Enantiomer excesses of rare and common sugar derivatives in carbonaceous meteorites

Abstract

Biological polymers such as nucleic acids and proteins are constructed of only one-the D or L-of the two possible nonsuperimposable mirror images (enantiomers) of selected organic compounds. However, before the advent of life, it is generally assumed that chemical reactions produced 50:50 (racemic) mixtures of enantiomers, as evidenced by common abiotic laboratory syntheses. Carbonaceous meteorites contain clues to prebiotic chemistry because they preserve a record of some of the Solar System's earliest (∼4.5 Gy) chemical and physical processes. In multiple carbonaceous meteorites, we show that both rare and common sugar monoacids (aldonic acids) contain significant excesses of the D enantiomer, whereas other (comparable) sugar acids and sugar alcohols are racemic. Although the proposed origins of such excesses are still tentative, the findings imply that meteoritic compounds and/or the processes that operated on meteoritic precursors may have played an ancient role in the enantiomer composition of life's carbohydrate-related biopolymers.carbonaceous meteorites | sugar acids | enantiomer excesses | aldonic acids | polyols T he organic phase of carbonaceous meteorites comprises an insoluble "macromolecular" material (1-3), a complex mixture of largely uncharacterized solvent-extractable compounds (4), as well as discrete (identified) soluble organic compounds such as amino acids (3, 5) nucleobases (6, 7), and sugar derivatives (8). Analyses of this prebiotic organic carbon have been important in understanding its early Solar System synthesis and history: Characteristics of the organic phase suggest that it consists of both products and survivors of interstellar/presolar grain irradiation (9) and subsequent encapsulation and aqueous reactions (10, 11) in asteroids "parent bodies."Several identified meteoritic organic compounds are chiral: They can exist as two nonsuperimposable mirror image compounds called enantiomers, commonly designated D and L. To date, most chiral meteoritic compounds are reported to be racemic mixtures, i.e., their D and L enantiomers are equal in abundance (3). Racemic compounds are expected in nature because typical abiotic synthetic processes are (historically) thought to occur in the absence of asymmetric influences. Racemic mixtures are equated with pristine/uncontaminated abiotic samples when discussing meteoritic compounds. However, some meteoritic amino acids that are rare on Earth, i.e., they are not constituents of proteins and therefore less likely to be contaminants, have been confirmed to contain L enantiomer excesses (EE) (3, 12, 13). The origins of such excesses are unknown.Sugars, aldehydes, or ketones that contain multiple carbon hydroxyl (carbon alcohol) groups, are also chiral and were likely necessary for the origin of life. In the majority of extant biological sugars and derivatives ("polyols"), the D enantiomers are significantly more abundant than the L enantiomers (we will note exceptions in Results and Natural Occurrence of Relevant Sugar Derivatives and Enantiomers)...

Chiral Sugars Drive Enantioenrichment in Prebiotic Amino Acid Synthesis

“…45 Derivatives of these sugars recently identified on meteoritic samples were shown to exhibit significant enantiomeric excesses toward the natural ( d ) enantiomer, 46 and their formation in interstellar ice analogues, albeit without indication of their stereochemistry, has recently been reported. 47 …”

“…It is important to note that sugars synthesized under prebiotically plausible conditions 50 or extracted from meteoritic samples 46 yield derivatives of a mixture of the pentoses shown in Table 1, even though in modern biology these pentoses do not play equal roles and are not formed in similar relative concentrations. For example, enantioenriched derivatives of the biologically rare lyxose were found in similar abundance to derivatives of ribose on two separate meteorites.…”

The Aguas Zarcas (CM2) meteorite: New insights into early solar system organic chemistry

“…Consistent with the prebiotic synthesis of the above C4 compounds, the C4 branched compounds, hydroxy methylglyceric acid (HMG), and hydroxy methylglycerol (HMGly) are also present (Fig. 6): these compounds are biologically rare and therefore indicative of indigenous chemical synthesis (Cooper and Rios 2016). Also, present (not shown) are traces of C5 and C6 sugar alcohols and C5 sugar acids.…”

“…Threitol, a chiral alcohol, is approximately racemic: the enantiomers of erythronic acid (C4) were not resolvable under the employed chromatographic conditions; however, threonic acid contained a small D‐enantiomer excess. This excess is apparently smaller than D‐threonic acid excesses from multiple other carbonaceous meteorites (Cooper and Rios 2016). The latter excesses were thought to be indigenous, that is, not a product of contamination; more chiral analyses of Zarcas extracts will be performed.…”

Monosaccharides and Their Derivatives in Carbonaceous Meteorites: A Scenario for Their Synthesis and Onset of Enantiomeric Excesses

“…The most recent works on the search for meteoritic polyols reported the finding of one sugar (dihydroxyacetone), several sugar alcohols, sugar mono- and di-acids and deoxy sugar acids [ 17 , 31 ] ( Figure 3 ).…”