Increasing Generation Number Research Articles

Visualization of Dendrimer Molecules by Transmission Electron Microscopy (TEM): Staining Methods and Cryo-TEM of Vitrified Solutions

Individual dendrimer molecules of poly(amidoamine) (PAMAM) from generation 10 (G10) to G5 were imaged by conventional transmission electron microscopy (TEM) after staining with aqueous sodium phosphotungstate. The dendrimers were resolved as separate, beam-stable entities, and their sizes and distribution of sizes were statistically analyzed and compared with data from small-angle X-ray scattering in solution. Aqueous G10 dendrimers were also characterized by examination in the frozen, hydrated state after quench-freezing in liquid ethane (cryo-TEM) and the data compared with the results of the staining experiments. To a first approximation, the dendrimers appeared circular in projection and the diameters conformed to a Gaussian distribution which broadened somewhat with increasing generation number. Cryo-TEM, in general, confirmed the staining experiments but suggested that, in the native state, there is more variability in the shapes of the dendrimers, with polyhedral shapes occurring quite frequently.

Structure and Thermodynamics of Homogeneous-Dendritic-Polymer Solutions: Computer Simulation, Integral-Equation, and Lattice-Cluster Theory

We present some calculated structural and thermodynamic properties of homogeneous-dendritic-polymer solutions using computer simulation methods, integral-equation theory, and lattice-cluster theory. Monte-Carlo methods are used to sample conformations of polymer molecules. From these conformations, we first compute two properties of the polymer: the distribution of segments within the molecule and the radius of gyration. Simulations for nonattracting polymer pairs give the potential of mean force and the second virial coefficient. Given the potential of mean force between polymer molecules, we use integral-equation theory to calculate the equation of state of an athermal solution at low polymer concentrations. We apply lattice-cluster theory to obtain solvent activities and liquid- liquid equilibria for homogeneous-dendritic polymers in nonathermal concentrated solution. There is little difference between the vapor pressures of solutions of linear polymers and homogeneous-dendritic polymers. However, there is a modest difference between the liquid-liquid coexistence curve for linear-polymer solutions and homogeneous-dendrimer solutions. The critical temperatures of dendrimer solutions are lower than those of solutions containing corresponding linear polymers. This difference rises with increasing generation number and decreasing separator length.

Characterization of Pyrene-Labeled Arborescent Polystyrenes Using Fluorescence Quenching Techniques

The chain segmental density and diffusional properties of arborescent (highly branched) polystyrenes were investigated by labeling the polymers with pyrene and observing their quenching behavior. A series of arborescent polymers with short (5000 g·mol-1) branches and a linear polymer were investigated. Nitrobenzene and nitrated linear polystyrene were used as quenchers for the pyrene fluorescence. For the nitrobenzene quenching experiments, quencher diffusion coefficients were calculated using the Smoluchowski equation. Comparison of the diffusion coefficients indicates a reduced diffusion rate for small molecules inside arborescent polystyrenes relative to the linear polymer and a decreasing trend with increasing generation number. Quenching experiments with linear nitrated polystyrene led to downward curvature in the Stern−Volmer plots, suggesting that a portion of the chromophores were not accessible to the quencher groups. The fractional Stern−Volmer model was applied, and the quenching rate constants and fraction of accessible chromophores were obtained. The arborescent samples had reduced quenching rate constants, and the fraction of accessible chromophores decreased for higher generation polymers.

