Stability of nucleic acid bases in concentrated sulfuric acid: Implications for the habitability of Venus' clouds

Life on Venus?

Venus is not generally at the forefront when considering extraterrestrial life. Yet, based on the physical similarities and proximity to Earth and with the little knowledge of its evolutionary history, there is a possibility that Venus may have hosted life in the past on the surface if Venus had liquid water and perhaps even has water present in the clouds today. While the early suggestions during the beginning of the space exploration about life on Venus were mostly speculative due to limited data, recent interest has arisen from realizations: (i) the unexplained ultraviolet absorption spectrum of Venus resembles many organics, (ii) there is chemical disequilibria in the cloud layer, (iii) the cloud aerosols likely contain significant abundances of hydrated iron and magnesium sulfates, and (iv) the solar radiation received in the cloud layer contains the appropriate wavelengths and flux to support phototrophy. Considering the extreme environmental survival of many terrestrial microorganisms, the possibility remains that any extant life on Venus in the past could have adapted to survival in the cloud layer far above the surface where energy and nutrients are available, but the precise compositions of the cloud particles and water availability are still uncertain. The key to solving the mystery of life on Venus is to determine if Venus had liquid water on the surface in its past and to measure the precise chemical composition of the Venus atmosphere and the cloud particles. Missions which will be launched in the next few years will provide much needed data that should provide some answers we seek and will surely raise more questions. This perspective reviews recent developments.

Simple Lipids Form Stable Higher-Order Structures in Concentrated Sulfuric Acid

Venus has become a target of astrobiological interest because it is physically accessible to direct exploration, unlike exoplanets. So far this interest has been motivated not by the explicit expectation of finding life but rather by a desire to understand the limits of biology. The venusian surface is sterilizing, but the cloud deck includes regions with temperatures and pressures conventionally considered compatible with life. However, the venusian clouds are thought to consist of concentrated sulfuric acid. To determine if any fundamental features of life as we understand them here on Earth could in principle exist in these extreme solvent conditions, we tested several simple lipids for resistance to solvolysis and their ability to form structures in concentrated sulfuric acid. We find that single-chain saturated lipids with sulfate, alcohol, trimethylamine, and phosphonate head groups are resistant to sulfuric acid degradation at room temperature. Furthermore, we find that they form stable higher-order structures typically associated with lipid membranes, micelles, and vesicles. Finally, results from molecular dynamics simulations suggest a molecular explanation for the observed robustness of the lipid structures formed in concentrated sulfuric acid. We conclude with implications for the study of Venus as a target of experimental astrobiology.

Astrobiological implications of the stability and reactivity of peptide nucleic acid (PNA) in concentrated sulfuric acid

Recent renewed interest regarding the possibility of life in the Venusian clouds has led to new studies on organic chemistry in concentrated sulfuric acid. However, life requires complex genetic polymers for biological function. Therefore, finding suitable candidates for genetic polymers stable in concentrated sulfuric acid is a necessary first step to establish that biologically functional macromolecules can exist in this environment. We explore peptide nucleic acid (PNA) as a candidate for a genetic-like polymer in a hypothetical sulfuric acid biochemistry. PNA hexamers undergo between 0.4 and 28.6% degradation in 98% (w/w) sulfuric acid at ~25°C, over the span of 14 days, depending on the sequence, but undergo complete solvolysis above 80°C. Our work is the first key step toward the identification of a genetic-like polymer that is stable in this unique solvent and further establishes that concentrated sulfuric acid can sustain a diverse range of organic chemistry that might be the basis of a form of life different from Earth's.

