ALBERT

Notes: One of the basic prerequisites for rational and high quality production of plastic parts is a tool layout tailored to the production process. To date, both design and construction have generally been based on values acquired by experience. This first, necessitates highly qualified personnel and second, involves what is frequently time consuming and costly finishing work. Experience acquired so far with computer-aided layout of injection molds shows that even a designer with little experience reaches the target more quickly and more reliably. At the same time he is able to draw on the results of intricate calculation and simulation methods which he was unable to apply in the past for reasons of time alone. This paper thus sets out the possibilities currently open and the experience available for computer-aided mold layout. The chief point of focus here is a system for the layout of injection molds. Working on from this system, however, the potentials for computer application are presented for blow mold- and foaming mold-design. After finding the mold principle, mold layout essentially divides up into two major areas, namely dimensioning calculations (CAE) and compilation of production documents (CAD). In dimensioning, the different functional elements of the mold are calculated. The aids that have been developed and the potentials of computer-aided dimensioning are presented with examples from the fields of rheological, thermal, and mechanical mold layout. Computer-aided rheological layout divides up into two steps. The first gives information on qualitative filling behavior (filling picture, flow paths) and the second provides quantitative results (pressures, shear stresses, temperatures). Computer-aided thermal layout similarly divides up into part steps. These are a rough overall energy balance, a rough layout of the tempering system, a segmented layout, and a homogeneity check, which involves simulating the temperature conditions in the mold by means of difference methods. When it comes to mechanical layout of the mold, programs are available for deformation calculations on basic cases and these will frequently be sufficient. For more complex cases of loading and deformation, a finite element program is used. Graphic data processing units can be used to supply extra facilities - first, to provide an aid for the simulation programs in dimensioning and second, to rationalize the compilation of the production documents. An illustration of a CAD workplace is given, incorporating the necessary computer configuration and peripherals. Compiling production documents is essentially a problem of variant design. The variants in this case are the individual components of the mold and a number of standardized accessories that can be called up as “standardized components.” The mold cavity, however, always has to be a free design. All programs are dialogue driven and are in a standardized manner so that even designers with no data processing experience can use the computer as an aid. The CAD/CAE system presented duly fulfils all these requirements. It allows the designer, at a single computer work station, to carry out both simulation and dimensioning calculations, to obtain information on material data, and to compile production documents on the basis of variant and free design. This provides the designer with a readily manageable aid and makes a considerable contribution towards improving the design result. Finally, the capacity of different computer concepts and the CAE/CAD/CAM systems on the market are discussed. For the future it will be possible to establish a computer-aided link between the different areas of design, from development of the molded part, via mold design and production, through setting the processing parameters of the injection molding machine.

Notes: Thermo-oxidative ageing processes were investigated in films (100 μm thick) made from isotactic polypropylene with different antioxidant concentrations between 0% and 0.1% using isothermal long-term differential thermal analysis (ILDTA). The end of the oven life of polypropylene coincided with the loss of mechanical properties which was confirmed by tensile tests at temperatures far below the crystallite melting range. Already prior to the beginning of autocatalytic decomposition, ILDTA experiments permits conclusions to be drawn on the thermal history of the specimens and statements made regarding the end of thermo-oxidative stability of the polymer, which is accompanied by a deterioration in the mechanical properties.In consequence of the correlation between oxidation time and antioxidant concentration the local antioxidant concentration in sheets or films can be measured by ILDTA. Thus, investigating the diffusion of an antioxidant in polypropylene using ILDTA is possible.

Notes: Infrared spectroscopy was used for the determination of the base-pairing content of four specific tRNAs in deuterium oxide solution. Infrared spectra were obtained in the 1750-1550 cm-1 region at various temperatures ranging from about 15 to 90°C. Melting curves were constructed by plotting the molar extinction coefficient at ν = 1657 cm-1 versus temperature. These transition curves enabled us to determine the ranges of temperature which correspond to the ordered (partially double-stranded) or randomly coiled structure of the tRNA. For a set of wavenumbers the extinction coefficients at these temperatures were used for the calculation of the base-pairing content. The procedure employed here is based on a method described earlier by Thomas [(1969) Biopolymers 7, 325-334]. For the conditions selected for this investigation (Mg2+-free D2O-buffer; 0.01M tris-DCl, 0.015M NaCl, pD 7.5) the results of this determination agree within the limits of errors with the number of base pairs predicted by the cloverleaf model.