Shapes of Dendrimers from Rotational-Echo Double-Resonance NMR

The solid-state shape, size, and intermolecular packing of a fifth-generation dendritic macromolecule were determined by a combination of site-specific stable-isotope-labeling, rotational-echo double-resonance (REDOR) NMR and distance-constrained molecular dynamics simulations. REDOR experiments measured dipolar couplings between 13C atoms located near the chain ends and an 19F label placed at the core of benzyl ether dendrimers (generations 1−5) based on 3,5-dihydroxybenzyl alcohol as the monomeric repeat unit. Intramolecular 13C−19F coupling was distinguished from intermolecular coupling by dilution with nonlabeled dendrimer. The average intramolecular 13C−19F distances for generations 3−5 were each approximately 12 Å, which indicates inward-folding of chain ends with increasing generation number. The average intermolecular 13C−19F dipolar coupling decreased with increasing generation number, consistent with decreased interpenetration for larger dendrimers. The measured intra- and intermolecular distances for the fifth-generation dendrimer were used as constraints on energy minimizations and molecular dynamics simulations, which resulted in visualizations of the dendrimer packing and an estimate of density in the solid state.

Molecular Dynamics Study of Dendrimer Molecules in Solvents of Varying Quality

The properties of dendrimers (“starburst” molecules) under varying solvent conditions are studied using molecular dynamics simulations. The dendrimers are found to have a compact (space filling) structure under all solvent conditions, with a radius of gyration which scales with the number of monomers as RG ∝ N1/3. For high generation number dendrimers, there is a distinct region of constant monomer density. The density in this region depends only on the solvent quality and is independent of the generation number. When the contributions of the different generations to the overall density profile are separated, we find that the monomers which belong to the first few generations are stretched and spatially localized. Later generations are less localized and penetrate well into the central regions of the dendrimer. The different primary branches (“dendrons”) of the dendrimers are found to be segregated. The amount of spatial overlap between the different dendrons decreases with increasing generation number of the dendrimer and increasing solvent quality. The relaxation times of the fluctuations of the internal structure were evaluated as well, to ensure that the simulations were run long enough for adequate sampling.

Dendrophanes: Water‐Soluble Dendritic Receptors. Preliminary communication

AbstractThe dendritic cyclophanes (dendrophanes )1–3 containing a [6.1.6.1]paracyclophane as the initiator core embedded in dendritic poly(ether‐amide) shells of first (1), second (2), and third (3) generation were prepared and characterized. The X‐ray crystal‐structure analyses of esters 7 and 4, derivatives of cyclophane core 9 and first‐generation dendrophane 1, respectively, displayed open cavity binding sites suitable for the inclusion complexation of aromatic substrates. With their carboxylate surface groups, dendrophanes 1–3 were readily soluble in aqueous phosphate buffer (pH 8.0), and the complexation of naphthalene derivatives was investigated by 1H‐NMR and fluorescence titrations. The binding studies demonstrated that the cyclophane cavity remains open and accessible to appropriate substrates even at higher dendritic generations. The 1:1 complexes formed in aqueous buffer were of similar stability to those formed by the unbranched core 9 (Ka between 1000 and 10000 1 mol−1, 300 K). Investigations with the fluorescent probe 6‐(p‐toluidino)naphthalene‐2‐sulfonate (12) showed that the micropolarity at the dendrophane core decreases with increasing generation number.

Nonlinear phenomena in flows of viscoelastic polymer fluids

Highly ordered honeycomb films were produced from arborescent graft polystyrene (AGPS) solutions using breath figures (BF). The structure of the BF pattern (whether single layered or multilayered) of pores depended on several factors such as the type of solvent used (CS2 or CHCl3), the length of the side chains, the generation number and the concentration of the polymer. When solutions of AGPS with shorter side chains (Mw ∼ 5000 g/mol) were used for the BF process, arborescent polymers generated a multilayer pore structure. However, when polymers with longer side chains (Mw ∼ 30,000 g/mol) were used, monolayer pore structures were mostly generated. The regularity and surface quality of the honeycomb pattern became worsened as the multilayered structures formed. The polymer concentration (ϕ) also played a critical role in determining the morphology of the BF films. Higher ϕ led to a multilayer pore structure while lower ϕ resulted in a monolayer pore structure. In general, the regularity of top layer pores was found to decrease with increasing generation number. The introduction of multiwall carbon nanotubes (MWCNTs) into AGPS solutions and application of the BF process using the solution blends yielded patterned hybrid films. SEM images showed that some CNTs were loaded into the honeycomb pattern of AGPS, but that most MWCNTs precipitated to form the bottom layer. In this work, the influence of concentration and surface modification of MWCNTs on the BF patterns obtained is discussed.

