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.

Keywords: aldonic acids; carbonaceous meteorites; enantiomer excesses; polyols; sugar acids.

Fig. 1.

Structures of sugar derivatives analyzed in this work. The compounds are composed of two to six carbons, 2C−6C. Sugar alcohols are commonly defined by the corresponding (parent) sugars, e.g., ribitol is the reduced form of ribose. Deoxy sugar acids (*) are acids with at least one carbon not bonded to an −OH group. Sugar alcohols are underlined.

Fig. 2.

GC-MS analysis of 3C and 4C sugar acids and enantiomers from various meteorites. (A) Racemic glyceric acid (Upper) and 4C acids including a d-excess in threonic acid (Lower) from GRA 95229. (B−D) Analyses from the indicated meteoritic samples: Murch 39 (B), glyceric acid (Upper) and 4C acids (Lower); Murch 38 (C), 4C acids; and ALH 831102 (D), 4C acids; the rare branched compound HMG is ubiquitous (see Results). Erythronic acid also contains a d EE (Table 1). All compounds were analyzed as isopropyl ester/trifluoroacetyl ester (i-Pr/TFA) derivatives (see SI Materials and Methods).

Fig. S1.

Additional meteoritic 3C and 4C sugar acid analyses. (A) Glyceric acid and 4C acids from the Murray meteorite. Murray has a much longer residence time on Earth than Murchison, possibly resulting in more contamination of l-threonic acid. HMG was not identified in Murray; this may be due to a general lower abundance of compounds in this sample. (B) HMG and 4C acids from a sample of Murchison 52. (separate from that in Fig. 4C)

Fig. S2.

The 4C deoxy sugar acids from the same sample and GC-MS run of Murchison 39 as that in Figs. 2B and 4A. A small amount of 3,4-dihydroxybutyric acid might be present and coelute with the second enantiomer of 2-methylglyceric acid.

Fig. 3.

GC-MS analysis of 4C and 5C sugar alcohols including enantiomer separation of d and l threitol from various meteorites. Meteorites analyzed are (A) Murchison (Murch) 57, (B) equimolar standards of erythritol and threitol (total, d+l) run under the same GC parameters as Murch 57, (C) GRA 95229, and (D) ALH 85013. The rare hydroxymethylglycerol (HMGly), the reduced analog of HMG (Fig. 1), was observed in most meteorite samples. (E) 5C Sugar alcohols from GRA 95229 (Upper) compared with corresponding compounds in soil/dust (Lower). The soil/dust sample was extracted for 20 h under an ambient (air) atmosphere. All compounds were analyzed as TFA derivatives.

Fig. 4.

The 5C sugar acid enantiomer analysis of the Murchison meteorite. (A) A sample of Murch 39 (same sample as in Fig. 2B) analyzed immediately after extraction and preparation. (B) Murch 39-2, a separate portion of the Murch 39 extract, was stored until methods were developed to (reliably) separate enantiomers of arabinonic acid (Upper). The corresponding compounds in a sample of soil/dust (from the same sample as in Fig. 3E) (Lower). The change in d-l elution order of xylonic acid enantiomers from A was due to use of a different GC column (see SI Materials and Methods). A possible trace amount of lyxonic acid is present in the soil/dust sample; this would likely be due to the breakdown/oxidation of the common 6C sugar, galactose (see homologous relationships in Fig. S5) as none of the rare parent sugar of lyxonic acid, lyxose, was found. (C) The 5C acids of Murch 52. Methods for the consistent enantiomer separation of ribonic acid were not yet developed. No l enantiomers of lyxonic acid or xylonic acid were seen—likely due to overall low abundances. The change in d-l elution order of arabinonic acid enantiomers from that of B was due to use of a different GC column and compound derivative. Murch 39 samples were analyzed in their i-Pr/TFA forms; Murch 52 compounds were analyzed in ethyl esters (Et/TFA) forms (see SI Materials and Methods for derivatization details).

Fig. 5.

