Showing papers by "ETH Zurich published in 1989"

The motion of a finite mass of granular material down a rough incline

Abstract: Rock, snow and ice masses are often dislodged on steep slopes of mountainous regions. The masses, which typically are in the form of innumerable discrete blocks or granules, initially accelerate down the slope until the angle of inclination of the bed approaches the horizontal and bed friction eventually brings them to rest. The present paper describes an initial investigation which considers the idealized problem of a finite mass of material released from rest on a rough inclined plane. The granular mass is treated as a frictional Coulomb-like continuum with a Coulomb-like basal friction law. Depth-averaged equations of motion are derived; they bear a superficial resemblance to the nonlinear shallow-water wave equations. Two similarity solutions are found for the motion. They both are of surprisingly simple analytical form and show a rather unanticipated behaviour. One has the form of a pile of granular material in the shape of a parabolic cap and the other has the form of an M-wave with vertical faces at the leading and trailing edges. The linear stability of the similarity solutions is studied. A restricted stability analysis, in which the spread is left unperturbed shows them to be stable, suggesting that mathematically both are possible asymptotic wave forms. Two numerical finite-difference schemes, one of Lagrangian, the other of Eulerian type, are presented. While the Eulerian technique is able to reproduce the M-wave similarity solution, it appears to give spurious results for more general initial conditions and the Lagrangian technique is best suited for the present problem. The numerical predictions are compared with laboratory experiments of Huber (1980) involving the motion of gravel released from rest on a rough inclined plane. Although in these experiments the continuum approximation breaks down at large times when the gravel layer is only a few particle diameters thick, the general features of the development of the gravel mass are well predicted by the numerical solutions.

Practical fast 1-D DCT algorithms with 11 multiplications

Abstract: A class of practical fast algorithms is introduced for the discrete cosine transform (DCT). For an 8-point DCT only 11 multiplications and 29 additions are required. A systematic approach is presented for generating the different members in this class, all having the same minimum arithmetic complexity. The structure of many of the published algorithms can be found in members of this class. An extension of the algorithm to longer transformations is presented. The resulting 16-point DCT requires only 31 multiplications and 81 additions, which is, to the authors' knowledge, less than required by previously published algorithms. >

Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode

Abstract: ROOTH1 proposed that the Younger Dryas cold episode, which chilled the North Atlantic region from 11,000 to 10,000 yr BP, was initiated by a diversion of meltwater from the Mississippi drainage to the St Lawrence drainage system. The link between these events is postulated to be a turnoff, during the Younger Dryas cold episode, of the North Atlantic's conveyor-belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the North Atlantic region2. This turnoff is attributed to a reduction in surface-water salinity, and hence also in density, of the waters in the region where North Atlantic Deep Water (NADW) now forms. Here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from Orca Basin core EN32-PC4 which reveal a significant reduction in meltwater flow through the Mississippi River to the Gulf of Mexico from about 11,200 to 10,000 radiocarbon years ago. This finding is consistent with the record for Lake Agassiz which indicates that the meltwater from the southwestern margin of the Laurentide Ice Sheet was diverted to the northern Atlantic Ocean through the St Lawrence valley during the interval from ~11,000 to 10,000 years before present (yr BP).

Stereospecific nuclear magnetic resonance assignments of the methyl groups of valine and leucine in the DNA-binding domain of the 434 repressor by biosynthetically directed fractional 13C labeling.

Abstract: Stereospecific 1H and 13C NMR assignments were made for the two diastereotopic methyl groups of the 14 valyl and leucyl residues in the DNA-binding domain 1-69 of the 434 repressor. These results were obtained with a novel method, biosynthetically directed fractional 13C labeling, which should be quite widely applicable for peptides and proteins. The method is based on the use of a mixture of fully 13C-labeled and unlabeled glucose as the sole carbon source for the biosynthetic production of the protein studied, knowledge of the independently established stereoselectivity of the pathways for valine and leucine biosynthesis, and analysis of the distribution of 13C labels in the valyl and leucyl residues of the product by two-dimensional heteronuclear NMR correlation experiments. Experience gained with the present project and a previous application of the same principles with the cyclic polypeptide cyclosporin A provides a basis for the selection of the optimal NMR experiments to be used in conjunction with biosynthetic fractional 13C labeling of proteins and peptides.