Mechanism for Peptide Bond Solvolysis in 98% w/w Concentrated Sulfuric Acid

We propose a mechanism for the solvolysis of the peptide bond in 98% (w/w) concentrated sulfuric acid based on the assessment of reactivity of four dipeptides at room temperature: l-alanyl-l-alanine (1), glycylglycine (2), glycyl-l-alanine (3), and l-alanylglycine (4). We find that dipeptides (2) and (4) are stable for at least two months in 98% w/w sulfuric acid, with no signs of reactivity. The dipeptides (1) and (3) are unstable and immediately begin complex solvolysis, which is mechanistically different from acid-catalyzed peptide bond hydrolysis. We show that the solvolysis of dipeptides (1) and (3) in 98% w/w sulfuric acid leads to the formation of alaninamide (6) and glicinamide (7), respectively. We propose that the mechanism of solvolysis of dipeptides (1) and (3) proceeds via dehydrogenation of the side chain methyl group (-CH3) of the C-terminal alanine. Consequently, the substitution of the -CH3 group of the C-terminal alanine with -CF3 stabilizes the l-alanyl-DL-trifluoroalanine dipeptide (5) to solvolysis in 98% w/w sulfuric acid.

Venus cloud catcher as a proof of concept aerosol collection instrument

We report on the proof-of-concept of a low-mass, low-power method for collecting micron-sized sulfuric acid aerosols in bulk from the atmosphere of Venus. The collection method uses four wired meshes in a sandwich structure with a deposition area of 225 cm2. It operates in two modes: passive and electrostatic. During passive operation, aerosols are gathered on the deposition surface by aerodynamic force. During electrostatic operation, a tungsten needle discharges a high voltage of - 10 kV at the front of the grounded mesh structure. The discharge ionizes aerosols and attracts them to the mesh by Coulomb forces, resulting in improved efficiency and tentative attraction of submicron aerosols. We describe the instrument construction and testing in the laboratory under controlled conditions with aerosols composed of 25%, 50%, 70%, 80%, 90% and 98%* concentration by volume of sulfuric acid, the rest water. We demonstrated the following: (i) both modes of operation can collect the entire range of sulfuric acid solutions; (ii) the collection efficiency increases steadily (from a few percent for water to over 40% for concentrated sulfuric acid) with the increased concentration of sulfuric acid solution in water in both modes; (iii) the relative improvement in the collection of the electrostatic mode decreases as the sulfuric acid concentration increases. We also demonstrated the operation of the instrument in the field, cloud particle collection on Mt. Washington, NH, and crater-rim fumaroles' particle collection on Kīlauea volcano, HI. The collection rate in the field is wind-speed dependent, and we observed collection rates around 0.1 ml[Formula: see text] in low wind environments (1-2 m[Formula: see text]), and around 1 ml[Formula: see text] in stronger wind (7-9 m[Formula: see text]).

Alternative Solvents for Life: Framework for Evaluation, Current Status, and Future Research

Life is a complex, dynamic chemical system that requires a dense fluid solvent in which to take place. A common assumption is that the most likely solvent for life is liquid water, and some researchers argue that water is the only plausible solvent. However, a persistent theme in astrobiological research postulates that other liquids might be cosmically common and could be solvents for the chemistry of life. In this article, we present a new framework for the analysis of candidate solvents for life, and we deploy this framework to review substances that have been suggested as solvent candidates. We categorize each solvent candidate through the following four criteria: occurrence, solvation, solute stability, and solvent chemical functionality. Our semiquantitative approach addresses all the requirements for a solvent not only from the point of view of its chemical properties but also from the standpoint of its biochemical function. Only the protonating solvents fulfill all the chemical requirements to be a solvent for life, and of those only water and concentrated sulfuric acid are also likely to be abundant in a rocky planetary context. Among the nonprotonating solvents, liquid CO2 stands out as a planetary solvent, and its potential as a solvent for life should be explored. We conclude with a discussion of whether it is possible for a biochemistry to change solvents as an adaptation to radical changes in a planet's environment. Our analysis provides the basis for prioritizing future experimental work to explore potential complex chemistry on other planets. Key Words: Habitability-Alternative solvents for life-Alternative biochemistry. Astrobiology 24, 1231-1256.

