Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Computable error bounds for pointwise derivatives of a Neumann problem

In this paper we discuss the recovery of derivatives and thecomputation of rigorous and useful upper bounds for the pointwiseerror in the recovered derivatives, for finite element approximationsof the Laplace equation with Neumann boundary conditions, especiallyat points close to or on a smooth, curved boundary. We analyzethe dipole image technique for the case of curved boundaries,and show how to compute reliable recovered derivatives and errorbounds even in the limiting case of points lying on the curvedboundary. Numerical experiments show reasonably tight errorbounds for points both close to and away from a curved boundary.     

Optical properties of a nematic twist cell

We show that many of the complicated static and dynamic optical transmission characteristics of a Schadt-Helfrich twist cell, including the “bounce” in transmission that is observed at negative oblique angles of incidence after switching it off, can be predicted quite well from a very simple dynamic model. The model ignores flow and has only a viscous resistance to angular reorientation, in addition to the Ossen-Frank elastic properties. The bounce observed in some cases with normally incident light cannot be predicted by the model. As expected, the model yields equilibrium configurations at any applied voltage that give the observed transmission very accurately. Dynamic observations in the upper part of the nematic temperature range, where the observed bounce at normal incidence is small, are well reproduced by the model. At lower temperatures the discrepancies are larger. When interpreted with the model, both dynamic and static measurements concur in indicating that the directors of the liquid crystal adjacent to the surfaces assumed a more or less permanent tilt of a few degrees in our cell. Alignment was obtained by rubbing the conducting electrodes in one direction with linen cloth.     

Determination of the tilt angles at surfaces of substrates in liquid crystal cells

A method to determine tilt angles of nematic liquid crystal directors at the surfaces of parallel substrates is described for homeotropic and homogeneous orientation as well as for TN-cells. Graphs which can be used to evaluate the tilt angles for a cyano-biphenyl mixture from simple transmission measurements are given. The direction of extreme optical path difference between plane polarized modes tilts much more than the tilt angle of the molecules.