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B Giese

Publications and source records attributed to B Giese.

At least 19 recordsLinked to original sources
PubMed2005

Cassini Imaging Science: initial results on Saturn's atmosphere.

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of approximately 375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.

Atmosphere↗
PubMed2005

Cassini Imaging Science: initial results on Phoebe and Iapetus.

The Cassini Imaging Science Subsystem acquired high-resolution imaging data on the outer Saturnian moon, Phoebe, during Cassini's close flyby on 11 June 2004 and on Iapetus during a flyby on 31 December 2004. Phoebe has a heavily cratered and ancient surface, shows evidence of ice near the surface, has distinct layering of different materials, and has a mean density that is indicative of an ice-rock mixture. Iapetus's dark leading side (Cassini Regio) is ancient, heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief. Local albedo variations within and bordering Cassini Regio suggest mass wasting of ballistically deposited material, the origin of which remains unknown.

Extraterrestrial Environment↗
PubMed2005

Cassini Imaging Science: initial results on Saturn's rings and small satellites.

Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem (ISS) during the first 9 months of Cassini operations at Saturn have produced many new findings. These include new saturnian moons; refined orbits of new and previously known moons; narrow diffuse rings in the F-ring region and embedded in gaps within the main rings; exceptionally fine-scale ring structure in moderate- to high-optical depth regions; new estimates for the masses of ring-region moons, as well as ring particle properties in the Cassini division, derived from the analysis of linear density waves; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings.

Journal Article↗
PubMed2002

Observations of comet 19P/Borrelly by the miniature integrated camera and spectrometer aboard Deep Space 1.

The nucleus of the Jupiter-family comet 19P/Borrelly was closely observed by the Miniature Integrated Camera and Spectrometer aboard the Deep Space 1 spacecraft on 22 September 2001. The 8-kilometer-long body is highly variegated on a scale of 200 meters, exhibiting large albedo variations (0.01 to 0.03) and complex geologic relationships. Short-wavelength infrared spectra (1.3 to 2.6 micrometers) show a slope toward the red and a hot, dry surface (</=345 kelvin, with no trace of water ice or hydrated minerals), consistent with approximately 10% or less of the surface actively sublimating. Borrelly's coma exhibits two types of dust features: fans and highly collimated jets. At encounter, the near-nucleus coma was dominated by a prominent dust jet that resolved into at least three smaller jets emanating from a broad basin in the middle of the nucleus. Because the major dust jet remained fixed in orientation, it is evidently aligned near the rotation axis of the nucleus.

Carbon↗
PubMed2001

Direct observation of hole transfer through DNA by hopping between adenine bases and by tunnelling.

The function of DNA during oxidative stress and its suitability as a potential building block for molecular devices depend on long-distance transfer of electrons and holes through the molecule, yet many conflicting measurements of the efficiency of this process have been reported. It is accepted that charges are transported over long distances through a multistep hopping reaction; this 'G-hopping' involves positive charges moving between guanines (Gs), the DNA bases with the lowest ionization potential. But the mechanism fails to explain the persistence of efficient charge transfer when the guanine sites are distant, where transfer rates do not, as expected, decrease rapidly with transfer distance. Here we show experimentally that the rate of charge transfer between two guanine bases decreases with increasing separation only if the guanines are separated by no more than three base pairs; if more bridging base pairs are present, the transfer rates exhibit only a weak distance dependence. We attribute this distinct change in the distance dependence of the rate of charge transfer through DNA to a shift from coherent superexchange charge transfer (tunnelling) at short distances to a process mediated by thermally induced hopping of charges between adenine bases (A-hopping) at long distances. Our results confirm theoretical predictions of this behaviour, emphasizing that seemingly contradictory observations of a strong as well as a weak influence of distance on DNA charge transfer are readily explained by a change in the transfer mechanism.

Adenine↗
PubMed2001

Signal transducer gp130: biochemical characterization of the three membrane-proximal extracellular domains and evaluation of their oligomerization potential.

Glycoprotein 130 (gp130) is a type I transmembrane protein and serves as the common signal-transducing receptor subunit of the interleukin-6-type cytokines. Whereas the membrane-distal half of the gp130 extracellular part confers ligand binding and has been subject to intense investigation, the structural and functional features of its membrane-proximal half are poorly understood. On the basis of predictions of tertiary structure, the membrane-proximal part consists of three fibronectin-type-III-like domains D4, D5 and D6. Here we describe the bacterial expression of the polypeptides predicted to comprise each of these three domains. The recombinant proteins were refolded from solubilized inclusion bodies in vitro, purified to homogeneity and characterized by means of size-exclusion chromatography and CD spectroscopy. For the first time the prediction of three individual membrane-proximal protein domains for gp130 has been verified experimentally. The three domains do not show intermediate-affinity or high-affinity interactions between each other. Mapping of a neutralizing gp130 monoclonal antibody against D4 suggested a particular functional role of this domain for gp130 activation, because above that an intrinsic tendency for low-affinity oligomerization was demonstrated for D4.

