Identifying the wide diversity of extraterrestrial purine and pyrimidine nucleobases in carbonaceous meteorites

The study addresses why pyrimidine nucleobases have been rarely reported in meteorites despite prebiotic models predicting their formation from meteoritic precursors. Recent samples from C- and B-type asteroids (Ryugu and Bennu) highlight the importance of carbonaceous bodies as reservoirs of diverse organic molecules relevant to the origin of life. Prior work identified nucleobases in carbonaceous chondrites but largely reported purines as dominant; uracil was the only pyrimidine previously quantified in Murchison water extracts. The authors aim to systematically search for and quantify a broader diversity of purine and pyrimidine nucleobases and isomers in carbonaceous meteorites using highly sensitive extraction and LC–HRMS methods that minimize degradation of fragile compounds and reduce contamination, thereby testing the hypothesis that pyrimidines are present but were previously undetected due to methodological limitations and sample heterogeneity.

• Carbonaceous chondrites contain diverse suites of organic molecules, including amino acids and sugars; ultra-high-resolution mass spectrometry of Murchison revealed hundreds of thousands of soluble organic species, underscoring substantial chemical diversity.
• Nucleobases have been reported in meteorites, with purines generally more abundant than pyrimidines. Uracil had been the only pyrimidine quantified in Murchison (~30 ppb in H2O extracts), with additional detections in harsher formic acid extracts.
• Prior extractions often used severe conditions (e.g., 95% formic acid, 100 °C, >24 h), which may enhance yields but can degrade fragile molecules (e.g., sugars, hexamethylenetetramine), complicating interpretation and possibly biasing observed distributions.
• Sample heterogeneity is known in meteorites (e.g., amino acid concentrations vary widely across Tagish Lake lithologies), likely affecting nucleobase distributions and concentrations.
• Photochemical experiments simulating interstellar ice processing produce nucleobases and many isomers, often yielding higher relative abundances of pyrimidine derivatives compared to purines, suggesting an ISM photochemical contribution to meteoritic organics.

Samples: Two Murchison (CM2) specimens (Murchison #1 from the University of Chicago; Murchison #2 from a meteorite trading company), Murray (CM2), and Tagish Lake (C2) powders were analyzed. Portions of 0.5–2 g were used.

Extraction and cleanup: For Murchison #2, Murray, and Tagish Lake, finely powdered samples were water-extracted by ultrasonication (10 min on crushed ice) with ultrapure water at room temperature, repeated three times with centrifugation between steps; supernatants were combined, frozen, and dried under reduced pressure. Extracts were cleaned by ion-exchange chromatography (AG1-X8) to isolate nucleobases and N-heterocycles, then dried, re-dissolved, and filtered (0.20 μm PTFE). Recovery of nucleobases and N-heterocycles with this procedure was validated previously using standards.

Instrumental analysis: Liquid chromatography–electrospray ionization–high-resolution mass spectrometry (HPLC-(ESI)-HRMS) at 140,000 resolving power over m/z 50–2000. Columns included inertSustain PFP and Hypercarb (2.1 × 250 mm). Exact-mass extracted ion chromatograms (±3 ppm) were used for target m/z, with MS/MS fragmentation for structural confirmation against standards. Formic acid in the mobile phase enhanced protonation and detectability. Chromatographic baseline resolution was achieved for N-heterocycles.

Contamination control: Procedural blanks (2 g deionized water) were processed identically. A soil sample from the Murchison strewn field (20–30 cm depth) was analyzed to assess terrestrial contamination. All glassware was baked at 450 °C for 3 h before use.

Photochemical product synthesis (comparative analogs): Interstellar ice analogs (H2O, CO, NH3, CH3OH) were deposited at 10 K and irradiated with deuterium lamps in a high-vacuum setup (SAMRA). After irradiation, residues were warmed to room temperature, extracted with H2O/CH3OH (1:1), and processed as above. These products served to compare distributions with meteoritic extracts.

Data analysis: Peak identification by retention time, accurate mass, and MS/MS matching to standards; quantification at ppb to ppt levels via calibration with standards; evaluation of isomer distributions and homolog series (e.g., alkylated imidazoles).

