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  • 1
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    Physical Chemistry of the H2SO4/HNO3/H2O System: Implications for Polar Stratospheric Clouds (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-09-10
    Description: Polar stratospheric clouds (PSCs) play a key role in stratospheric ozone depletion. Surface-catalyzed reactions on PSC particles generate chlorine compounds that photolyze readily to yield chlorine radicals, which in turn destroy ozone very efficiently. The most prevalent PSCs form at temperatures several degrees above the ice frost point and are believed to consist of HNO(3) hydrates; however, their formation mechanism is unclear. Results of laboratory experiments are presented which indicate that the background stratospheric H(2)SO(4)/H(2)O aerosols provide an essential link in this mechanism: These liquid aerosols absorb significant amounts of HNO(3) vapor, leading most likely to the crystallization of nitric acid trihydrate (NAT). The frozen particles then grow to form PSCs by condensation of additional amounts of HNO(3) and H(2)O vapor. Furthermore, reaction probability measurements reveal that the chlorine radical precursors are formed readily at polar stratospheric temperatures not just on NAT and ice crystals, but also on liquid H(2)SO(4) solutions and on solid H(2)SO(4) hydrates. These results imply that the chlorine activation efficiency of the aerosol particles increases rapidly as the temperature approaches the ice frost point regardless of the phase or composition of the particles.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Molina, M J -- Zhang, R -- Wooldridge, P J -- McMahon, J R -- Kim, J E -- Chang, H Y -- Beyer, K D -- New York, N.Y. -- Science. 1993 Sep 10;261(5127):1418-23.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17745351" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
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    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 2
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    Uncovering of functional alternative CTLA-4 counter-receptor in B7-deficient mice (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-11-05
    Description: B7 delivers a costimulatory signal through CD28, resulting in interleukin-2 secretion and T cell proliferation. Blockade of this pathway results in T cell anergy. The in vivo role of B7 was evaluated with B7-deficient mice. These mice had a 70 percent decrease in costimulation of the response to alloantigen. Despite lacking B7 expression, activated B cells from these mice bound CTLA-4 and GL1 monoclonal antibody, demonstrating that alternative CTLA-4 ligand or ligands exist. These receptors are functionally important because the residual allogenic mixed lymphocyte responses were blocked by CTLA4Ig. Characterization of these CTLA-4 ligands should lead to strategies for manipulating the immune response.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Freeman, G J -- Borriello, F -- Hodes, R J -- Reiser, H -- Hathcock, K S -- Laszlo, G -- McKnight, A J -- Kim, J -- Du, L -- Lombard, D B -- CA 40216/CA/NCI NIH HHS/ -- New York, N.Y. -- Science. 1993 Nov 5;262(5135):907-9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Division of Hematologic Malignancies, Dana-Farber Cancer Institute, Boston, MA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/7694362" target="_blank"〉PubMed〈/a〉
    Keywords: Abatacept ; Animals ; Antigens, CD ; Antigens, CD80/genetics/*immunology/metabolism ; Antigens, Differentiation/immunology/*metabolism ; B-Lymphocytes/*immunology ; Base Sequence ; CTLA-4 Antigen ; Cell Line ; *Immunoconjugates ; Interleukin-2/secretion ; Isoantigens/immunology ; Lymphocyte Activation ; Lymphocyte Culture Test, Mixed ; Mice ; Mice, Inbred BALB C ; Mice, Inbred C57BL ; Mice, Knockout ; Molecular Sequence Data ; Mutation ; T-Lymphocytes/*immunology ; Transfection
    Print ISSN: 0036-8075
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  • 3
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    Electrostatic screening of charge and dipole interactions with the helix backbone (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-04-09
    Description: Electrostatic interactions in proteins are potentially quite strong, but these interactions are mitigated by the screening effects of water, ions, and nearby protein atoms. The early work of Kirkwood and Westheimer on small organic molecules showed that the extent of the screening may depend on whether charged or dipolar groups are involved. The dielectric and ionic screening of the interactions between the dipolar backbone amide groups of monomeric alpha helices and either (i) solvent-exposed charges or (ii) solvent-exposed dipoles at the amino terminus was measured. The dielectric screening effects are an order of magnitude greater for the backbone-charge interactions than for the backbone-dipole interactions, and the ionic strength dependence is substantially different in the two cases. These results suggest that interactions that involve the dipolar groups of proteins may be relatively more important for stability and function than is generally thought.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Lockhart, D J -- Kim, P S -- New York, N.Y. -- Science. 1993 Apr 9;260(5105):198-202.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Biology, Massachusetts Institute of Technology, Cambridge Center 02142.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8469972" target="_blank"〉PubMed〈/a〉
