What are the different adsorption mechanisms of L-histidine onto saponite at acidic and basic pH levels?

Label:chem

Topic
Understanding the adsorption mechanisms of L-histidine onto saponite is crucial for interpreting its stability and potential detection on Mars. The study explores how L-histidine interacts with saponite at different pH levels, which can influence the adsorption process and the molecule's resistance to UV radiation.

From: "Photodegradation of the biomarker L-histidine induced by edge sites of a clay mineral in the Martian harsh UV environment", Icarus, Volume 442, December 2025, 116757
Answer
At acidic pH (pH 4), L-histidine is positively charged and can be intercalated into the interlayers of saponite through cation exchange with sodium cations. Additionally, interactions occur at the edges of the saponite through the carboxylate group of L-histidine with protonated edge groups (AlOH2+ and SiOH2+). At basic pH (pH 9.6), L-histidine is negatively charged and cannot intercalate into the interlayers due to electrostatic repulsion. Instead, it interacts with the edges of the saponite through hydrogen bonding with the imidazole NH group. These different adsorption mechanisms affect the stability and detectability of L-histidine under UV radiation.
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