General instability of dipeptides in concentrated sulfuric acid as relevant for the Venus cloud habitability

Recent renewed interest in the possibility of life in the acidic clouds of Venus has led to new studies on organic chemistry in concentrated sulfuric acid. We have previously found that the majority of amino acids are stable in the range of Venus' cloud sulfuric acid concentrations (81% and 98% w/w, the rest being water). The natural next question is whether dipeptides, as precursors to larger peptides and proteins, could be stable in this environment. We investigated the reactivity of the peptide bond using 20 homodipeptides and find that the majority of them undergo solvolysis within a few weeks, at both sulfuric acid concentrations. Notably, a few exceptions exist. HH and GG dipeptides are stable in 98% w/w sulfuric acid for at least 4 months, while II, LL, VV, PP, RR and KK resist hydrolysis in 81% w/w sulfuric acid for at least 5 weeks. Moreover, the breakdown process of the dipeptides studied in 98% w/w concentrated sulfuric acid is different from the standard acid-catalyzed hydrolysis that releases monomeric amino acids. Despite a few exceptions at a single concentration, no homodipeptides have demonstrated stability across both acid concentrations studied. This indicates that any hypothetical life on Venus would likely require a functional substitute for the peptide bond that can maintain stability throughout the range of sulfuric acid concentrations present.

The Concept of Life on Venus Informs the Concept of Habitability

An enduring question in astrobiology is how we assess extraterrestrial environments as being suitable for life. We suggest that the most reliable assessments of the habitability of extraterrestrial environments are made with respect to the empirically determined limits to known life. We discuss qualitatively distinct categories of habitability: empirical habitability that is constrained by the observed limits to biological activity; habitability sensu stricto, which is defined with reference to the known or unknown limits to the activity of all known organisms; and habitability sensu lato (habitability in the broadest sense), which is circumscribed by the limit of all possible life in the universe, which is the most difficult (and perhaps impossible) to determine. We use the cloud deck of Venus, which is temperate but incompatible with known life, as an example to elaborate and hypothesize on these limits.

A Quadruple Revolution: Deciphering Biological Complexity with Artificial Intelligence, Multiomics, Precision Medicine, and Planetary Health

A quiet quadruple revolution has been in the making in systems science with convergence of (1) artificial intelligence, machine learning, and other digital technologies; (2) multiomics big data integration; (3) growing interest in the "variability science" of precision/personalized medicine that aims to account for patient-to-patient and between-population differences in disease susceptibilities and responses to health interventions such as drugs, nutrition, vaccines, and radiation; and (4) planetary health scholarship that both scales up and integrates biological, clinical, and ecological contexts of health and disease. Against this overarching background, this article presents and highlights some of the salient challenges and prospects of multiomics research, emphasizing the attendant pivotal role of systems medicine and systems biology. In addition, we emphasize the rapidly growing importance of planetary health research for systems medicine, particularly amid climate emergency, ecological degradation, and loss of planetary biodiversity. Looking ahead, we anticipate that the integration and utilization of multiomics big data and artificial intelligence will drive further progress in systems medicine and systems biology, heralding a promising future for both human and planetary health.

Formation of Chromophores from cis-Pinonaldehyde Aged in Highly Acidic Conditions

Sulfuric acid in the atmosphere can participate in acid-catalyzed and acid-driven reactions, including those within secondary organic aerosols (SOA). Previous studies have observed enhanced absorption at visible wavelengths and significant changes in the chemical composition when SOA was exposed to sulfuric acid. However, the specific chromophores responsible for these changes could not be identified. The goals of this study are to identify the chromophores and determine the mechanism of browning in highly acidified α-pinene SOA by following the behavior of specific common α-pinene oxidation products, namely, cis-pinonic acid and cis-pinonaldehyde, when they are exposed to highly acidic conditions. The products of these reactions were analyzed with ultra-performance liquid chromatography coupled with photodiode array spectrophotometry and high-resolution mass spectrometry, UV-vis spectrophotometry, and nuclear magnetic resonance spectroscopy. cis-Pinonic acid (2) was found to form homoterpenyl methyl ketone (4), which does not absorb visible radiation, while cis-pinonaldehyde (3) formed weakly absorbing 1-(4-(propan-2-ylidene)cyclopent-1-en-1-yl)ethan-1-one (5) and 1-(4-isopropylcyclopenta-1,3-dien-1-yl)ethan-1-one (6) via an acid-catalyzed aldol condensation. This chemistry could be relevant for environments characterized by high sulfuric acid concentrations, for example, during the transport of organic compounds from the lower to the upper atmosphere by fast updrafts.

