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G C Hill

Publications and source records attributed to G C Hill.

At least 19 recordsLinked to original sources
PubMed2006

Limits on the high-energy gamma and neutrino fluxes from the SGR 1806-20 giant flare of 27 December 2004 with the AMANDA-II detector.

On 27 December 2004, a giant gamma flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5) TeV-1 m;{-2} s;{-1} for gamma=-1.47 (-2) in the gamma flux and 0.4(6.1) TeV-1 m;{-2} s;{-1} for gamma=-1.47 (-2) in the high-energy neutrino flux.

Journal Article↗
PubMed2004

Search for extraterrestrial point sources of neutrinos with AMANDA-II.

We present the results of a search for point sources of high-energy neutrinos in the northern hemisphere using AMANDA-II data collected in the year 2000. Included are flux limits on several active-galactic-nuclei blazars, microquasars, magnetars, and other candidate neutrino sources. A search for excesses above a random background of cosmic-ray-induced atmospheric neutrinos and misreconstructed downgoing cosmic-ray muons reveals no statistically significant neutrino point sources. We show that AMANDA-II has achieved the sensitivity required to probe known TeV gamma-ray sources such as the blazar Markarian 501 in its 1997 flaring state at a level where neutrino and gamma-ray fluxes are equal.

Journal Article↗
PubMed2003

Limits on diffuse fluxes of high energy extraterrestrial neutrinos with the AMANDA-B10 detector.

Data from the AMANDA-B10 detector taken during the austral winter of 1997 have been searched for a diffuse flux of high energy extraterrestrial muon neutrinos. This search yielded no excess events above those expected from background atmospheric neutrinos, leading to upper limits on the extraterrestrial neutrino flux measured at the earth. For an assumed E-2 spectrum, a 90% classical confidence level upper limit has been placed at a level E2Phi(E)=8.4 x 10(-7) cm(-2) s(-1) sr(-1) GeV (for a predominant neutrino energy range 6-1000 TeV), which is the most restrictive bound placed by any neutrino detector. Some specific predicted model spectra are excluded. Interpreting these limits in terms of the flux from a cosmological distributions of sources requires the incorporation of neutrino oscillations, typically weakening the limits by a factor of 2.

Journal Article↗
PubMed2001

Observation of high-energy neutrinos using Cerenkov detectors embedded deep in Antarctic ice.

Neutrinos are elementary particles that carry no electric charge and have little mass. As they interact only weakly with other particles, they can penetrate enormous amounts of matter, and therefore have the potential to directly convey astrophysical information from the edge of the Universe and from deep inside the most cataclysmic high-energy regions. The neutrino's great penetrating power, however, also makes this particle difficult to detect. Underground detectors have observed low-energy neutrinos from the Sun and a nearby supernova, as well as neutrinos generated in the Earth's atmosphere. But the very low fluxes of high-energy neutrinos from cosmic sources can be observed only by much larger, expandable detectors in, for example, deep water or ice. Here we report the detection of upwardly propagating atmospheric neutrinos by the ice-based Antarctic muon and neutrino detector array (AMANDA). These results establish a technology with which to build a kilometre-scale neutrino observatory necessary for astrophysical observations.

Journal Article↗
PubMed1998

Biochemical and molecular properties of the Trypanosoma brucei alternative oxidase.

The protozoal parasite Trypanosoma brucei depends on a mitochondrial non-cytochrome terminal oxidase known as the trypanosome alternative oxidase (TAO) in its mammalian host. We have recently cloned the cDNA from T. brucei bloodstream form and have characterized a 33 kDa mitochondrial protein as TAO. Here we report that the TAO is a single copy gene in T. brucei and its expression is down regulated at the level of transcript abundance during differentiation from the bloodstream to the procyclic trypanosomes. Like other alternative oxidases (AOXs) cloned from different plants and fungi, TAO possesses the conserved sequences at the centrally located predicted membrane spanning domains and the signature sequence at the C-terminal hydrophilic domain for a pair of putative iron binding motifs (E-X-X-H). Phylogenetic analysis of the deduced protein sequences of eight different alternative oxidases cloned from different plants and fungi revealed that TAO is more closely related to the alternative oxidases of the fungi clade than that of plants. TAO has been functionally expressed in Escherichia coli. In the first of the two putative iron binding motifs, site-directed mutagenesis of E215 to A, L, N and Q resulted in the loss of the ability of the TAO gene to complement the heme deficiency of the E. coli mutants (SASX41B and GE1387) by conferring on them a CN-insensitive pathway of respiration. The conservative substitution of E215 by aspartate and histidine reduced the growth of the E. coli auxotrophs by approximately 80%. The mutations apparently did not have any effect on the stability of the expressed protein as revealed by the immunoblot analysis of the bacterial protein using TAO monoclonal antibody, which we have developed. Together, these points suggest that E215 plays an important role in the function of TAO. The steady state level of TAO mRNA is down-regulated in the procyclic stage presumably accounting for the low levels of TAO protein in these forms.

