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91.
An empirical projection method is described to predict the magnitudes and signs of two-bond 13C-13C spin-coupling constants (2JCC) in aldopyranosyl rings. The method has been applied primarily to the interpretation of 2JCCC values, although the behavior of 2JCOC has also been examined in light of the new approach, producing results which may prove useful in the conformational analysis of O-glycosidic linkages in oligosaccharides. High-level ab initio calculations of 2JCC values in model compounds were found to be in agreement with the predictions. 相似文献
92.
Nienow AW Langheinrich C Stevenson NC Emery AN Clayton TM Slater NK 《Cytotechnology》1996,22(1-3):87-94
Because of concern for cell damage, very low agitation energy inputs have been used in industrial animal cell bioreactors, typical values being two orders of magnitude less than those found in bacterial fermentations. Aeration rates are also very small. As a result, such bioreactors might be both poorly mixed and also unable to provide the higher oxygen up-take rates demanded by more intensive operation. This paper reports experimental studies both of K
L
a and of mixing (via pH measurements) in bioreactors up to 8 m3 at Wellcome and of scaled down models of such reactors at Birmingham. Alongside these physical measurements, sensitivity of certain cell lines to continuously controlled dO2 has been studied and the oxygen up-take rates measured in representative growth conditions. An analysis of characteristic times and mixing theory, together with other recent work showing that more vigorous agitation and aeration can be used especially in the presence of Pluronic F-68, indicates ways of improving their performance. pH gradients offer a special challenge. 相似文献
93.
A general assay for restriction endonucleases and other DNA-modifying enzymes with plasmid substrates 总被引:2,自引:0,他引:2
I. Barry Vipond Geoffrey S. Baldwin Mark Oram Symon G. Erskine Lois M. Wentzell Mark D. Szczelkun Timothy J. Nobbs Stephen E. Halford 《Molecular biotechnology》1995,4(3):259-268
A procedure for measuring the activities of enzymes that alter the covalent structure of DNA is described. The assay utilizes
covalently closed circles of DNA as the substrate and yields quantitative data on the fraction of this DNA converted to both
open-circle and linear forms. 相似文献
94.
Abstract. Eragrostis intermedia (Plains lovegrass) is a midheight perennial bunchgrass native to semi-arid grasslands of the southwestern USA, that becomes an abundant and dominant component of these grasslands in areas long protected from livestock grazing. Substantial mortality of plains lovegrass occurred on a large livestock exclosure in southeastern Arizona, after a period of declining precipitation, but only in areas that had not burned in the previous three years. Lovegrass abundance subsequently increased on both undisturbed and burned sites, but remained substantially higher on the burned area. Long-term abundance of plains lovegrass may depend on episodic fire, particularly during periods of reduced precipitation. 相似文献
95.
Susann Teneberg Timothy R. Hirst Jonas Ångström Karl-Anders Karlsson 《Glycoconjugate journal》1994,11(6):533-540
The binding specificities of cholera toxin andEscherichia coli heat-labile enterotoxin were investigated by binding of125I-labelled toxins to reference glycosphingolipids separated on thin-layer chromatograms and coated in microtitre wells. The binding of cholera toxin was restricted to the GM1 ganglioside. The heat-labile toxin showed the highest affinity for GM1 but also bound, though less strongly, to the GM2, GD2 and GD1b gangliosides and to the non-acid glycosphingolipids gangliotetraosylceramide and lactoneotetraosylceramide. The infant rabbit small intestine, a model system for diarrhoea induced by the toxins, was shown to contain two receptor-active glycosphingolipids for the heat-labile toxin, GM1 ganglioside and lactoneotetraosylceramide, whereas only the GM1 ganglioside was receptor-active for cholera toxin. Preliminary evidence was obtained, indicating that epithelial cells of human small intestine also contain lactoneotetraosylceramide and similar sequences. By computer-based molecular modelling, lactoneotetraosylceramide was docked into the active site of the heat-labile toxin, using the known crystal structure of the toxin in complex with lactose. Interactions which may explain the relatively high toxin affinity for this receptor were found.Abbreviations CT
cholera toxin
- CT-B
B-subunits of cholera toxin
- LT
Escherichia coli heat-labile enterotoxin
- hLT
humanEscherichia coli heat-labile enterotoxin
- pLT
porcineEscherichia coli heat-labile enterotoxin
- EI
electron ionization 相似文献
96.
97.
