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

Maxwell D. Seager, Sara Seager+2 more|PubMed|Jan 2, 2024

Planetary Science and Exploration69 references4 citations

TL;DR

This study investigates the stability of 20 biogenic amino acids in concentrated sulfuric acid at Venusian cloud conditions (81–98% w/w H₂SO₄, 30–50 °C). After four weeks, 19 amino acids retained their backbone integrity, with only minor side-chain modifications, indicating that amino acid structures can persist in Venus's acidic clouds—challenging prior assumptions about biochemical incompatibility and expanding the potential for sulfuric acid-based biochemistry.

ABSTRACT

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.

Motivation & Objective

• To assess the chemical stability of all 20 biogenic amino acids under simulated Venusian cloud conditions.
• To determine whether amino acid backbones can survive in concentrated sulfuric acid, a solvent previously considered incompatible with life.
• To evaluate the implications of amino acid stability for the potential habitability of Venus's cloud decks.
• To identify which amino acids undergo structural modification and under what conditions (acid concentration, temperature).
• To inform future missions by identifying biochemically relevant molecules likely to persist in Venus's atmospheric environment.

Proposed method

• Exposure of individual biogenic amino acids to 81% and 98% w/w sulfuric acid solutions at 30 °C and 50 °C for up to 28 days.
• Use of high-performance liquid chromatography (HPLC) to quantify remaining intact amino acids post-exposure.
• Analysis of reaction products via mass spectrometry to detect side-chain modifications.
• Comparison of degradation rates and structural stability across the 20 amino acids under varying acid concentrations.
• Controlled experiments to isolate the effects of temperature and acid concentration on amino acid stability.
• Statistical analysis of survival rates and modification patterns across the amino acid set.

Experimental results

Research questions

• RQ1 Can biogenic amino acids maintain their backbone structure in concentrated sulfuric acid at Venusian cloud conditions?
• RQ2 Which amino acids undergo degradation or side-chain modification in 81–98% H₂SO₄ over four weeks?
• RQ3 How do temperature and acid concentration influence the stability of amino acids in this environment?
• RQ4 To what extent do structural features of amino acids (e.g., side-chain polarity, charge) affect their stability in sulfuric acid?
• RQ5 What are the implications of amino acid stability for the potential existence of sulfuric acid-based biochemistry in Venus's clouds?

Key findings

• Thirteen of the 20 biogenic amino acids remained chemically unreactive in 98% w/w sulfuric acid after four weeks.
• Twelve of the 20 amino acids remained unreactive in 81% w/w sulfuric acid after four weeks.
• For the remaining amino acids, only side-chain modifications occurred; the amino acid backbone remained intact.
• The stability of amino acid backbones in concentrated sulfuric acid challenges the long-held assumption that such solvents are incompatible with life-related chemistry.
• The results suggest that a wide range of biologically relevant molecules could persist in Venus’s cloud layers, supporting the plausibility of alternative biochemistries.
• The findings significantly broaden the scope for considering concentrated sulfuric acid as a potential solvent for extraterrestrial life.