    Keywords: Amino Acid Sequence ; Circular Dichroism ; Electrochemistry ; Hydrogen-Ion Concentration ; Magnetic Resonance Spectroscopy ; Molecular Sequence Data ; Osmolar Concentration ; Peptides/*chemistry ; Protein Structure, Secondary ; Thermodynamics
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  • 4
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    Japan's SAT? (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-04-16
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kim, S W -- New York, N.Y. -- Science. 1993 Apr 16;260(5106):278.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22606755" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
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    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 5
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    Potassium Channels in Samanea saman Protoplasts Controlled by Phytochrome and the Biological Clock (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-05-14
    Description: Leaflet movement in legumes depends on rhythmic, light-regulated ion fluxes in opposing regions of the leaf-moving organ. In flexor and extensor protoplasts from Samanea saman Merrill, opening and closing of K(+) channels were rhythmic in constant darkness. When channels were open in flexor protoplasts they were closed in extensor protoplasts, and vice versa. The rhythms were shifted by a delay in the onset of constant darkness, a response typical of endogenous circadian rhythms. During the light period, the channels in flexor protoplasts were sensitive to red light that was followed by premature darkness; phytochrome was implicated as the photoreceptor.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kim, H Y -- Cote, G G -- Crain, R C -- New York, N.Y. -- Science. 1993 May 14;260(5110):960-2.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17818385" target="_blank"〉PubMed〈/a〉
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  • 6
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    A switch between two-, three-, and four-stranded coiled coils in GCN4 leucine zipper mutants (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-11-26
    Description: Coiled-coil sequences in proteins consist of heptad repeats containing two characteristic hydrophobic positions. The role of these buried hydrophobic residues in determining the structures of coiled coils was investigated by studying mutants of the GCN4 leucine zipper. When sets of buried residues were altered, two-, three-, and four-helix structures were formed. The x-ray crystal structure of the tetramer revealed a parallel, four-stranded coiled coil. In the tetramer conformation, the local packing geometry of the two hydrophobic positions in the heptad repeat is reversed relative to that in the dimer. These studies demonstrate that conserved, buried residues in the GCN4 leucine zipper direct dimer formation. In contrast to proposals that the pattern of hydrophobic and polar amino acids in a protein sequence is sufficient to determine three-dimensional structure, the shapes of buried side chains in coiled coils are essential determinants of the global fold.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Harbury, P B -- Zhang, T -- Kim, P S -- Alber, T -- GM44162/GM/NIGMS NIH HHS/ -- GM48958/GM/NIGMS NIH HHS/ -- New York, N.Y. -- Science. 1993 Nov 26;262(5138):1401-7.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8248779" target="_blank"〉PubMed〈/a〉
    Keywords: Amino Acid Sequence ; Crystallography, X-Ray ; *DNA-Binding Proteins ; Fungal Proteins/*chemistry/genetics ; Hydrogen Bonding ; *Leucine Zippers ; Molecular Sequence Data ; Mutation ; Protein Conformation ; Protein Kinases/*chemistry/genetics ; Protein Structure, Secondary ; *Saccharomyces cerevisiae Proteins
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  • 7
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    The nitrogenase FeMo-cofactor and P-cluster pair: 2.2 A resolution structures (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-05-07
    Description: Structures recently proposed for the FeMo-cofactor and P-cluster pair of the nitrogenase molybdenum-iron (MoFe)-protein from Azotobacter vinelandii have been crystallographically verified at 2.2 angstrom resolution. Significantly, no hexacoordinate sulfur atoms are observed in either type of metal center. Consequently, the six bridged iron atoms in the FeMo-cofactor are trigonally coordinated by nonprotein ligands, although there may be some iron-iron bonding interactions that could provide a fourth coordination interaction for these sites. Two of the cluster sulfurs in the P-cluster pair are very close together (approximately 2.1 angstroms), indicating that they form a disulfide bond. These findings indicate that a cavity exists in the interior of the FeMo-cofactor that could be involved in substrate binding and suggest that redox reactions at the P-cluster pair may be linked to transitions of two cluster-bound sulfurs between disulfide and sulfide oxidation states.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Chan, M K -- Kim, J -- Rees, D C -- 1F32 GM15006/GM/NIGMS NIH HHS/ -- New York, N.Y. -- Science. 1993 May 7;260(5109):792-4.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena 91125.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8484118" target="_blank"〉PubMed〈/a〉