Animals↗
PubMed2000

A new photocleavable linker in solid-phase chemistry for ether cleavage

We have designed a new linker (1) for the solid-phase synthesis that cleaves ether bonds photolytically. The linker was prepared in nine steps and anchored to the support via an amide bond. Photocleavage is a two-step process in which the immobilized alcohols are released by photolytic generation of a radical that undergoes a spontaneous beta-bond scission. The pivaloyl linker (1) was found to cleave off alcohols in high yields and purities. Only traces of acid (pH approximately 5.5) are necessary for an efficient cleavage.

Journal Article↗
PubMed2000

Electron transfer in DNA from guanine and 8-oxoguanine to a radical cation of the carbohydrate backbone.

Photolysis of a 4'-pivaloyl-substituted nucleotide in single- and double-stranded DNA (1) generated an enol ether radical cation 4 that was reduced to enol ether 17 by electron transfer from the nearest guanoside (G). Variation of the nucleotide sequence demonstrated a strong distance dependence of this electron-transfer rate with beta = 1.0 +/- 0.1 A(-1). When 8-oxoguanosine (G(oxo)) was used as the electron donor, the rate of the electron transfer increased by a factor of 4 but the distance dependence of the transfer remained unchanged within experimental error. In single strands, the number of intervening A, T, and C nucleotides had a much smaller effect; the rate remained nearly constant for two, three, or four intervening nucleotides. This is explained by the flexibility of the single-stranded oligonucleotides.

Carbohydrate Metabolism↗
PubMed2000

Cytokine-induced impairment of short-chain fatty acid oxidation and viability in human colonic epithelial cells.

Pro-inflammatory cytokines may directly influence the viability and metabolic function of colonic epithelial cells (CEC) as an early event in the development of inflammatory bowel disease. We report here that TNF-alpha+IFN-gamma induced a synergistic, concentration-dependent decline in butyrate oxidation, an essential energy supply, in HT-29 and DLD-1 cells. TNF-alpha+IFN-gamma induced a parallel profound decline in cell viability in HT-29 cells, but not in DLD-1 cells, where impairment of butyrate oxidation seemed to precede later occurrence of cell damage. TNF-alpha+INF-gamma induced CEC damage was independent on NO formation and involved the IFN-gamma signalling pathway as well as induction of apoptosis. If cytokines have similar effects in vivo, these may lead to energy deficiency and thus contribute to CEC damage and disturbance of the epithelial integrity.

Apoptosis↗
PubMed2000

Long-distance charge transport in DNA: the hopping mechanism.

Long-distance charge transport from a guanine radical cation (G(+*)) to a G-rich sequence is of biological importance. This reaction was studied by selective charge injection into a G, monitoring the charge transport to a GGG sequence by competing H(2)O-trapping. The efficiency of the charge transport diminished dramatically with increasing number of A:T base pairs between G(+*) and GGG. But in DNA strands where G's are located between the G(+*) and GGG sequence, long-distance charge transport occurred by a multistep hopping mechanism.

Base Pairing↗
PubMed2000

A simple method to establish short-term cultures of normal human colonic epithelial cells from endoscopic biopsy specimens. Comparison of isolation methods, assessment of viability and metabolic activity.

BACKGROUND: Abnormalities in colonic epithelial cell function have been implicated in the pathogenesis of various intestinal disorders, especially inflammatory bowel disease (IBD). The mechanisms, however, remain obscure owing to the lack of representative human colonic epithelial cell models. The aim of this study was to develop and validate a method for establishment of short-term culture of normal human colonic epithelial cells from endoscopic biopsies. METHODS: Epithelial cells were isolated from colonoscopic biopsies by means of ethylenediaminetetraacetic acid/ethylene glycol tetraacetic acid (EDTA/EGTA) (10 or 60 min) or by enzyme treatment and cultured in collagen-coated wells. Viability was measured with a methyltetrazoleum conversion assay, confocal laser, and electron microscopy. Metabolic function was measured by means of butyrate oxidation, 14C-leucine and 3H-glucosamine incorporation; DNA synthesis by means of 3H-thymidine incorporation, and apoptosis with an enzyme-linked immunosorbent assay (ELISA) for histone-associated DNA fragments. Cell types were identified by immunocytochemistry. RESULTS: Ten minutes of EDTA/EGTA treatment released intact crypts and was superior to both the 60-min treatment and enzymatic treatment in terms of viability and nonepithelial cell contamination, respectively. Despite activation of detachment-induced apoptosis, a median 51% of the isolated cells was viable after 24 h of culture and metabolically active as judged by 3H-thymidine, 14C-leucine, and 3H-glucosamine incorporation. Butyrate oxidation followed more complex kinetics (substrate activation) than observed previously in other models. The apparent Km values (medians) were 0.7 mM and 4.5 mM in low and high concentration ranges, respectively. CONCLUSION: We report a simple method to establish culture of human colonic epithelial cells from endoscopically obtained biopsy specimens, producing sufficient viable cells to perform metabolic studies pertinent to the pathogenesis of IBD and related human disorders.

