenerate electron-hole plasmas in Si and Ge

At low crystal temperatures, pulsed-laser excitation of Si and Ge can produce a mobile electron-hole plasma with a Fermi energy much larger than k_BT. The motion of this degenerate plasma away from the excitation surface depends intimately on its interactions with high-frequency phonons. Momentum damping and phonon-wind forces are principal factors which determine the plasma motion on nanosecond and longer time scales. A variety of luminescence and heat-pulse experiments are reviewed here which characterize the transport behavior of photo-generated electron-hole plasma in these indirect-gap semiconductors.