Aldehyde Oxidoreductases↗
PubMed1995

Pentamidine congeners. 3. Crystal structure and molecular modeling studies of trans-1,4-bis(4-amidinophenoxy)-2-butene.

X-ray diffraction was used to confirm the geometry of trans-1,4-bis(4-amidinophenoxy)-2-butene dihydrochloride dihydrate (trans-butenamidine). trans-Butenamidine is a semirigid analogue of pentamidine that has demonstrated good anti-Pneumocystis carinii activity in rats. Molecular modeling studies revealed that unlike pentamidine or propamidine, trans-butenamidine does not discriminate between AT and TA sequences in its binding to the minor groove of DNA. Crystal data: [C18H22N4O2(2+)][Cl(-)]2[H2O]2, triclinic space group, P1, a = 9.443(1) A, b = 11.400(1) A, c = 11.919(1) A, alpha = 62.19(1) degree, beta = 81.10(1) degree, gamma = 72.19(1) degree, V = 1080.3(3) A3, Z = 2, R = 0.054 for 1149 observed reflections with I > 3 sigma (1).

Base Sequence↗
PubMed1994

High-field NMR and restrained molecular modeling studies on a DNA heteroduplex containing a modified apurinic abasic site in the form of covalently linked 9-aminoellipticine.

Two-dimensional NMR methods were used to model the possible solution structure of an intercalative complex of 9-aminoellipticine (Aell), a polycyclic pyridocarbazolamine, covalently bound to an apurinic ring-opened deoxyribose site of a duplex DNA fragment in the reduced Schiff base form. The required oligonucleotide single strand containing covalently attached aminoellipticine was obtained by reductive amination in the presence of sodium cyanoborohydride. The combined NMR-energy minimization methods were employed to refine the model structures of two distinct forms, intrahelical and extrahelical, of a control 9-mer duplex DNA, d(CGTG.dr.GTGC).d(GCACTCACG), which contains an apurinic site positioned opposite a dT residue on the complementary strand. The model structure of an aminoellipticine conjugate with the same DNA sequence, derivatized via the aforementioned covalent attachment, was also obtained by incorporating intermolecular drug-DNA and intra- and internucleotide NOE-derived proton-proton distance estimates as restraints in energy minimization routines. The indole ring system of aminoellipticine, which is inserted at the apurinic site, intercalates between and is parallel to flanking GC base pairs. The pyridinic ring of aminoellipticine, in protonated form, also stacks between cytidine and thymidine bases on the complementary strand, which is consistent with the observation that the normal sequential NOE connectivity at the 5'-C13-T14 step is broken and indeed diverted through the ellipticine moiety, e.g., C13-Aell-T14 connectivities through the Aell-H4/C5Me protons. Interestingly, the partial stacking of the pyridinic ring is observed only between the 5'-CT step vs an adjacent 5'-TC step, owing to inherently weak stacking interactions associated with the former. In the absence of any potential groups that can participate in electrostatic or hydrogen-bonding interactions with the nucleic acid, pi-pi stacking and hydrophobic contacts at the intercalation site appear to be the important factors in determining stability and conformation of the aminoellipticine-DNA conjugate. Stacking interactions in such a bistranded intercalative complexation of aminoellipticine apparently govern the formation of a single intrahelical form of a right-handed B-type DNA duplex. The overall structural features lead us to propose working models for an enzyme-like DNA cleavage activity of 9-aminoellipticine and the observed inhibition of the AP endonuclease-dependent DNA excision-repair pathway.

Base Sequence↗
PubMed1992

Template-directed design of a DNA-DNA cross-linker based upon a bis-tomaymycin-duplex adduct.

