SUGAR-RELATED COMPOUNDS IN METEORITES

George Cooper, Novelle Kimmich, Josh Sarinana, Katrina Brabham, Laurence Garrel, and Warren Belisle

A goal of NASA is to understand the origin and evolution of life. Carbonaceous meteorites provide the only record yet available for the laboratory study of organic compounds that were synthesized very early in the Solar System and delivered to the planets. Until now sugars and related compounds (polyols), one the most critical classes of compounds necessary for all current lifeforms, had not been definitively identified in extraterrestrial samples. Ribose and deoxyribose, five-carbon sugars, are central to the role of contemporary nucleic acids, DNA and RNA. Glycerol, a three-carbon sugar alcohol, is a constituent of all known biological membranes. Part of the scientific research performed at Ames is directed towards determining if such compounds are part of the organic content of meteorites. This report described the results of the search for such compounds.

Results are reported from analysis of water extracts of the Murchison and Murray carbonaceous meteorites. The means of identification of compounds was gas chromatography-mass spectrometry (GC-MS). Compounds were prepared for GC-MS as their trimethylsilyl and/or tertiary butyl-dimethylsilyl derivatives. Our analyses of Murchison and Murray extracts show that a variety of polyols are present in carbonaceous meteorites (Figure 1). The identified compounds include a sugar, dihydroxy acetone; sugar-alcohols; sugar mono-acids; sugar di-acids; and "deoxy" sugar acids (or "saccharinic" acids). In general the compounds follow the abiotic synthesis pattern of other meteorite classes of organic compounds: decreasing abundance with increasing carbon number within a class of compounds and many, if not all, possible isomers are present at a given carbon number.

A plausible synthetic origin for at least of some of the polyols in Murchison and Murray is the photolysis of interstellar gases on interstellar grains. Another possible origin is the condensation of alkaline aqueous solutions of formaldehyde – which is known to produce polyols. Formaldehyde is a relatively abundant and ubiquitous molecule in interstellar space and comets. Extracts of Murchison and Murray show that the aqueous solution on the parent body(ies) was slightly alkaline. Once produced, further chemistry under alkaline and/or oxidizing can oxidized sugars to a variety of acids of the type in Figure 1.

The fact that a suite of related sugar derivatives and dihydroxyacetone are present in meteorites makes it likely that more sugars were, at one time, also present. Other bodies (comets or asteroids), perhaps in different stages of aqueous alteration or oxidation, may have delivered intact sugars to planets in the early Solar System. However dihydroxyacetone alone is capable of producing larger sugars in aqueous solution. The finding of these compounds in some of the oldest objects in the Solar System suggests that polyhdroxylated compounds were, at the very least, available for incorporation into the first living organisms.

Figure 1. Polyols identified in the Murchison and Murray carbonaceous meteorites. Compounds were identified by gas chromatography-mass spectrometry as their trimethylsilyl and/or tertiary butyl-dimethylsilyl derivatives. *6-C Sugars monomers were not seen but may be present in bound forms.