Quantitative analysis of the effect of selection history on sugar yield adaptation of sugarcane clones

An objective of the CSR sugarcane breeding programme in Australia was to assess the scope for broadening the genetic base of the commercial sugarcane germ plasm through interspecific hybridization with Saccharum spontaneum clones. The contribution of both selection history and S. spontaneum to sugar yield and its components was investigated in the germ plasm pool assembled. The analysis was conducted on a data-set of 256 clones, consisting of parents and full-sib families generated from 32 biparental crosses, tested in six environments. The minimum number of generations back to S. spontaneum ancestor in the clone's pedigree was used as a germ plasm score. The geographical origin and selection history of each parent and their use in the biparental crosses were used to develop a selection history score for parents and offspring. The variation for seven attributes, cane yield, commercial cane sugar %, sugar yield, stalk number per stool, stalk weight, fibre % and ash % juice was partitioned according to the germ plasm and selection history scores. Significant (P<0.05) clone variation and clone x environment interaction for all attributes was present. The germ plasm scores accounted for a significant (P<0.05) component of the clone variation for all of the attributes except cane yield. There was an increase in sugar yield with an increase in the minimum number of generations back to a S. spontaneum clone. The selection history groups accounted for a high proportion of the variation among parental clones for all of the attributes except cane yield. This suggested that parents were the outcome of strong selection pressure for the commercial cane attributes. However, the selection history groups for the offspring produced by random mating of parents did not account for a high proportion of the variation for the attributes. Using the mixture method of classification we partitioned the 256 clones into five groups for patterns of performance for the seven attributes across the six environments. The five groups emphasized major differences in the patterns of performance for the seven attributes across environments. The distribution of germ plasm and selection history scores in each of the five groups indicated that their patterns of performance were associated with selection history and minimum generations to S. spontaneum. Therefore, both the analysis on selection history and germ plasm scores (extrinsic classification) and the analysis on the mixture method of classification (intrinsic classification) emphasized the influence of selection history on the sugar yield of sugarcane.

Variability in life history traits of the aphid, Acyrthosiphon pisum (Harris), from sexual and asexual populations.

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7-13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.

Effect of plasma waves on the optical properties of a multilayered metallic Fibonacci superlattice.

Within the hydrodynamic model of electron dynamics, the optical properties of a metallic Fibonacci superlattice have been studied for the region of p-polarized soft x rays and the extreme ultraviolet. By using the 4 x 4 transfer-matrix formalism and taking into account retardation effects and the coupling between transverse and longitudinal waves at the metal boundaries, we have discussed the electromagnetic normal modes for the quasiperiodic superlattice in the rational approximation. We found that the dispersion curves are mainly of two types, and, similar to the reflectivities, both the real and imaginary parts of the dispersion-relation pattern have a rich structure of self-similarity. With increasing generation number, the electromagnetic modes all become critical.

On the variety of electron diffraction patterns from a face-centred icosahedral quasicrystal

The discovery of the icosahedral phase by Shechtman et al. (i) has led us into an exciting world of quasicrystals. Chattopadhyay et al. (2) reported on the variety of electron diffraction patterns from a primitive icosahedral phase of the AI-Mn system. They identified important zone axes by choosing a set of strong reflections. From the stereogram, six important zone axes were identified by the combination of relrods corresponding to the vertex, edge and face vectors of an icosahedron. Intensities were compared with those calculated from the Landau generation scheme of Nelson and Sachdev (3), where a combination of star vectors pointing to the vertices of an icosahedron sequentially generate all the reciprocal vectors, with a decrease in intensity with increasing generation number.

