Tubulin rings: curved filaments with limited flexibility and two modes of association

Tubulin rings have been previously identified as composed of linear polymers of tubulin subunits, equivalent to a protofilament in the microtubule wall but in a curved rather than a straight conformation. We have examined and measured a number of different ring structures obtained under different conditions. The preferred curvature is indicated by a single ring of 380 A outside diameter. Radially double rings consist of two coplanar rings of 460 A and 350 A outside diameter, held together by a pattern of eight identical contacts between the 40 A subunits in the inner and outer rings. In some circumstances a larger ring, 570 A diameter, can be added to the outside, or a smaller ring, 240 A diameter, may be added to the inside of the radially double ring, in both cases repeating the pattern of eight radial contacts. The distortion of the filament from its relaxed 380 A diameter curvature apparently can be made without disrupting the longitudinal bond between subunits in the filament, but must be stabilized by the energy of the radial contact. All of these rings (single and radially double and triple) are observed to associate axially to form pairs or in some cases larger stacks. The radially double rings or an axially associated pair of these (quadruple ring) may also associate to form crystals. These are thin plates, up to 100 micrometers in extent and several micrometers thick which have been of limited use so far in diffraction studies because of irregularities in the packing of adjacent rings.     

1,2-Cycloheptanedionedioxime as a micro-gravimetric reagent for nickel

Summary 1,2-Cycloheptanedionedioxime (heptoxime), previously reported as a water-soluble gravimetric reagent for nickel, has been applied succesfully to the micro-gravimetric determination of this ion. The procedure, modified from the macro-determination, consists of precipitating nickelheptoxime in a micro-filter beaker and weighing the precipitate after drying at 110–120° C. The commonly encountered cations and metallic ions do not interfere or may be masked by complexing agents. The method has been tested on standard nickel solutions and standard steel samples.

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Zusammenfassung Über die Eignung des 1,2-Cycloheptandiondioxims (Heptoxim) als wasserlösliches Reagens für die gravimetrische Bestimmung von Nickel wurde in einer früheren Arbeit berichtet. Nun wird gezeigt, daß es sich auch für die Mikrobestimmung des Nickels eignet. Das Nickelheptoxim wird in einem Filterbecher gefällt und nach Trocknen bei 110 bis 120° C gewogen. Die gewöhnlich das Nickel begleitenden Metalle hindern die Bestimmung entweder nicht oder können durch Überführung in geeignete Komplexe zufriedenstellend maskiert werden. Das Mikroverfahren wurde an Nickellösungen bekannten Gehaltes und an Stahlproben des National Bureau of Standards geprüft.

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Résumé On a rapporté dans un travail précédent, la possibilité d'emploi de la cyclo-1,2 heptanedionedioxime (heptoxime) comme réactif hydrosoluble pour le dosage gravimétrique du nickel. On a montré maintenant qu'elle est convenable pour le microdosage du nickel. L'heptoximate de nickel est précipité au bécher-filtre et pesé après séchage à 110–120°. Les métaux qui accompagnent habituellement le nickel, ou bien ne gênent pas ou bien peuvent être masqués d'une façon satisfaisante. Le procédé microchimique a été essayé sur des solutions de nickel de titre connu ou sur des échantillons d'acier du National Bureau of Standards.

     

Intraatomic exchange and the violation of Hund's rule in twisted ethylene

It is demonstrated that the reason the singlet state lies below the triplet state in twisted ethylene (in violation of Hund's rule) is because there remains a significant component of the antiferromagnetic coupling of two triplet methylenes optimal for the methylene—methylene separated limit. We find that this effect can be completely described using a simple generalized valence bond wavefunction.     

Models for capacitated lot-sizing problem with backlogging,setup carryover and crossover

Setup operations are significant in some production environments. It is mandatory that their production plans consider some features, as setup state conservation across periods through setup carryover and crossover. The modelling of setup crossover allows more flexible decisions and is essential for problems with long setup times. This paper proposes two models for the capacitated lot-sizing problem with backlogging and setup carryover and crossover. The first is in line with other models from the literature, whereas the second considers a disaggregated setup variable, which tracks the starting and completion times of the setup operation. This innovative approach permits a more compact formulation. Computational results show that the proposed models have outperformed other state-of-the-art formulation.     

Stability and convergence of the spectral Lagrange-Galerkin method for mixed periodic/non-periodic convection-dominated diffusion problems

We present a convergence analysis of the spectral Lagrange-Galerkinmethod for mixed periodic/non-periodic convection-diffusionproblems. The scheme is unconditionally stable, independentof the diffusion coefficient, even in the case when numericalquadrature is used. The theoretical predictions are illustratedby a series of numerical experiments. For the periodic case,our results present a significant improvement on those givenby Süli & Ware (1991) SIAM J. Numer.Anal.28, 423-445).     

Structure and mobility of defects formed from collision cascades in MgO

We study radiation-damage events in MgO on experimental time scales by augmenting molecular dynamics cascade simulations with temperature accelerated dynamics, molecular statics, and density functional theory. At 400 eV, vacancies and mono- and di-interstitials form, but often annihilate within milliseconds. At 2 and 5 keV, larger clusters can form and persist. While vacancies are immobile, interstitials aggregate into clusters (In) with surprising properties; e.g., an I4 is immobile, but an impinging I2 can create a metastable I6 that diffuses on the nanosecond time scale but is stable for years.     

Synchronization of trajectories in canonical molecular-dynamics simulations: observation, explanation, and exploitation

For two methods commonly used to achieve canonical-ensemble sampling in a molecular-dynamics simulation, the Langevin thermostat and the Andersen [H. C. Andersen, J. Chem. Phys. 72, 2384 (1980)] thermostat, we observe, as have others, synchronization of initially independent trajectories in the same potential basin when the same random number sequence is employed. For the first time, we derive the time dependence of this synchronization for a harmonic well and show that the rate of synchronization is proportional to the thermostat coupling strength at weak coupling and inversely proportional at strong coupling with a peak in between. Explanations for the synchronization and the coupling dependence are given for both thermostats. Observation of the effect for a realistic 97-atom system indicates that this phenomenon is quite general. We discuss some of the implications of this effect and propose that it can be exploited to develop new simulation techniques. We give three examples: efficient thermalization (a concept which was also noted by Fahy and Hamann [S. Fahy and D. R. Hamann, Phys. Rev. Lett. 69, 761 (1992)]), time-parallelization of a trajectory in an infrequent-event system, and detecting transitions in an infrequent-event system.