Biopsy↗
PubMed1999

Long-range charge hopping in DNA.

The fundamental mechanisms of charge migration in DNA are pertinent for current developments in molecular electronics and electrochemistry-based chip technology. The energetic control of hole (positive ion) multistep hopping transport in DNA proceeds via the guanine, the nucleobase with the lowest oxidation potential. Chemical yield data for the relative reactivity of the guanine cations and of charge trapping by a triple guanine unit in one of the strands quantify the hopping, trapping, and chemical kinetic parameters. The hole-hopping rate for superexchange-mediated interactions via two intervening AT base pairs is estimated to be 10(9) s(-1) at 300 K. We infer that the maximal distance for hole hopping in the duplex with the guanine separated by a single AT base pair is 300 +/- 70 A. Although we encounter constraints for hole transport in DNA emerging from the number of the mediating AT base pairs, electron transport is expected to be nearly sequence independent because of the similarity of the reduction potentials of the thymine and of the cytosine.

Journal Article↗
PubMed1999

Probing interactions between HIV-1 reverse transcriptase and its DNA substrate with backbone-modified nucleotides.

BACKGROUND: To gain a molecular understanding of a biochemical process, the crystal structure of enzymes that catalyze the reactions involved is extremely helpful. Often the question arises whether conformations obtained in this way appropriately reflect the reactivity of enzymes, however. Rates that characterize transitions are therefore compulsory experiments for the elucidation of the reaction mechanism. Such experiments have been performed for the reverse transcriptase of the type 1 human immunodeficiency virus (HIV-1 RT). RESULTS: We have developed a methodology to monitor the interplay between HIV-1 RT and its DNA substrate. To probe the protein-DNA interactions, the sugar backbone of one nucleotide was modified by a substituent that influenced the efficiency of the chain elongation in a characteristic way. We found that strand elongation after incorporation of the modified nucleotide follows a discontinuous efficiency for the first four nucleotides. The reaction efficiencies could be correlated with the distance between the sugar substituent and the enzyme. The model was confirmed by kinetic experiments with HIV-1 RT mutants. CONCLUSIONS: Experiments with HIV-1 RT demonstrate that strand-elongation efficiency using a modified nucleotide correlates well with distances between the DNA substrate and the enzyme. The functional group at the modified nucleotides acts as an 'antenna' for steric interactions that changes the optimal transition state. Kinetic experiments in combination with backbone-modified nucleotides can therefore be used to gain structural information about reverse transcriptases and DNA polymerases.

Acetylation↗
PubMed1999

Memory of Chirality in Photochemistry.

The helical-chiral character of the diradical intermediate 2, which cyclizes quicker than it equilibrates, explains the memory effect of chirality that occurs during the enantioselective photocyclization of alanine derivative 1 to give the proline derivative 3. Ts=H(3)CC(6)H(4)SO(2).

Journal Article↗
PubMed1998

4'-Acylated thymidine 5'-triphosphates: a tool to increase selectivity towards HIV-1 reverse transcriptase.

4'-Acylated thymidines represent a new class of DNA chain terminators, since they have been shown to act as post-incorporation chain-terminating nucleotides despite the presence of a free 3'-hydroxyl group. Here, we describe the action of the 4'-acetyl- (MeTTP) and 4'-propanoylthymidine 5'-triphosphate (EtTTP) on HIV-1 reverse transcriptase in RNA- and DNA-dependent DNA synthesis and on DNA synthesis catalyzed by the cellular DNA polymerases alpha, beta, delta and epsilon. MeTTP exhibits a high selectivity towards HIV-1 reverse transcriptase. By the use of the bulkier propanoyl group as the 4'-substituent of the nucleoside 5'-triphosphate, selectivity towards HIV-1 reverse transcriptase could be increased without affecting substrate efficiency. Thus, 4'-modifications may serve as a tool to increase selectivity towards HIV-1 reverse transcriptase.

Acylation↗