Genetic and non-genetic factors affecting pre-weaning survivability in kids

ABSTRACTThe present study was carried out on 1243 kids (379 Black Bengal; 568 Jamunapari × Black Bengal and 296 Beetal × Black Bengal) born between 1981 and 1986. Data were analysed to study the influence of genetic group, sex, season of birth, type of birth, birth weight, dam's weight at kidding, filial generation and parity of dam (as covariate) on survivability of kids during the pre-weaning period (0 to 90 days of age). The overall survival rates during 0 to 15, 16 to 30, 31 to 60, 61 to 90 and 0 to 90 days of age were 81·68 (s.e. 2·89), 95·55 (s.e. 2·00), 90·77 (s.e. 2·33), 93·31 (s.e. 2·59) and 69·94 (s.e. 3·65) % respectively. Variation in survival rates from 0 to 15 days of age was significant due to genetic group (P &lt; 0·01), birth weight (P &lt; 0·01), dam's weight at kidding and parity of dams as a covariate (P &lt; 0·05). Season of birth (P &lt; 0·05), weight of kids (P &lt; 0·01) and dam's weight at kidding (P &lt; 0·01) had significant effects on survival rate during 0 to 90 days of age. Sex and type of birth had no significant effect on survival rates. The interaction effects of genetic groups with season of birth and sex on pre-weaning survival rates were not significant in all ages except the effect between genetic group and sex during 31 to 60 days of age. An improvement in survival rates during the period of study was noticed with the increase in filial generation number from Fj to F3, although the difference was not significant except from 61 to 90 days of age (P &lt; 0·05). Birth weight of kids had a positive linear relationship with survivability during the pre-weaning period. Summer-born kids had a significantly lower survival rate than those born in the winter and monsoon periods from 16 to 30, 61 to 90 and from 0 to 90 days of age. A negative linear relationship was observed between dam's weight at kidding and survival rates of kids from 0 to 90 days of age except the 25 to 30 kg dams deviate. Heritability estimates of survival rates during pre-weaning periods were not significantly different from zero, indicating that improvement by simple selection is probably limited.

Free-carrier absorption from Fibonacci sequences of delta -doped layers in silicon.

Theoretical results are reported for the dynamical resistivity and free-carrier absorption from Fibonacci sequences of \ensuremath{\delta}-doped layers in silicon. Distinct peaks develop, with increasing generation number, in the region of plasmon excitations and the spectra show self-similarity properties. Numerical results are presented for sequences with generation numbers 5, 10, and 15, corresponding to a total number of layers approximately equal to 10, 100, and 1000, respectively.

Preferential distribution of leukocytes in rat mesentery microvessel networks

Distribution of leukocytes in rat mesenteric microvessel networks was studied using intravital fluorescence video microscopy. A digital image analysis system was used to measure vessel diameters, flow velocities and leukocyte fluxes in 306 capillaries of 8 networks. Capillaries were defined as vessel segments connecting divergent to convergent branch points. Their topological position within the network was quantified by a generation number defined as the number of bifurcations between the capillary and the arteriole feeding the network. Proximal capillaries (generation numbers 4 and 5) were slightly but significantly smaller in diameter (8.9 +/- 0.4 micron, mean +/- SEM) than distal ones (generation numbers 20 and 21, 10.1 +/- 0.4 micron). Average capillary flow velocity decreased markedly from 2.0 +/- 1.0 mm.s-1 in proximal to 0.41 +/- 0.06 mm.s-1 in distal capillaries. Average leukocyte concentration was 3.4 +/- 0.5.10(9) 1(-1) and thus significantly below systemic values (6.0.10(9) 1(-1] in proximal capillaries, and above in distal ones (11.7 +/- 2.6.10(9) 1(-1). The analysis of flow and leukocyte flux partition at 138 bifurcations showed preferential distribution of leukocytes to the daughter capillary with higher flow rate. This suggests a tentative explanation for the observed leukocyte accumulation along the microvascular tree: due to their low fractional flow, proximal capillaries draw relatively leukocyte-poor blood from the arteriole feeding the network; this leads to an increased leukocyte concentration in distal capillaries. As a consequence of the concomitant increase of capillary diameter with increasing generation number, leukocytes are preferentially flowing through larger capillaries and are excluded from small ones.

