L-histidine in Saponite: Detection, Characterization and UV Degradation Studies for Biosignature Identification on Mars

Laboratory simulations of the Martian conditions are essential in order to interpret results collected on Mars exploration missions. Among them, the study of the degradation and/or the evolution of possible organic biomarkers adsorbed on minerals is very important. The thin atmosphere of Mars allows ionizing radiations to reach the surface of the planet and drive the photochemical production of strong oxidants in the soil which possibly degrade the organics on the Mars´ surface¹. However, traces of ancient life forms may be found in old mineral deposits preserved within protected environments recently accessible due to erosion or impacts.It is known that mid-UV radiation is among the main degradation agents on Mars². Thus, inspection the stability and reactivity of possible biomarkers in the eolian-mobile layer and in the different fresh subsurface minerals exposed at the surface of Mars is important. Laboratory simulations of the harsh Martian conditions can evaluate the likelihood of preservation of potential biomarkers on different minerals on Mars. This provides a critical support to rover missions on Mars, helping to select the most appropriate minerals to find organic molecules and therefore, possible biomarkers.

 

In this work, the interaction and stability of L-histidine in saponite are studied. The saponite mineral was chosen because is a common product of low-temperature reaction between water and the mafic minerals on Mars. In addition, saponite in old cold basaltic crust is favourable for microbial life.³ L-histidine, an α- amino acid used in the biosynthesis of proteins, was chosen as biomarker because it is very diagnostic of life. Moreover, it presents different protonation states depending on the pH, which might result in different interaction with the mineral at different pHs.

 

The samples were prepared using the equilibrium adsorption method. In particular, pure synthetic saponite was put in contact with different solutions of L-histidine at pH 2.7 and 9.6, in order to investigate the effect of acidic or alkaline conditions on the adsorption process. The suspensions were kept for 24 hours on rotation to reach the equilibrium state during molecular adsorption, and then they were dried in an oven at 40^◦C.

 

The samples were analyzed by X-Ray Diffraction to understand if any intercalation of L-histidine in the negatively-charged interlayer sites of saponite occurs at acidic pH. Further characterization of molecule-mineral interactions was carried out by Transmittance and Diffuse Reflectance InfraRed Fourier Transform Spectroscopy. Finally, the samples were processed under Martian-like UV irradiation conditions using an experimental setup which allows to monitor the degradation kinetics in situ by infrared spectroscopy analysis during UV irradiation. These experiments allowed us to investigate the catalytic/protective behaviour of saponite towards L-hystidine in different pH conditions, and obtain the degradation cross section of L-histidine adsorbed on saponite compared to the pure molecule, along with the half-lifetime of degradation under Martian UV flux.

 

References

 ¹Quinn, R. C.; Martucci, H. F. H.; Miller, S. R.; Bryson, C. E. Perchlorate Radiolysis on Mars and the Origin of Martian Soil Reactivity. Astrobiology 2013, 13 (6), 515–520.

²Fornaro, T.; Steele, A.; Brucato, J. R. Catalytic/Protective Properties of Martian Minerals and Implications for Possible Origin of Life on Mars. Life 2018, 8.

³Y. Sueoka, S. Yamashita, M. Kouduka, Y. Suzuki, Deep Microbial Colonization in Saponite-Bearing Fractures in Aged Basaltic Crust: Implications for Subsurface Life on Mars, Frontiers in Microbiology. 10 (2019).