Electron microscopy of a hemagglutinin from Limulus polyphemus.
Macromolecular ring shaped components corresponding to hemagglutinin studied in Limulus polyphemus hemolymph by electron microscopy
Edelman, G. M.↗
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Structural Differences Between Insoluble Organic Matter in Matrix and Chondrules: Potential Insights into Pre-Parent Body Thermal Alteration
Carbon is a major molecular constituent of primitive chondrites of low petrologic grade. A majority of the carbon (>70%) is in the insoluble organic matter (IOM) fraction that cannot be extracted from the meteorite sample using polar or nonpolar solvents. The IOM is comprised of a kerogen-like macromolecular carbon (MMC) structure that consists of aromatic rings connected by highly branching aliphatic chains. Raman spectroscopy is highly sensitive to IOM due to a strong resonance enhancement that occurs for all visible excitation wavelengths, making Raman a powerful in-situ probe of IOM. The Raman spectrum of IOM consists of two primary bands, the G band originates from all carbon-carbon bonds in the MMC structure and the D band that originates from carbon 6-member ring breathing motions. The peak position and width of the G and D bands probe the structure of the IOM. Using Raman spectroscopy, the IOM structure has been shown to correlate with the thermal alteration history of the meteorite, allowing for petrologic grade classification to the tenths place in the 3.0-3.9 range. Here, we have collected Raman images of GRA 06100,41 (CR2) for the examination of the IOM spatial distribution and observe significant differences between the IOM in the matrix and chondrules.
R S Jakubek↗
Irradiated Benzene Ice Provides Clues to Meteoritic Organic Chemistry
Aromatic hydrocarbons account for a significant portion of the organic matter in carbonaceous chondrite meteorites, as a component of both the low molecular weight, solvent-extractable compounds and the insoluble organic macromolecular material. Previous work has suggested that the aromatic compounds in carbonaceous chondrites may have originated in the radiation-processed icy mantles of interstellar dust grains. Here we report new studies of the organic residue made from benzene irradiated at 19 K by 0.8 MeV protons. Polyphenyls with up to four rings were unambiguously identified in the residue by gas chromatography-mass spectrometry. Atmospheric pressure photoionization Fourier transform mass spectrometry was used to determine molecular composition, and accurate mass measurements suggested the presence of polyphenyls, partially hydrogenated polyphenyls, and other complex aromatic compounds. The profile of low molecular weight compounds in the residue compared well with extracts from the Murchison and Orgueil meteorites. These results are consistent with the possibility that solid phase radiation chemistry of benzene produced some of the complex aromatics found in meteorites.
benzene↗
Structural Studies of Human Pyruvate Dehydrogenase
Human pyruvate dehydrogenase (E1) catalyzes the irreversible decarboxylation of pyruvate in the presence of Mg(2+) and thiamin pyrophosphate (TPP) followed by the rate-limiting reductive acetylation of the lipoyl moiety linked to dihydrolipoamide acetyltransferase. The three-dimensional structure of human E1 is elucidated using the methods of macromolecular X-ray crystallography. The structure is an alpha, alpha', beta and beta' tetramer with the protein units being in the tetrahedral arrangement. Each 361-residue alpha-subunit and 329-residue beta-subunit is composed of a beta-sheet core surrounded by alpha-helical domains. Each subunit is in extensive contact with all the three subunits involving TPP and magnesium cofactors, and potassium ions. The two binding sites for TPP are at the alpha-beta' and alpha'-beta interfaces, each involving a magnesium ion and Phe6l, His63, Tyr89, and Met200 from the alpha-subunit (or alpha'-subunit), and Met81 Phe85, His128 from the beta-subunit (or beta'-subunit). K+ ions are nestled between two beta-sheets and the end of an alpha-helix in each beta-subunit, where they are coordinated by four carbonyl oxygen groups from Ile12, Ala160, Asp163, and Asnl65, and a water molecule. The catalytic C2 carbon of thiazolium ring in this structure forms a 3.2 A contact with a water molecule involved in a series of H-bonds with other water molecules, and indirectly with amino acids including those involved in the catalysis and regulation of the enzyme.
Ciszak, Ewa↗