A template-directed approach to the design of a DNA-DNA interstrand cross-linker based upon the structure of a bis-tomaymycin-duplex adduct has been carried out. Tomaymycin is a member of the pyrrolo[1,4]benzodiazepines antitumor antibiotics. In a previous study (F.L. Boyd et al., Biochemistry 1990, 29, 2387-2403), we have shown that two tomaymycin molecules can be covalently bound to a 12-mer duplex molecule, where the drug molecules are on opposite strands six base-pairs apart, and the stereochemistry at the drug bonding site, and orientation in the minor groove, was defined by high-field NMR. This bis-tomaymycin 12-mer duplex adduct maintains the self-complementarity of the duplex and a B-type structure. In the present study we have shown using high-field NMR that this same 12-mer sequence can be truncated by two base pairs so that the two tomaymycin-modified guanines are now only four base-pairs apart, the two species of tomaymycin molecules are still bound with the same stereochemistry and orientation, and the 10-mer duplex adduct maintains its self-complementarity. In a second 10-mer duplex we have shown that changing the bonding sequence from 5'CGA to 5'AGC does not significantly affect the structure of the bis-tomaymycin-duplex adduct. However, when the sequence is rearranged so that the drugs point in a tail-to-tail orientation rather than in the previous head-to-head configuration, there are more than one species of tomaymycin bound to DNA, and, as a consequence, the bis-tomaymycin 10-mer duplex adduct loses its self-complementarity. Last, we have used the 10-mer duplex containing the 5'CGA sequence, in which the tomaymycin molecules are oriented head to head, to design an interstrand cross-linking species in which the two drug molecules are linked together with a flexible linker molecule.

Base Sequence↗
PubMed1991

Characterization of a novel developmentally regulated gene from Trypanosoma brucei encoding a potential phosphoprotein.

We have isolated a cDNA clone corresponding to a single-copy nuclear gene that is upregulated at the mRNA level during in vitro differentiation of bloodstream trypomastigotes of strains of both Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense to procyclic forms. Transcript levels begin to increase within minutes of introduction of bloodstream forms into culture and peak well before cultures exhibit a procyclic morphology. This increase in transcript levels was found to occur both in the absence of protein synthesis and in a nontransforming strain blocked very early in the developmental program, both conditions under which accumulation of procyclic acidic repetitive protein (PARP) transcripts did not occur in control experiments. DNA sequence analysis reveals an open reading frame sufficient to encode a protein of approximately 50 kDa within the cDNA, but data base searches for homology at either the amino acid or nucleotide level revealed no related sequences. A high density of kinase consensus target sites in the deduced amino acid sequence suggests that the gene product may be a phosphoprotein.

Amino Acid Sequence↗
PubMed1991

Computer simulation of the binding of saframycin A to d (GATGCATC)2.

The binding of Saframycin A to the octanucleotide duplex d(GATGCATC)2 was investigated using molecular dynamics. For covalent binding at N2 of the central guanine, only the R configuration at the alkylating carbon (C7) was permitted for B DNA and the 3' direction in the minor groove was preferred by 50.6 kcal/mol. The dihydroquinone form of saframycin A gave stronger binding than the quinone, in agreement with the literature. Addition of solvent and counterions made no significant change in the geometry model. The proposed mechanism of DNA alkylation, involving iminium ion intermediates from the dihydroquinone or quinone, was investigated by modeling these species. They gave models with good net binding enthalpies, and C7 was in close proximity to N2 of guanine. The noncovalent binding of saframycin A and its dihydroquinone in the vicinity of guanine also was favorable in the 3' direction.

Antibiotics, Antineoplastic↗
PubMed1991

Computer simulation of the binding of naphthyridinomycin and cyanocycline A to DNA.