    Keywords: Azotobacter vinelandii/*enzymology ; Iron/*chemistry ; Models, Molecular ; Molybdoferredoxin/*chemistry ; Nitrogenase/*chemistry ; Oxidation-Reduction ; Sulfur/*chemistry ; X-Ray Diffraction
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  • 8
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    Crystal structure of neocarzinostatin, an antitumor protein-chromophore complex (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-11-12
    Description: Structures of the protein-chromophore complex and the apoprotein form of neocarzinostatin were determined at 1.8 angstrom resolution. Neocarzinostatin is composed of a labile chromophore with DNA-cleaving activity and a stabilizing protein. The chromophore displays marked nonlinearity of the triple bonds and is bound noncovalently in a pocket formed by the two protein domains. The chromophore pi-face interacts with the phenyl ring edges of Phe52 and Phe78. The amino sugar and carbonate groups of the chromophore are solvent exposed, whereas the epoxide, acetylene groups, and carbon C-12, the site of nucleophilic thiol addition during chromophore activation, are unexposed. The position of the amino group of the chromophore carbohydrate relative to C-12 supports the idea that the amino group plays a role in thiol activation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kim, K H -- Kwon, B M -- Myers, A G -- Rees, D C -- CA47148/CA/NCI NIH HHS/ -- GM45162/GM/NIGMS NIH HHS/ -- New York, N.Y. -- Science. 1993 Nov 12;262(5136):1042-6.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena 91125.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8235619" target="_blank"〉PubMed〈/a〉
    Keywords: Apoproteins/chemistry ; Computer Graphics ; Computer Simulation ; Crystallography, X-Ray ; Hydrogen Bonding ; Models, Molecular ; Protein Conformation ; Protein Structure, Secondary ; Zinostatin/*chemistry
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  • 9
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    Condensed-matter energetics from diatomic molecular spectra (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-04-30
    Description: Analyses of molecular spectra and compression data from crystals show that a single function successfully describes the dependence on interatomic separation of both the potential energy of diatomic molecules and the cohesive binding energy of condensed matter. The empirical finding that one function describes interatomic energies for such diverse forms of matter and over a wide range of conditions can be used to extend condensed-matter equations of state but warrants further theoretical study.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kim, I H -- Jeanloz, R -- Jhung, K S -- New York, N.Y. -- Science. 1993 Apr 30;260(5108):649-52.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17812221" target="_blank"〉PubMed〈/a〉
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  • 10
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    Gene replacement in Toxoplasma gondii with chloramphenicol acetyltransferase as selectable marker (1993)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1993-11-05
    Description: A system for stable transformation of Toxoplasma gondii tachyzoites was developed that exploited the susceptibility of Toxoplasma to chloramphenicol. Introduction of the chloramphenicol acetyltransferase (CAT) gene fused to Toxoplasma flanking sequences followed by chloramphenicol selection resulted in parasites stably expressing CAT. A construct incorporating the tandemly repeated gene, B1, targeted efficiently to its homologous chromosomal locus. Knockout of the single-copy gene, ROP1, was also successful. Stable transformation should permit the identification and analysis of Toxoplasma genes important in the interaction of this opportunistic parasite with its host.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kim, K -- Soldati, D -- Boothroyd, J C -- New York, N.Y. -- Science. 1993 Nov 5;262(5135):911-4.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Microbiology and Immunology, Stanford University School of Medicine 94305.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8235614" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; Chloramphenicol/pharmacology ; Chloramphenicol O-Acetyltransferase/*genetics ; Drug Resistance ; *Genes, Protozoan ; Genetic Markers ; Multigene Family ; Plasmids ; Recombination, Genetic ; Toxoplasma/drug effects/*genetics ; Transfection ; *Transformation, Genetic
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