Human pituitary and placental hormones control human insulin-like growth factor II secretion in human granulosa cells.

Human granulosa cells cultured with calf serum actively proliferated for 18-20 generation and secreted progesterone into the medium; progesterone levels appeared to decline with increase in generation number. Cells cultured under serum-free conditions secreted significant amounts of progesterone and insulin-like growth factor II (IGF-II). The progesterone secretion was enhanced by the addition of human follitropin, lutropin, and chorionic gonadotropin but not by growth hormone. These cells, when challenged to varying concentrations of human growth hormone, human chorionic somatomammotropin, human prolactin, chorionic gonadotropin, follitropin, and lutropin, secreted IGF-II into the medium as measured by specific IGF-II RIA. Among these human hormones, chorionic gonadotropin, follitropin, and lutropin were most effective in inducing IGF-II secretion from these cells. When synthetic lutropin-releasing hormone and alpha-inhibin-92 were tested, only lutropin-releasing hormone was effective in releasing IGF-II. The results described suggest that cultured human granulosa cells can proliferate and actively secrete progesterone and IGF-II into the medium. IGF-II production in human granulosa cells was influenced by a multi-hormonal complex including human growth hormone, human chorionic somatomammotropin, and prolactin.

Studies on sugar isomerases of Lactobacillus sp.: Whole cell immobilization of d-glucose isomerase

A new soil isolate of Lactobacillus sp. grown in Yamanaka medium under submerged conditions showed the presence of d-glucose, d-xylose and d-ribose isomerases in washed cell suspension and cell free extracts. d-Xylose isomerase ( d-xylose ketol-isomerase, EC 5.3.1.5) and d-ribose isomerase ( d-ribose ketol-isomerase, EC 5.3.1.20) activities reached a maximum in 48 h of growth and then declined. d-Glucose isomerase ( d-glucose 6-phosphate isomerase, d-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) activity was maximum after 72 h and remained constant for ∼120 h of growth. d-Glucose isomerase activity increased with the increase in number of generations of culture and reached a maximum in 5–6 generations, whereas d-xylose and d-ribose isomerase activities decreased. The washed and starved whole cells could be heat treated and immobilized on the rough surface of glass rods or glass slides using acetone treatment. The heat treated immobilized cells showed only the presence of d-glucose isomerase activity and showed no d-xylose and d-ribose isomerase activities. d-Glucose isomerase activity of heat treated immobilized cells was inhibited less by sorbitol, mannitol, sodium arsenate, cysteine and calcium ions than the free d-glucose isomerase activity in fresh untreated washed whole cells and cell free extracts. EDTA inhibition had the same effect for both forms. Ca 2+ inhibition could be reversed by adding Mg 2+ ions.

The use of free-living nematodes as a bioassay for estuarine sediments

A bioassay technique using free-living nematodes has been developed to assess the environmental quality of estuarine sediments. Nematodes offer the advantages of (a) generally being the most abundant metazoan in sediments; (b) having short life histories and no pelagic larvae; and (c) being relatively easy to maintain in laboratory culture. Small individual body size and rapid generation time (two weeks) render nematodes especially useful in evaluating effects of potential toxicants on populations, rather than on single individuals. The bioassay technique was developed using two species, Chromadorina germanisa and Diplolaimella punicea, and sediments from the Hudson-Raritan estuarine area. The life histories and feeding habits of both species are known. Test sediments were taken from eight stations that represent a gradient from lightly to heavily impacted. Concentrations of potential toxicants measured in these sediments included polychlorinated hydrocarbons, polynuclear aromatic hydrocarbons, and heavy metals. Control sediments, taken from 100 km distant, were almost totally toxicant-free. Nematodes were incubated in the dark at 25°C for two weeks at 1:1 and 1:10 concentrations (volume/volume) of sediments prepared with sterile sea water. Food consisted of bacteria from nematode stock cultures. Intrinsic rate of natural increase ( r) of the nematodes was used as the indicator of sediment quality; r has units (daily increase in number of generations), is easily computed, and can be treated statistically. Both species grew better in control and other sediments containing low concentrations of PCBs, PAHs, and heavy metals. In sediments having TIF PCB and PAH concentrations (ppb) of more than 270 and 8700, respectively, population growth of both species tended to be less than half that found at lower concentrations. Significant inverse correlations between r and toxicant concentration occurred at 1:1 sediment-water concentrations. At 1:10 concentrations the correlations remained negative, but not at the 0·05 level of significance. Use of the nematodes Chromadorina germanica and Diplolaimella punicea offers a quick, easy and statistically testable means of assessing the quality of marine sediments on populations, rather than single individuals, of marine benthic invertebrates.