Christine M. Kampny Timothy A. Dickinson Nancy G. Dengler 《American journal of botany》1994,81(10):1343-1353
Intraspecific variation in floral development was studied in four morphologically distinct strains of Pseudolysimachion longifolium and two of P. spicatum. Size increase of whole buds from living plants of P. longifolium was followed in absolute time. Growth rates were homogeneous within inflorescences and within individual plants, and significantly different between individual plants. Organ growth trajectories (plotted as organ size over gynoecium size) differed little among individuals of Pseudolysimachion belonging to morphologically distinct strains; variation was most pronounced in the calyx. Corolla tube as well as calyx and corolla lobe measurements varied more during the early stages of development than near maturity, suggesting constraints on mature flower form. In addition to intraspecific variation, floral development in Pseudolysimachion was compared with that in related Veronica chamaedrys and Veronicastrum virginicum, using data from a previous study. Mature flowers of Pseudolysimachion have long corolla tubes similar to those of Veronicastrum and long corolla lobes similar to those of Veronica. The trajectories of most organs of Pseudolysimachion were intermediate between those of Veronica and Veronicastrum in position as well as in type of curve, i.e., different trajectories lead to similar end products; thus, the shapes of the mature organs are not strictly homologous and not indicative of close relationship. 相似文献
98.
Lawrence G. Harshman Timothy Prout 《Evolution; international journal of organic evolution》1994,48(3):758-766
In this paper we show that when Drosophila melanogaster females are mated twice, the semen of the second male causes a reduction of the effective number of resident sperm from the previous mating. This is demonstrated by two different kinds of experiments. In one set of experiments, mated females were remated to two different kinds of sterile males, one with normal semen and the other with deficient semen. The effect on the resident sperm was determined from the number of remaining progeny after mating to the sterile male, with the result that the normal semen reduced the amount of resident sperm in comparison with matings to the males with deficient semen. The second set of experiments employed interrupted matings. These experiments were based on the observation that semen is delivered before sperm during the first 5 min of copulation. The second matings were interrupted instantly, 2 min, and 4 min after the initiation of copulation. Compared to the instant interruptions, the two later interruptions had the effect of reducing the amount of resident sperm. The results of these two experiments clearly indicate that a sperm-incapacitation process plays a role in the well-documented phenomenon of sperm displacement (last-male advantage) in this species. Such a process could play a role in sperm displacement in the many cases where the mechanism is unknown. 相似文献
99.
Antonio Cuadrado Wolfgang Issing Timothy P. Fleming Christopher J. Molloy 《Journal of cellular physiology》1994,159(3):434-440
Protein kinase C (PKC) represents a family of structurally related Ser/Tre kinases which are involved in mitogenic signalling and may contribute to human neoplasia. To address this issue, the messenger RNA and protein levels of PKC isoenzymes α and β were analyzed in several human sarcoma- and carcinoma-derived cell lines. Carcinomas contained low or undetectable levels of either PKC-α or PKC-β. Sarcomas exhibited similar or increased PKC expression compared to human diploid fibroblasts. Moreover, sarcoma cell lines expressing one PKC isoform did not contain detectable levels of the other. When PKC was depleted from the tumor cells, we observed that the PKC overexpressing sarcomas had reduced their malignant properties as determined by their ability to grow in semisolid medium. In addition, epidermal growth factor-stimulated and erbB2-transformed fibroblasts exhibited enhanced cell growth in the absence of PKC. We propose a model for the effect of PKC as a negative regulator of proliferation in epithelial cells and a growth promoter in fibroblasts. © 1994 wiley-Liss, Inc. 相似文献
100.
Dan W. Urry D. Channe Gowda Shaoqing Peng Timothy M. Parker Naijie Jing R. Dean Harris 《Biopolymers》1994,34(7):889-896
Commonly a key element enabling proteins to function is an amino acid residue or residues with functional side chains having shifted pKa values. This article reports the results on a set of protein-based polymers (model proteins) that exhibit hydrophobic folding and assembly transitions, and that have been designed for the purpose of achieving large hydrophobic-induced pKa shifts by selectively replacing Val residues by Phe residues. The high molecular weight polypentapeptides, actually poly (tricosapeptides) with six varied pentamers in fixed sequence, were designed and synthesized to have the same amino acid compositions but different proximities between a single aspartic acid residue and 5 Phe residues per 30 residues. With the 5 Phe residues distal from the Asp residue, the observed pKa shift was 2.9 when compared to the Val-containing reference. With the 5 Phe residues within 1 nm of the Asp residue, the pKa shift was 6.2. This represents a free energy of interaction of 8 kcal/moles. To our knowledge, this is the largest pKa shift documented for an Asp residue in a polypeptide– or protein–water system. Data are reviewed that do not support the usual electrostatic arguments for pKa shifts of charge–charge repulsion and/or unfavorable ion self-energies arising from displacement of water by hydrophobic moieties, but rather the data are interpreted to indicate the presence of an apolar–polar repulsive free energy of hydration, which results from a potentially highly cooperative competition between apolar and polar species for hydration. © 1994 John Wiley & Sons, Inc. 相似文献