Cartilage contains mixed fibrils of collagen types II, IX, and XI.

Abstract: The distribution of collagen XI in fibril fragments from 17-d chick embryo sternal cartilage was determined by immunoelectron microscopy using specific polyclonal antibodies. The protein was distributed throughout the fibril fragments but was antigenically masked due to the tight packing of collagen molecules and could be identified only at sites where the fibril structure was partially disrupted. Collagens II and IX were also distributed uniformly along fibrils but, in contrast to collagen XI, were accessible to the antibodies in intact fibrils. Therefore, cartilage fibrils are heterotypically assembled from collagens II, IX, and XI. This implies that collagen XI is an integral component of the cartilage fibrillar network and homogeneously distributed throughout the tissue. This was confirmed by immunofluorescence.

Production of soluble recombinant proteins in bacteria

Abstract: Production of recombinant proteins in bacteria is limited by the formation of cytoplasmic aggregates (inclusion bodies or “IBs”). This review summarizes what is known about why IBs form and ways of increasing the production of soluble protein in bacterial systems. The easiest way to lower IB formation is to reduce the growth temperature of the bacteria. IB formation is not directly correlatable with the production rate, nor with the size of the produced protein. The primary sequences of a few proteins that do not form IBs at higher production temperatures contain either a low content of proline residues or stretches of acidic amino acids. Metal ion binding may also lower the tendency to form IBs at growth temperatures above 30°C. Three aspects of protein synthesis in mammalian cells, compartmentation, interprotein interactions (sortases, foldases, unfoldases, and chaperonins), and post-translational modifications, have significant effects on the solubility of the proteins produced. Possibilities for mimicking these mechanisms in bacteria via secretion, cloning of mammalian foldases, and mutation of the post-translational modification systems of the host bacteria are discussed.

Nature and site of phospholamban regulation of the Ca2+ pump of sarcoplasmic reticulum

Abstract: The rapid removal of Ca2+ ions from the cytosol, necessary for the efficient relaxation of cardiac muscle cells, is performed by the Ca2+-pumping ATPase of the sarcoplasmic reticulum. The calcium pump is activated by cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban, an integral membrane protein of the sarcoplasmic reticulum. Using a heterobifunctional crosslinking agent which can be cleaved and photoactivated, we provide evidence for a direct interaction between the two proteins. Only the non-phosphorylated form of phospholamban interacts with the ATPase, demonstrating that phospholamban is an endogenous inhibitor that is removed from the ATPase by phosphorylation. Non-phosphorylated phospholamban interacts only with the calcium-free conformation of the ATPase and is released when it is converted to the calcium-bound state. We localized the site of interaction to a single peptide isolated after cyanogen bromide cleavage of the ATPase. The peptide derives from a domain just C-terminal to the aspartyl phosphate of the active site. This domain is unique to ATPases of the sarcoplasmic reticulum in that it has no homology with any other phosphorylation-type ion pump. The domain occurs in both slow- and fast-twitch isoforms of the ATPase, even though phospholamban is not expressed in fast-twitch muscles.

Modern metabolism as a palimpsest of the RNA world.

Abstract: An approach is developed for constructing models of ancient organisms using data from metabolic pathways, genetic organization, chemical structure, and enzymatic reaction mechanisms found in contemporary organisms. This approach is illustrated by a partial reconstruction of a model for the "breakthrough organism," the last organism to use RNA as the sole genetically encoded biological catalyst. As reconstructed here, this organism had a complex metabolism that included dehydrogenations, transmethylations, carbon-carbon bond-forming reactions, and an energy metabolism based on phosphate esters. Furthermore, the breakthrough organism probably used DNA to store genetic information, biosynthesized porphyrins, and used terpenes as its major lipid component. This model differs significantly from prevailing models based primarily on genetic data.