Characterization of Adult Bovine Adrenocortical Cells Throughout Their Life Span in Tissue Culture*

The characteristics of adult bovine adrenocortical cells were studied throughout their life span of 55-65 generations in monolayer culture. Over this period, the cells maintained the capacity to synthesize steroids when tested with repeated maximal doses of ACTH, prostaglandin E1, monobutyryl cAMP, or cholera toxin. Prostaglandin E1 stimulated cAMP production and steroidogenesis, and inhibited DNA synthesis, as measured by incorporation of [3H]thymidine, with dose-response characteristics that did not vary over the first 50 generations in culture. In contrast, the maximal rate of cAMP production stimulated by ACTH declined exponentially at a rate of 7% per generation. In primary and secondary cultures, ACTH stimulated steroidogenesis maximally and inhibited [3H]thymidine incorporation into DNA completely at a half-maximal effective concentration (ED50) of 0.08 nM which was two orders of magnitude less than the ED50 of 8 nM for stimulation of cAMP production. As the ACTH-stimulated maximal rate of cAMP production fell with increasing generation number, the ED50 for ACTH stimulation of steroidogenesis and inhibition of DNA synthesis increased. From about the 20th generation onward, the ability of ACTH to inhibit DNA synthesis maximally declined so that by the 40th generation, cells were completely resistant to the growth-inhibitory effects of ACTH. High-dose ACTH continued, however, to stimulate steroid production maximally over the 50 generations studied. In late passage cells, the ED50 for ACTH stimulation of steroidogenesis was 8 nM, identical to that for cAMP production. Although ACTH-stimulated cAMP production was related to both stimulation of steroidogenesis and inhibition of DNA synthesis, higher cAMP levels appeared required for inhibition of DNA synthesis than for stimulation of steroidogenesis. Mitogenic responses to fibroblast growth factor and to angiotensin II were retained throughout long term growth in culture. The progressive loss of ACTH-responsiveness was specific and a function of aging of bovine adrenocortical cells in culture.

The potential number of redial generations in echinostomatids (trematoda)

By carrying out ‘chain-transplantations’ of rediae of Isthmiophora melis from infected to uninfected snails ( Lymnaea stagnalis) it has been shown to date that these rediae have passed through a minimum of 42, but possibly 102, successive generations. It is suggested that a high mortality due to transplantation procedures is responsible for the fact that only 11·94 per cent of transplanted rediae survive and give rise to further generations of rediae. Since rediae of different generation numbers do not differ significantly with regard to (a) their infectivity to recipient snails; (b) the time from their transplantation to the emergence of cercariae; and (c) the infectivity of nmetacercariae derived from them it is argued that the multiplication potential of daughter rediae of I. melis is unlimited. Similar results have been obtained with the rediae of Echinoparyphium aconiatum. Preliminary data for the rediae of Echinostoma revolutum suggest similar trends to the other species, although rediae of increasing generation number show a progressive decrease in their infectivity to recipient snails. It is concluded that the limiting factors for redial multiplication in the intermediate host must be the size and life span of the intermediate host.