Year-Long Stability of Nucleic Acid Bases in Concentrated Sulfuric Acid: Implications for the Persistence of Organic Chemistry in Venus' Clouds

We show that the nucleic acid bases adenine, cytosine, guanine, thymine, and uracil, as well as 2,6-diaminopurine, and the "core" nucleic acid bases purine and pyrimidine, are stable for more than one year in concentrated sulfuric acid at room temperature and at acid concentrations relevant for Venus clouds (81% w/w to 98% w/w acid, the rest water). This work builds on our initial stability studies and is the first ever to test the reactivity and structural integrity of organic molecules subjected to extended incubation in concentrated sulfuric acid. The one-year-long stability of nucleic acid bases supports the notion that the Venus cloud environment-composed of concentrated sulfuric acid-may be able to support complex organic chemicals for extended periods of time.

Astrobiological Potential of Venus Atmosphere Chemical Anomalies and Other Unexplained Cloud Properties

Long-standing unexplained Venus atmosphere observations and chemical anomalies point to unknown chemistry but also leave room for the possibility of life. The unexplained observations include several gases out of thermodynamic equilibrium (e.g., tens of ppm O2, the possible presence of PH3 and NH3, SO2 and H2O vertical abundance profiles), an unknown composition of large, lower cloud particles, and the "unknown absorber(s)." Here we first review relevant properties of the venusian atmosphere and then describe the atmospheric chemical anomalies and how they motivate future astrobiology missions to Venus.

Stability of 20 Biogenic Amino Acids in Concentrated Sulfuric Acid: Implications for the Habitability of Venus' Clouds

Scientists have long speculated about the potential habitability of Venus, not at the 700K surface, but in the cloud layers located at 48-60 km altitudes, where temperatures match those found on Earth's surface. However, the prevailing belief has been that Venus' clouds cannot support life due to the cloud chemical composition of concentrated sulfuric acid-a highly aggressive solvent. In this work, we study 20 biogenic amino acids at the range of Venus' cloud sulfuric acid concentrations (81% and 98% w/w, the rest water) and temperatures. We find 19 of the biogenic amino acids we tested are either unreactive (13 in 98% w/w and 12 in 81% w/w) or chemically modified in the side chain only, after 4 weeks. Our major finding, therefore, is that the amino acid backbone remains intact in concentrated sulfuric acid. These findings significantly broaden the range of biologically relevant molecules that could be components of a biochemistry based on a concentrated sulfuric acid solvent.

Stability of nucleic acid bases in concentrated sulfuric acid: Implications for the habitability of Venus' clouds

What constitutes a habitable planet is a frontier to be explored and requires pushing the boundaries of our terracentric viewpoint for what we deem to be a habitable environment. Despite Venus' 700 K surface temperature being too hot for any plausible solvent and most organic covalent chemistry, Venus' cloud-filled atmosphere layers at 48 to 60 km above the surface hold the main requirements for life: suitable temperatures for covalent bonds; an energy source (sunlight); and a liquid solvent. Yet, the Venus clouds are widely thought to be incapable of supporting life because the droplets are composed of concentrated liquid sulfuric acid-an aggressive solvent that is assumed to rapidly destroy most biochemicals of life on Earth. Recent work, however, demonstrates that a rich organic chemistry can evolve from simple precursor molecules seeded into concentrated sulfuric acid, a result that is corroborated by domain knowledge in industry that such chemistry leads to complex molecules, including aromatics. We aim to expand the set of molecules known to be stable in concentrated sulfuric acid. Here, we show that nucleic acid bases adenine, cytosine, guanine, thymine, and uracil, as well as 2,6-diaminopurine and the "core" nucleic acid bases purine and pyrimidine, are stable in sulfuric acid in the Venus cloud temperature and sulfuric acid concentration range, using UV spectroscopy and combinations of 1D and 2D 1H 13C 15N NMR spectroscopy. The stability of nucleic acid bases in concentrated sulfuric acid advances the idea that chemistry to support life may exist in the Venus cloud particle environment.