High pressure freezing comes of age

Abstract: High pressure freezing permits the successful cryoimmobilization of thick biological specimens (up to approx. 500 microns). A very high yield of adequately frozen specimens, in which no segregation patterns due to ice crystal formation is apparent after freeze-substitution or freeze-fracturing, is obtained with suspensions of microorganisms as well as plant and animal tissue. This very high yield is attributed to an optimized transfer of pressure and cold to the biological specimen. This is achieved by replacement of extraspecimen water or buffer by 1-hexadecene, a chemically inert, hydrophobic paraffin oil of low viscosity and low surface tension.

Structure and Dynamics of Chiral Molecules

Abstract: Could there be chiral methane? What is the characteristic structural feature (in a physicochemical sense) of a molecule? This question dates back to Louis Pasteur, the discoverer of molecular chirality, and since the work of van’t Hoff and Le Be1 is generally considered by chemists as solved. In the present article it is pointed out that there exist fundamentally conflicting theoretical views of the physical origin of molecular chirality. These views predict consequences that could, in principle, be distinguished experimentally, but at present there is no conclusive experimental evidence available. Possible experiments are suggested that test different hypotheses. The importance of the magnitude of the parity-violating energy difference AEpv in molecules due to the weak nuclear force for both the structure and spectra of chiral molecules and for the kinetics of racemization is discussed. The chemical relaxation rate coefficient of chiral molecules with some appreciable energy of excitation is derived for several limiting cases of a simple statistical mechanical model, which takes AEpv into account.

Framework density distribution of zeolite-type tetrahedral nets

Abstract: The porosity of zeolite-type frameworks can be expressed in terms of the framework density (FD), defined as the number of tetra-hedral atoms per 1,000 A3. For the known zeolites and zeolite-type materials, FD values range from 12.5 to 20.2. Here we show that the range of the observed FD values depends on the type and relative number of the smallest rings in the tetrahedral networks. The frameworks of lowest density are those with a maximum number of 4-rings. The minimum framework density increases with the size of the smallest rings in the network. There is a clear gap in the FD values between zeolite and dense frameworks, and this also depends on the type of smallest rings present. These findings are of considerable predictive value in the important field of zeolites and molecular sieves; in particular, the minimum possible FD can be predicted for categories of frameworks defined by the smallest rings. The results of this study can serve as a guide for future efforts to synthesize low-density framework structures.

A General Equation for the Terminal Fall Speed of Solid Hydrometeors

Abstract: A comprehensive yet simple formula is presented for the terminal fall velocity of solid precipitation particles. It depends on three particle parameters: mass, the mean circumscribed area presented to the flow, and the mean effective projected area presented to the flow. This formula is deduced from a single mean Davies number-Reynolds number relation based on boundary layer theory and therefore includes environmental conditions (air density and temperature). Terminal velocity is predicted with errors ≲10% for a wide variety of particles, e.g., various planar and columnar crystals, rimed and unrimed aggregates, graupel (lump, conical, and hexagonal) and hail up to over 10cm in diameter. In view of its general form and the broad base of examination, the formula is felt to be generally applicable to all kinds of natural solid precipitation particles including shapes not tested in this study.

The −24/−12 promoter comes of age

Abstract: A new bacterial promoter type has been identified in the last few years. Originally designated as nif (= nitrogen fixation) or ntr (= nitrogen regulation) consensus promoter, it is now evident that this promoter occurs in many different bacterial species and is used not only for genes involved in nitrogen assimilation but also for genes determining many other unrelated metabolic functions. The general features of this type of promoter are (i) the conserved -24(GG)/-12(GC) consensus sequence, (ii) its recognition by a specific RNA polymerase sigma factor, sigma 54, which is encoded by the ntrA gene (synonyms: glnF, rpoN, rpoE), and (iii) the requirement for a transcriptional regulatory protein to activate the expression of the associated genes. In addition, many (but not all) of these genes possess a promoter-upstream activator sequence (enhancer) which is the target site for the binding of the activating protein and is required for maximal expression. In some cases, in which gene expression does not appear to be dependent on the presence of upstream binding sites, the activating protein may interact directly with the RNA polymerase-promoter complex. In conclusion, the expression from all -24/-12 consensus promoters known to date is positively controlled.

