Unconstrained Direct Optimization of Spacecraft Trajectories Using Many Embedded Lambert Problems

Journal of Optimization Theory and Applications - Direct optimization of many-revolution spacecraft trajectories is performed using an unconstrained formulation with many short-arc, embedded...     

Selectively N‐Protected Enantiopure 2,5‐Disubstituted Piperazines: Avoiding the Pitfalls in Solid‐Phase Fukuyama–Mitsunobu Cyclizations

Robust protocol! A diverse array of piperazine scaffolds was obtained by a robust solid‐phase‐synthesis protocol involving multistep elaboration of a resin‐bound aziridine (see scheme). Microwave‐assisted on‐resin protectinggroup introduction and manipulation as well as intramolecular Fukuyama–Mitsunobu cyclization conditions were key features.

     

Measurement of mechanical input power with an application to wall structures

This paper describes a simple method for measuring mechanical input power to a structure by direct analogue multiplication and integration of force and velocity signals. The influence of phase errors is controlled through measurement of both the real and the imaginary power components. The method is applied to some measurements on partition walls and agreement is obtained between input power and dissipated power computed from velocity and loss factor measurement on the wall surfaces.It is argued that the method is also applicable for a steady state measurement of the loss factor and, furthermore, that the power conversion efficiency, ζ, the ratio between the radiated sound power and input power, is a most relevant factor for characterising the radiation properties of mechanically excited partition walls.     

Oxidation profiles of thermally aged nitrile rubber

The thermal degradation of a commercial, stabilized, unfilled nitrile (Buna-N) rubber material was investigated at temperatures in the range 85–140 °C. The resulting heterogeneous oxidation, due to diffusion limitations in oxygen availability, was studied using infrared microscopy and modulus profiling. Degradation-related spectral changes were observed primarily in the hydroxyl, carbonyl and ester regions; quantitative analysis revealed identical oxidation profiles for these chromophores. These chemical oxidation profiles (carbonyl formation) were correlated with mechanical modulus (hardness) profiles. Degradation of the sample proceeds via a linear increase in the carbonyl concentration, but an exponential increase in the modulus with time. It is concluded that the profile development and aging behavior can be described by a diffusion-limited autoxidation mechanism which can be modeled computationally. The results are compared to those of a previously studied carbon-black-filled material.     

Structure of microcin J25, a peptide inhibitor of bacterial RNA polymerase, is a lassoed tail

Microcin J25 (MccJ25) is a 21-amino acid peptide inhibitor active against the DNA-dependent RNA polymerase of Gram negative bacteria. Previously, the structure of MccJ25 was reported to be a head-to-tail circle, cyclo(-G(1)GAGHVPEYF(10)VGIGTPISFY(20)G-). On the basis of biochemical studies, mass spectrometry, and NMR, we show that this structure is incorrect, and that the peptide has an extraordinary structural fold. MccJ25 contains an internal lactam linkage between the alpha-amino group of Gly1 and the gamma-carboxyl of Glu8. The tail (Tyr9-Gly21) passes through the ring (Gly1-Glu8), with Phe19 and Tyr20 straddling each side of the ring, sterically trapping the tail in a noncovalent interaction we call a lassoed tail.     

Iron-catalyzed cross-coupling of imidoyl chlorides with Grignard reagents

[reaction: see text] A general, high yielding rapid iron-catalyzed cross-coupling reaction between Grignard reagents and imidoyl chlorides is described. These reactions are typically completed within 5 min, resulting in high yields of 71-96% using 5% iron catalyst in a THF-NMP solvent mixture. Functionalized imidoyl chlorides (e.g., R = CO(2)Me) gave excellent yields (89%).