Identification of diamino acids in the Murchison meteorite

Abstract

C hondritic meteorites, in particular the CM-type carbonaceous chondrites, make up a unique subset of primitive meteorites, which are of particular interest in the context of origins of life because of their relatively high carbon content and because most of this carbon is present as organic matter. This material is a diverse mixture of compounds that in particular includes carboxylic acids, dicarboxylic acids, hydroxy acids, sulfonic acids, phosphonic acids, and amino acids in the form of monoamino alkanoic acids and monoamino alkandioic acids (1). Among these classes of compounds, a very small fraction of the meteoritic isomers is believed to support prebiotic evolutionary processes. We analyzed hydrolyzed hot-water extracts of a fresh sample of the Murchison CM-chondrite by GC-MS with a chiral column. With an efficient derivatization method and an improved detection technique we focused on compounds with more than two functional groups. MethodsA 1.19-g sample (MPI 320͞14) was taken from the interior of the Murchison meteorite by using a stone crusher. The sample provided fresh fracture surfaces. The sample was powdered for 2 ϫ 2 min at 600 rpm by a planetary micro mill (Pulverisette 7, Fritsch, Idar-Oberstein, Germany) in a positive-pressure ''class 100'' clean room. A 5-mg aliquot of the sample was imaged by a raster electron microscope (CS44, CamScan, Cranberry Township, PA), showing a homogeneous particle size distribution in the low micrometer range. A cold-water (water for organic trace analysis, Fluka) extraction was performed with a 130-mg aliquot that was subjected to diamino acid analysis. In parallel, 347 mg of the powdered sample was extracted with 700 l of water for 20 h at 100°C. After centrifugation (Eppendorf safe-lock tubes in Biopur-quality), the liquid phase was split. One hundred microliters was taken for direct derivatization by using the procedure described ref.2, leading to N,NЈ-diethoxycarbonyl diamino acid ethyl ester (ECEE) derivatives. Another aliquot of 100 l was hydrolyzed in 6 M HCl (hydrochloric acid for amino acid analysis, ampoule, Fluka) at 110°C for 24 h (3). After evaporation of the 6 M HCl, the residue was dissolved in 0.1 M HCl and derivatized by the above protocol. The ECEE derivatives obtained in this way were subjected to enantioselective GC-MS (Varian Chrompack Chirasil-L-Val capillary column; 12 m ϫ 0.25-mm inner diameter, 0.12-m film thickness, 250°C inlet temperature, pulsed splitless injection, 1.5 ml⅐min Ϫ1 constant f low of He carrier gas). The oven temperature program applied for the solvent trapping mode started at 50°C with 10°C⅐min Ϫ1 up to 90°C and then with 2°C⅐min Ϫ1 to 110°C, then was 10°C⅐min Ϫ1 to 180°C, where it was held constant for 39 min. The Agilent 6890͞5973 GC-MSD system was used. The identities of the diamino acid peaks obtained via enantioselective GC-MS were verified by comparing the retention times and the mass spectra with literature data (4) and external standards purchased from Fluka. A serpentine sample was taken as a blank (5). Results a...

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