Cyanocycline A was found to have a pKa of 6.6. Protonation of N14 was established by 1H NMR spectroscopy. In strongly acidic solution the oxazolidine ring opened irreversibly. A model was derived for the binding of naphthyridinomycin and cyanocycline A to the hexanucleotide duplex d(ATGCAT)2, by using the molecular mechanics and dynamics modules of AMBER 3.0. It involved protonation on the oxazolidine-ring nitrogen, reduction of the quinone ring to a hydroquinone, formation of an iminium ion with loss of the C7 substituent, noncovalent binding in the minor groove with the hydroquinone ring in the 3'-direction from guanine, and covalent binding to the 2-amino group of this guanine with C7 adopting the R configuration. This model is consistent with the experimental evidence on the DNA binding of these drugs. An alternative binding mode based on opening of the oxazolidine ring and alkylation at C3a also was feasible according to molecular mechanics calculations. The geometry of naphthyridinomycin does not permit interstrand cross-linking involving both C3a and C7, but formation of a cross-link to protein appears possible. When the covalent naphthyridinomycin-d(ATGCAT)2 models were refined in the presence of water and counterions, the models with the most favorable net binding enthalpies were the same as those produced by simulation in vacuum. Qualitative estimates of the relative entropy changes resulting from adduct formation were based on the number of ordered (hydrogen bonded) water molecules released from d(ATGCAT)2 and from the drug. In all cases but one, d(ATGCAT)2 loses five water molecules. It loses six in the C3a covalent model with 5',S geometry. Naphthyridinomycin hydroquinone loses up to two water molecules, depending on the particular adduct. The 3',R model was again favored for the C7 covalent adduct. Among the C3a covalent models, the one with 5',R geometry lost the second most water molecules, but it had the best binding enthalpy.

Alkylation↗
PubMed1990

DNA binding by antitumor anthracene derivatives.

The relative DNA binding strengths of bisantrene and nine new analogues were measured by spectrophotometric titration and melt transition temperature (Tm) techniques. Data from the spectrophotometric titrations could not be fit by simple Scatchard plots. However, they were fit by a McGhee-von Hippel equation over part of the binding range. The entire range of data was fit by a smoothing cubic spline function. The first derivative of this function gave, for each compound, a curve whose intercept provided a measure of relative binding strength. The delta Tm values agreed qualitatively with the spectrophotometric titration results, although there was not a precise linear relationship. Determinations of macroscopic pKas revealed that most of the compounds were dications at pH 7.0, but a few were mixtures of monocations and dications. No correlation was found between these binding studies and antitumor potencies in a clonogenic assay, which suggests that factors other than DNA binding can determine cytotoxicity for some of the analogues.

Animals↗
PubMed1990

A comparison of the amino acid sequences of Crithidia fasciculata and Trypanosoma rhodesiense cytochromes c.

Apocytochrome c was isolated from procyclic trypomastigotes of Trypanosoma rhodesiense EATRO 1895 and purified on Amberlite IRC-50 ion-exchange resin. Tryptic peptides were generated from the purified apoprotein and a partial amino acid sequence was determined. A comparison of the amino acid sequence of Crithidia fasciculata with the partial amino acid sequence of T. rhodesiense reveals significant homology.

Amino Acid Sequence↗
PubMed1988

Computer simulation of the binding of quinocarcin to DNA. Prediction of mode of action and absolute configuration.

Computer-based models were derived for the covalent and noncovalent binding of the antitumor antibiotic quinocarcin to a representative DNA segment, d(ATGCAT)2. They showed that a mode of action, involving opening of the oxazolidine ring to give an iminium ion, followed by initial noncovalent binding in the minor groove and subsequent alkylation of the 2-amino group of guanine, was rational and attended by favorable interaction energies in each step. The best model had the aryl ring of quinocarcin lying in the 3' direction from the covalent binding site and an R configuration at the carbon involved in covalent bond formation. It also showed that the preferred absolute configuration for quinocarcin was the reverse of that arbitrarily assigned in the literature.

Computer Simulation↗
PubMed1986

Trypanosoma rhodesiense: mitochondrial proteins of bloodstream and procyclic trypomastigotes.

One- and two-dimensional gel electrophoresis of the solubilized mitochondrial proteins of bloodstream and procyclic trypomastigote Trypanosoma brucei rhodesiense and radiolabeling of proteins in the presence of cycloheximide were used to identify proteins synthesized in the trypanosome mitochondrion. The proteins which comprise the mitochondrion were found to be very similar in both bloodstream and procyclic trypomastigotes, but do differ in their level of synthesis. A protein putatively identified as subunit II of cytochrome oxidase (EC 1.9.3.1) was detected in mitochondria from both the procyclic and bloodstream organisms. The presence of this protein in bloodstream trypomastigotes and the overall similarity of protein content in the trypanosome mitochondria is noteworthy in view of the fact that bloodstream trypomastigotes have a repressed mitochondrion with no detectable tricarboxylic acid cycle or cytochrome electron transport chain.

Animals↗