Notes: Recently, it was suggested that parallel β-sheets have a significant dipole moment, in contrast to antiparallel sheets. Ab initio molecular-orbital (MO) calculations on parallel and antiparallel β-strands of tetra(Gly) show that they have very similar charge distributions. Interaction energies between two and three strands of tetra(Gly), obtained using the direct reaction field Hamiltonian, show that a particular choice of point charges is probably not crucial for calculating interactions within β-sheets, but that it might be for calculating interactions between these sheets and other parts of a protein, in particular, α-helices. The point-charge representation of our MO-SCF results will probably reduce the hazard of introducing artefacts in electrostatic calculations of protein conformational energies, provided the short-range interactions are treated in a more realistic way, i.e., such that intra- and interchain induction effects are included.

Notes: The solution structure of an α-amylase inhibitor, tendamistat, calculated from nmr data with the distance geometry program DISMAN is subjected to restrained energy minimization. To study the influence of force field parametrizations and the convergence behavior of refinement algorithms, two different programs were used. AMBER is an established software package including a steepest descent and/or conjugent gradient optimizer in the Euclidian space; the name AMBER also represents a force field. The program FANTOM (fast Newton-Raphson torsion angle energy minimizer) is a new restrained energy refinement implementation of the Newton-Raphson algorithm, which uses second derivatives of the conformational energy in dihedral angle space with the ECEPP/2 force field. For both programs the normal energy force field was supplemented with an additional potential of the form ΣA(di - ui)6 (if di 〉 ui), which enforces upper limits ui to selected distances di as measured by nmr. Improvements of the intramolecular interactions with a decrease of the internal energies of about 1000 kcal/mol could be achieved without increasing the distance constraint violations. The restrained energy refinements caused only small changes of the molecular geometries: The root mean square distance values for the backbone atoms between the initial DISMAN structure and the refined structures are about 0.5 Å for AMBER and about 0.7 Å for FANTOM. Local conformational changes during the restrained energy minimizations are analyzed with respect to hydrogen-bond formation, and with respect to comparisons of the solution structure and the crystal structure.

Notes: The collective internal dynamics of transfer RNAphe from brewer's yeast in solution was studied by depolarized dynamic light scattering (DDLS). Within the melting region of tRNA the depolarized spectra consist of two Lorentzian, where the narrow (slow) component describes the overall rotation of the macromolecule. The broad component is attributed to the collective reorientation of the bases within the biopolymer. At high temperature only this relaxation process is observed in the spectrum. The viscosity dependence of the collective internal relaxation process is described by the Stokes-Einstein-Debye equation for rotational diffusion. Estimates of the internal orientational pair correlation factor from the integral depolarized intensities of tRNAPhe solutions indicates that the observed dynamics correspond to the collective reorientation of approximately 5 bases. A comparison of the results presented with DDLS studies on the aggregation of the mononucleotide guanosine-5′-monophosphate confirms this result. For a further characterization of the relaxation process we studied the effect of hydrostatic pressure (1-1000 bar) on the depolarized spectra of tRNA. While other spectroscopic methods like nmr, fluorescence polarization anisotropy decay, or ESR give information about the very local motion of a single base within the DNA or RNA, this study shows that by DDLS one can characterize collective internal motions of macromolecules.

Notes: Photon correlation spectroscopy has been used to study the translational (D0)and internal dynamics of monodisperse linear and polydisperse superhelical plasmid-DNAs. Scattering vector dependent correlation functions were measured and analyzed with the inverse Laplace transform CONTIN written by S. Provencher. For scattering vectors (q) lower than 1.3 × 105cm-1, D0 can be separated from internal dynamics. Using the measured D0value, the q dependence of internal modes was determined. Extrapolation of the internal relaxation times for q → 0 yields the longest internal relaxation time t1. This time can be connected to the persistence length in terms of the Berg-Soda model, which describes the molecule as a semiflexible circular polymer with hydrodynamic interactions. The calculated length of 76 nm for DNA, I = 0.15 mol/L, is a little higher than the one obtained from static light scattering data without excluded volume corrections. A comparison of experimental and simulated correlation functions for the Berg-Soda model shows that the model gives a fairly good description of the dynamics of the linear molecule, whereas large discrepancies between model and experimental functions are observed for the superhelical DNA. Small differences between model and experimental functions are mainly attributed to the neglect of the torsional modes that may be coupled to bending and flexing modes. For the superhelical DNA the agreement is improved if the calculation is carried out with a linear molecule, with shorter contour length and increased diameter. Both quantities can be derived from the known superhelix tilt angle.

Notes: The enthalpy change accompanying the double helix-coil transition of polyriboadenylic acid (poly A) in aqueous solution has been measured optically and calorimetrically in the pH range 5.7-4.5. The course of this cooperative transition was followed optically by measuring changes in ultraviolet absorption as a function of temperature at different pH values, and calorimetrically by determining the heat capacity of the solution through the transition region. From the latter measurements, the enthalpy of transition was calculated. It is shown, that ΔH is dependent on pH as it is expected from the influence of protonation of the double helix of poly A.