Tortorella, Attila (2025) Stability and interaction determinants of biomolecules in the harsh conditions of the solar system. [Tesi di dottorato]

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Abstract

Life’s persistence depends on the ability of biomolecules to preserve structure, stability, and function under variable physicochemical conditions. Investigating how these processes occur in environments beyond terrestrial standards provides critical insight into the molecular boundaries of habitability. This PhD thesis examines how temperature, ionic composition, hydration and pressure affect the organization and energetics of biological macromolecules under Martian-like aqueous conditions, with the goal of identifying the physicochemical principles that enable molecular adaptation in extreme or extraterrestrial environments. Chapter 2 focuses on the behaviour of model bacterial membranes composed of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) in the presence of perchlorate and sulphate salts, both abundant in the Martian regolith. Using spectroscopic and calorimetric techniques, the effects of these salts on bilayer stability, phase behaviour, and fluidity were characterized. The results revealed that perchlorates favour the physiologically relevant fluid phase in the presence of high pressure, suggesting that a prototypical bacterium could thrive in such conditions. Remarkably, even in concentrated brines, PE/PG membranes maintained their integrity and exhibited reversible thermotropic transitions, demonstrating that simple phospholipid bilayers can remain stable under saline conditions analogous to those expected for Mars. Chapter 3 explores molecular recognition processes through a detailed thermodynamic characterization of the lysozyme–NAG3 complex as a model of protein–ligand interaction. Isothermal titration calorimetry and complementary spectroscopic analyses revealed that Martian-relevant salts modulate binding energetics primarily through hydration effects. Perchlorate ions weaken binding affinity by competitive inhibition, whereas other salts preserve binding. In all cases, a pronounced enthalpy–entropy compensation was observed, indicating that hydration rearrangements mitigate environmental perturbations and confer molecular resilience in high-salinity media. Finally, Chapter 4 investigates the interactions between model lipid membranes and G-quadruplex (G4) nucleic acid structures. These non-canonical nucleic acids were selected because of their exceptional thermodynamic stability under harsh conditions, including elevated temperature and high ionic strength. Given their intrinsic robustness, it is reasonable to hypothesize that such structures could have persisted in prebiotic environments and interacted with primitive membranes, potentially contributing to the molecular organization processes that preceded the emergence of microbial life. Experimental analyses demonstrated that both DNA and RNA G4s associate with zwitterionic membranes, while only RNA G4s display measurable binding to anionic membranes. Isothermal titration calorimetry revealed distinct thermodynamic mechanisms for DNA and RNA binding, reflecting differences in backbone chemistry and hydration dynamics. Furthermore, G4s preferred gel-phase membranes over fluid-phase ones, underscoring the role of membrane phase in this type of interaction. These findings suggest that prototypical biological membranes could have actively modulated the structure, stability, and organization of nucleic acids, influencing molecular evolution at the interface between chemistry and biology. In summary, this PhD thesis demonstrates that essential biological processes, compartmentalization, molecular recognition, and information storage, can persist under a wide range of non-terrestrial conditions. The results delineate the molecular strategies that underlie thermodynamic resilience and highlight membrane-associated systems as robust chemical frameworks capable of sustaining life in extraterrestrial aqueous environments.

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