• First comprehensive detection of a wide variety of pyrimidine nucleobases and isomers in carbonaceous meteorites, including cytosine, uracil, thymine, isocytosine, and multiple imidazolecarboxylic acids; adds to known purines (adenine, guanine) and related purines (xanthine, hypoxanthine).
• Murchison extracts (ppb-level): examples include guanine (72 ppb in #1; 6 ppb in #2), adenine (15 ppb in #1; 1 ppb in #2), xanthine (39 ppb in #1; 3 ppb in #2), hypoxanthine (24 ppb in #1; 1 ppb in #2); pyrimidines detected include uracil (15 ppb in #1; 1 ppb in #2), cytosine (~4 ppb in both), thymine (up to ~5 ppb), and isocytosine (≤1 ppb). 1-methyluracil and methyl-imidazolecarboxylic acids were also present at multi-ppb levels.
• Other meteorites: In Murray, guanine was most abundant among purines (~25 ppb). In Tagish Lake, adenine (~6 ppb) and 6-methyluracil (~17 ppb) were particularly abundant.
• Extremely high abundances of alkylated imidazole homologs inferred: estimated totals up to ~41,300 ppb (Murchison), ~500 ppb (Tagish Lake), and ~21,200 ppb (Murray), indicating alkylimidazoles are among the most abundant solvent-extractable organics in these meteorites.
• Structural isomer distributions and relative abundances of pyrimidine analogs in meteorites closely resemble products from photoprocessed interstellar ice analogs, supporting a contribution from ISM photochemistry.
• Absence (or very low presence) of pyrazole compared to abundant imidazole homologs; several peaks likely represent structural isomers of alkylated imidazole/pyrazole.
• Detection of nicotinamide and nicotinic acid (vitamin B3 forms) at tens of ppb levels, consistent with previous reports for some carbonaceous meteorites.
• Soil from the Murchison strewn field shows different profiles and, in some cases, higher levels of certain compounds, but the overall molecular diversity, racemic amino acids, and preserved sugars in meteoritic extracts support an indigenous extraterrestrial origin of the detected nucleobases.

The results resolve the longstanding paucity of reported pyrimidines in meteorites by demonstrating that diverse pyrimidine nucleobases and isomers are present at ppb to sub-ppb levels. Methodological advances—gentle water extraction, ion-exchange cleanup, optimized HPLC conditions with formic acid, high-resolution MS, and MS/MS confirmation—substantially enhanced detectability and minimized degradation, revealing a richer inventory than previously observed. Differences between Murchison #1 and #2 underscore sample heterogeneity but do not negate the broader finding of pyrimidine diversity.

Comparisons with photochemically produced interstellar ice residues show similar molecular distributions (e.g., dominance of pyrimidine analogs, patterns of alkylated homologue series), suggesting that a fraction of the nucleobase inventory and related N-heterocycles could originate from ISM photoprocessing, later incorporated into asteroid parent bodies. Parent-body aqueous alteration and hydrothermal reactions may further modify this inventory (e.g., conversion of cytosine to uracil via deamination), generating the observed suite.

The indigenous nature of these compounds is supported by: (i) co-detection of racemic amino acids and primordial sugars from the same extracts; (ii) the presence of dihydrouridine and other non-biological distributions; and (iii) distinct profiles relative to local soil controls. The large abundances of alkylimidazoles highlight previously underappreciated N-heterocycle classes in meteorites, with implications for prebiotic catalysis and cofactor chemistry.

Collectively, these findings strengthen the hypothesis that carbonaceous meteorites delivered a complex mixture of canonical and non-canonical nucleobases and precursors to the early Earth, potentially facilitating the emergence of genetic polymers.

This study identifies a wide diversity of purine and, crucially, pyrimidine nucleobases and isomers in carbonaceous meteorites (Murchison, Murray, Tagish Lake) using highly sensitive LC–HRMS workflows. The molecular distributions resemble those from photoprocessed interstellar ice analogs, indicating that ISM photochemistry likely contributed to meteoritic nucleobases, with additional parent-body processing shaping the final suite. The detection of canonical base pairs (adenine–uracil, guanine–cytosine, adenine–thymine) alongside numerous non-canonical analogs implies that meteorites could have supplied a broad set of informational building blocks to the early Earth.

Future work should: (i) perform comprehensive surveys of returned asteroid samples (Ryugu, Bennu) with similar or improved methods; (ii) integrate computational tools for homolog and isomer family assignment; (iii) use in silico reaction discovery and laboratory simulations to elucidate formation pathways; and (iv) assess the roles of aqueous alteration and thermal histories in modulating nucleobase inventories.

• Publication reports ppb–ppt quantification limits and numerous identifications, but many related peaks remain unidentified, limiting full molecular accounting.
• Mild extraction optimized to preserve fragile compounds may underestimate total nucleobase concentrations compared with harsher extractions used previously.
• Sample heterogeneity between meteorite pieces (e.g., Murchison #1 vs #2) affects concentration estimates and complicates direct comparisons.
• Formation pathways (ISM vs parent-body processes) are inferred from distributional similarities and not uniquely resolved; detailed mechanisms remain beyond the scope of this study.
• Potential terrestrial contamination was addressed via blanks and local soil controls, but cannot be completely excluded for trace-level compounds.