GC-MS chromatograms and mass spectra of 6C sugar acids; all compounds in this analysis were converted to their i-Pr/TFA derivatives. (A) Murch 39 (Upper) and the corresponding compounds from soil/dust (Lower, from same soil/dust sample as Figs. 4B and 3E); *, in Murch 39, d-allonic acid is of relatively low abundance and is considered a tentative identification; however, the major ions, 631, 563, and 543, are present in the mass spectrum (see B) at the correct GC retention time; †, the enantiomers of mannonic acid coelute under the employed GC conditions. Parentheses indicate that the l enantiomers were not detected; their known retention times are indicated. (B) Mass spectra of a 6C standard (gluconic acid) (Lower) and a 6C sugar acid (Upper) from Murchison. The shown mass spectra are nearly identical to all 6C aldonic acids. (C) Examples of enantiomer separation of standards.

Fig. 6.

Plot of the average d/l ratios of the majority of identified chiral meteoritic sugar acids from Table 1. The 5C values are only from Murch 39; this Murchison fragment was more pristine than Murch 52; *, the 6C value is extrapolated based on the d/l trend of 3C → 4C sugar acids.

Fig. S3.

GC-IRMS chromatogram of ¹³C isotope measurements of d+l glyceric acid (peak 6) and 2-methylglyceric acid (peak 5) after initial purification from a Murchison extract by ion chromatography (both compounds analyzed as TMS derivatives). The β-alanine (peak 7) is an internal standard. Peaks 3 and 4 are pulses of CO₂ reference gas.

Fig. S4.

Amino acid enantiomer analysis of meteorite GRA 95229. The sample is from the same extract as that found to have an excess of d-threonic acid and racemic glyceric acid (Fig. 2A). Other protein amino acids (not shown) were also racemic.

Fig. S5.

The structural relationships of homologous 3C through 6C straight-chained aldehyde (aldo) sugars (only d enantiomers are shown). Upon mild oxidation or alkaline hydrolysis of an individual sugar, a predictable set of homologous degradation products (sugar acids) is observed. For example, upon oxidation of mannose, observed compounds include mannonic acid (trace amounts) → arabinonic acid → erythronic acid → glyceric acid; for talose, compounds include talonic acid (trace amounts) → lyxonic acid → threonic acid → glyceric acid. Arrows are not definite indicators of the order of product formation. Degradation products are observed to retain the d/l ratio of the starting sugar (Fig. S6).

Fig. S6.

A few examples of degradation products from the oxidation and alkaline hydrolysis of sugar standards: (A) fructose, (B) allose, and (C) idose. The observed reactions in each chromatogram demonstrate that degradation products maintain the initial d/l ratio of the starting sugar. Glyceric acid enantiomers were not consistently resolvable (depending on column condition); the shown chromatograms illustrate a range of compounds.

Fig. S7.

The interconversion and retention of EE of 5C sugar acids under relatively mild alkaline conditions. In each compound, only carbon #2 can (relatively) easily invert configuration thus allowing epimerization (schemes A and B) but not compete racemization.

Fig. S8.

Additional contamination studies. (A) A typical profile of glyceric acid from water-extracted soil/dust (microorganisms)—as compared with racemic glyceric acid observed in the meteorites of this study. (B) The 4C acids in a known contaminated (crust) sample of Murchison; the GC column did not separate the enantiomers of erythronic acid. There is a significant increase in the l/d ratio of threonic acid relative to that of interior samples and a pristine meteorite such as GRA95229 (Table 1 and Fig. 2): The increase in the l enantiomer can be the result of microbial vitamin C metabolism and/or the result of selective microbial consumption of the d enantiomer as shown by laboratory experiments (Fig. S9).

Fig. S9.

Chromatograms showing microbial action on a standard mixture of dl-threonic acid. (A) Control sample containing only the standards in water. (B) The results of sustained microbial action on the same mixture after addition of a small amount of Murchison meteorite powder and incubation at 37 °C for 5 days. There is clearly preferential consumption of the d enantiomer. Theoretically, such an effect would lead to excesses of the l enantiomer (opposite to the meteoritic results in Table 1) if microbial consumption of meteoritic threonic acid under ambient (dry) meteorite storage conditions was significant; incubation results were similar for other polyol standards. In the case of dl-arabinonic acid, where the l enantiomer is biologically dominant, microbes indeed consumed more of the l enantiomer: ∼2–3 times less l is present (relative to d) after 10 days. However, all observed soil or contaminated meteorite samples show an excess of the l-arabinonic enantiomer (e.g., Fig. 4B, Lower). Such results indicate that preferential microbial consumption of enantiomers are unlikely to be significant, at least in the present results.