Reactions of lysine with montmorillonite at 80 °C: Implications for optical activity, H+ transfer and lysine–montmorillonite binding

Amino acid–smectite interaction may have catalyzed prebiotic reactions essential for the emergence of life. Lysine solutions (0.05 M) were reacted with Na-smectite in adsorption–desorption experiments. The lysine–smectite complexes were heated at 80 °C for 10 days to investigate (1) possible slow processes taking place at surface temperature that would be accelerated at higher temperature and (2) processes taking place in hydrothermal systems. Three sets of experiments were performed: thermal treatment in closed tubes and water added regularly; thermal treatment in closed tubes without adding water; and thermal treatment in open tubes and no added water. After lysine desorption (displacement with 0.1 M CaCl2), the solutions were investigated using circular dichroism (CD) and the smectite samples using FTIR and CHN elemental analysis. CD spectra were dependent on the solution pH, which was controlled by lysine protonation state. The lysine protonation state was altered by the adsorption–desorption process, with a higher Lys+/Lys± ratio after desorption. The CD and CHN analyses show that the thermal treatment in a moist state causes stronger smectite–lysine binding. FTIR data suggest that the stronger binding is caused by more or stronger H bonds between –NH+3 lysine groups and smectite basal O atoms.

Graphical abstractAdsorption–desorption from smectite changes lysine protonation state from dominantly a zwitterion to dominantly cationic. Thermal treatment of lysine–smectite complexes in moist state increases the binding forces between the two.Figure optionsDownload full-size imageDownload as PowerPoint slide