Induction and prevention of chondrocyte hypertrophy in culture

Abstract: Primary chondrocytes from whole chick embryo sterna can be maintained in suspension culture stabilized with agarose for extended periods of time. In the absence of FBS, the cells remain viable only when seeded at high densities. They do not proliferate at a high rate but they deposit extracellular matrix with fibrils resembling those of authentic embryonic cartilage in their appearance and collagen composition. The cells exhibit many morphological and biochemical characteristics of resting chondrocytes and they do not produce collagen X, a marker for hypertrophic cartilage undergoing endochondral ossification. At low density, cells survive in culture without FBS when the media are conditioned by chondrocytes grown at high density. Thus, resting cartilage cells in agarose cultures can produce factors required for their own viability. Addition of FBS to the culture media leads to profound changes in the phenotype of chondrocytes seeded at low density. Cells form colonies at a high rate and assume properties of hypertrophic cells, including the synthesis of collagen X. They extensively deposit extracellular matrix resembling more closely that of adult rather than embryonic cartilage.

Modification of nuclear lamin proteins by a mevalonic acid derivative occurs in reticulocyte lysates and requires the cysteine residue of the C-terminal CXXM motif

Abstract: The C-terminus of nuclear lamins (CXXM) resembles a C-terminal motif (the CAAX box) of fungal mating factors and ras-related proteins. The CAAX box is subject to different types of post-translational modifications, including proteolytic processing, isoprenylation and carboxyl methylation. By peptide mapping we show that both chicken lamins A and B2 are processed proteolytically in vivo. However, whereas the entire CXXM motif is cleaved from lamin A, at most three C-terminal amino acids are removed from lamin B2. Following translation of cDNA-derived RNAs in reticulocyte lysates, lamin proteins specifically incorporate a derivative of [14C]mevalonic acid (MV), i.e. the precursor of a putative isoprenoid modification. Remarkably, no MV is incorporated into lamin B2 translated from a mutant cDNA encoding alanine instead of cysteine in the C-terminal CXXM motif. These results implicate this particular cysteine residue as the target for modification of lamin proteins by an isoprenoid MV derivative, and they indicate that isoprenylation is amenable to studies in cell-free systems. Moreover, our observations suggest that C-terminal processing of newly synthesized nuclear lamins is a multi-step process highly reminiscent of the pathway elaborated recently for ras-related proteins.

Self-replicating micelles : a chemical version of a minimal autopoietic system

Abstract: Reverse micelles hosting the internal production of the surfactant are proposed as experimentally feasible models of simple (or ‘minimal’) autopoietic systems. We describe the conditions under which these may be formed and their possible biological implications. The micellar systems considered here turn out also to exhibit a capacity for self-reproduction through fragmentation under plausible conditions, thus constituting also a minimal experimental model for prebiotic self-reproduction.

Security enhancement using an optimal switching power flow

Abstract: The method presented allows the determination of the optimal and n-1 secure power system state by switching operations. A linear switching model is applied in an active way to model the control actions and in a more passive way to set up contingency constraints. Using the switching model in a dual role permits the integration of contingencies and control actions into one optimization algorithm. Thus, with exactly one optimization step the power system can be improved considering various security levels. A comparison is made between preventive conservative and postcontingency rescheduling. The main difference lies in the time-dependent performance of the topology changes in establishing a normal undisturbed system state if any contingency occurs. The preventive way of rescheduling has the advantage that there are no limit violations produced by a contingency. In postcontingency rescheduling short-term overloads are tolerated. This system state has the advantage of lower production costs. The performance of the algorithm is shown in a test case. >

Strong pressure dependence of Tc of the new 80 K phase YBa2Cu4O8+x

Abstract: The pressure dependence of the superconducting transition temperature Tc of the new structure YBa2Cu4O8 was measured on bulk material and single crystals as well. The differential dTc/dp was determined to be 0.55 K/kbar. Such a large change of Tc with increasing pressure was never observed before in compounds with a Tc of about 80 K. The results are discussed on the basis of Cryot's model and on polaronic based superconductivity.