Enantiomer–Racemate Separation in Alanine Aqueous Solutions Triggered by Crystallization of High-Pressure Ice

The exclusive use of l-amino acids in terrestrial life─known as biological homochirality─suggests the existence of a mechanism that can amplify small initial l-enantiomeric excesses, possibly of extraterrestrial origin, into near-complete chiral purity. In this study, we examined a potential physicochemical mechanism for such amplification involving the pressure-induced dissolution and crystallization behavior of enantiomer (l-alanine) and racemate (dl-alanine) in aqueous solutions. Solubilities were measured at high pressures using a diamond anvil cell, where a single crystal of l-alanine or dl-alanine was compressed in saturated solutions up to about 0.8 GPa and crystal volumes were measured. As pressure increased, the solubility of l-alanine significantly increased by 24%, whereas that of dl-alanine showed only a 2% increase. To further investigate enantiomeric separation, several aqueous solutions with varying l:d ratios were pressurized up to 2 GPa. The crystallization of alanine was accompanied by the crystallization of high-pressure ice phases (ice VI and VII). Precipitates were dependent on the initial l:d ratio of the solution; for example, in a l:d = 7:3 system, dl-alanine preferentially precipitated, while the excess l-alanine did not crystallize. These observations demonstrate a selective crystallization mechanism driven by enhanced solubility differences under pressure, which provides a plausible prebiotic pathway for the amplification of chiral asymmetry in amino acid systems, particularly in the interiors of icy planetary bodies.