This study has detected bound organic acids within the Murchison meteorite organic macromolecule. Benzoic acid was the most abundant compound; other abundant compounds include C1 and C2 benzoic acids. Their origin and significance will be discussed

Gilmour, I.

Sephton, M. A.

Watson, J. S.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsMacromolecular organic acids in the MurchisonmeteoriteConference or Workshop ItemHow to cite:Watson, J. S.; Sephton, M. A. and Gilmour, I. (2005). Macromolecular organic acids in the Murchison meteorite. In:36th Lunar and Planetary Science Conference, 14-18 Mar 2006, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1829.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukMACROMOLECULAR ORGANIC ACIDS IN THE MURCHISON METEORITE.  J. S. Watson, M A. Seph-ton, I. Gilmour, 1Planetary and Space Sciences Research Institute, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (j.watson@open.ac.uk).   Introduction:  There have been relatively few studies of polar moieties in the macromolecular or-ganic matter in carbonaceous chondrites (e.g. [1]). NMR data suggests that there are various oxygen-containing functionalities in Murchison [2]. This study aims to characterize the polar moieties associated with Murchison macromolecular material. Conventional pyrolysis gas chromatography mass spectrometry (Py-GC-MS) is unable to detect a large number of polar O- and N-containing compounds as these species are ei-ther not GC-amenable or defunctionalization occurs during pyrolysis.  This study has detected bound organic acids within the Murchison meteorite organic macromolecule. A HF/HCl residue from a sample of Murchison meteorite was subjected to online thermochemolysis followed by analysis by gas chromatography-mass spectrometry (GC-MS). Thermally assisted hydrolysis and methyla-tion (thermochemolysis) using tetramethylammonium hydroxide (TMAH) results in the cleaving of ether and ester bonds and methylation of the released products. Temperatures employed (280 °C) are below those re-quired for pyrolytic cracking of organic material. Thus thermochemolysis is an excellent method for determin-ing oxygen bound moieties within organic macromole-cules such as that in the Murchison meteorite. Fig 1: Total ion chromatogram of products released upon thermochemolysis of HF/HCl residue from Mur-chison.  The distribution of the C1 and C2 benzoic acids (BA) contain all possible structural isomers (except the ethyl BA). The most abundant isomers include 3,4-dimethylbenzoic acid (DMBA), 3,5-DMBA, 2,6-DMBA and phenylacetic acid. ). Unlike terrestrial samples extraterrestrial samples are expected to con-tain all possible isomers. Naphthoic acids are also pre-sent and represent the largest ring size aromatic acids detected. Results and discussion:  The distribution of com-pounds released by thermochemolysis are displayed in Fig 1. Benzoic acid was the most abundant compound released, other abundant compounds include methyl and dimethyl benzoic acids together with methoxy benzoic acids (Figs. 1 and 2). Short chain α,ω-dicarboxylic acids (C4-8) were also released from the macromolecule similar to those observed in solvent extracts of the same meteorite [3]. Other compounds detected include fluoranone, methoxymethylsulfanyl-benezenes and dimethylsulfone as well as aromatic hydrocarbons.     Fig 2: Partially reconstructed mass chromatogram (m/z 150 + 164) displaying the C1 and C2 benzoic ac-ids (as methyl esters) released upon thermochemolysis of HF/HCl residue from Murchison.  Hydrocarbons were also detected that where not present in the thermally desorbed (280 °C, no TMAH) sample. Compounds identified include naphthalene, Lunar and Planetary Science XXXVI (2005) 1829.pdfmethylnaphthalenes, biphenyl, methylbiphenyls, ace-naphthylene, acenaphthene, phenanthrene, anthracene, fluoranthene and pyrene (see Fig 3). This discrepancy between thermally desorbed and thermochemolysis products suggest that hydrocarbons are released when the HF/HCl residue is treated with TMAH and may be occluded in the organic framework but not covalently bound. Such an interpretation is supported by the pre-dominance of the parent molecule compared to the alkylated homologue. This is the first time that oc-cluded hydrocarbons have been suggested to be pre-sent within the macromolecular material.   Fig 3: Partially reconstructed mass chromatogram (m/z 128 + 142 + 152 + 154 +178 + 202) displaying aromatic hydrocarbons released upon thermochemoly-sis of HF/HCl residue from Murchison  There are two probable mechanisms for the forma-tion of organic acids firstly irradation of cosmic ices in the interstellar medium. The more likely process in-volves aqueous alteration upon the parent body, either oxidation of free hydrocarbons or oxidation of macro-molecular material. The distribution of the acids pre-sent will reflect whether they are derived from the free fraction or the macromolecule. Bound acids will be formed from free acids being transesterfied into the macromolecule. The aliphatic acids will probably be destroyed by further oxidation whereas the aromatic acids require ring cleavage for complete oxidation and are therefore more resistant to complete mineralisation.   References:   [1]Hayatsu, R., et al., (1980) Science, 207, 1202-1204. [2] Cody G. D. et al.. Geochimica et Cosmo-chimica Acta, 66, 1851-1865. [3] Lawless, J. G. and Yuen, G. U., (1979). Nature, 282, 396-398.  Lunar and Planetary Science XXXVI (2005) 1829.pdf

Macromolecular organic acids in the Murchison meteorite

http://oro.open.ac.uk/8150/1/Macromolecular_Organic_Acids_in_the_Murchison_Meteorite.pdf

Macromolecular organic acids in the Murchison meteorite

Abstract

Similar works

Full text

Available Versions

Open Research Online (The Open University)