5:15 PM - 7:15 PM
[MIS08-P02] In-situ observation of the crystallization behavior of L-excess alanine solutions under high pressure and the possibility of enantiomeric enrichment
Keywords:amino acid, high pressure, chirality
Amino acids have enantiomers, the L-form and the D-form. Since these enantiomers have nearly identical physical and chemical properties, such as melting points and solubility, the abiotic synthesis results in a racemic mixture with an equal ratio of L- and D-forms. On the other hand, amino acids in biological systems consist exclusively of the L-form. This phenomenon is known as "homochirality," and its origin has not been clarified yet.
Various hypotheses have been proposed to explain the origin of homochirality. One leading hypothesis is that circularly polarized ultraviolet light in space selectively induced the formation or decomposition of one enantiomer. However, the enantiomeric excess of L-amino acids generated in space is estimated to be only about 1%, and even the highest L-excess detected in meteorites is only 10–20%. So, these hypotheses are insufficient to fully explain the origin of homochirality. To solve the mystery of the origin of life, it is essential to elucidate how L-excess amino acids underwent enantiomericenrichment and ultimately led to homochirality after being incorporated into meteoritic parent bodies or comets.
In this study, we investigated the enantiomeric enrichment of L-amino acid under high-pressure conditions, simulating environments where water and amino acids coexist, such as inside icy planets.
Saturated aqueous solutions of L-alanine and D-alanine were mixed at volume ratios of 10:0, 9:1, 7:3, and 5:5. Each sample solution was pressurized up to about 3 GPa using the gas-loading diamond anvil cells. The pressure was determined by the ruby fluorescence method. In-situ X-ray diffraction measurements were performed at KEK-PF BL18C to observe the crystallization of alanine and high-pressure ice phases (ice VI, VII).
In all the four solutions, ice VI began to precipitate at 1.0–1.5 GPa and transformed into ice VII at 2.0–2.5 GPa. In contrast, the precipitation behavior of alanine crystals varied depending on the D/L ratio of the solutions. Only L-alanine crystals precipitated from the solutions with L:D = 10:0 and 9:1, while only DL-alanine precipitated from the solutions with L:D = 7:3 and 5:5. In the 7:3 solution, no diffraction peaks corresponding to excess L-alanine were observed, despite the excess L-alanine molecules. This suggests that L-alanine that did not crystallize under high pressure may exist in a non-crystalline form. Additionally, the unit cell volumes of ice VI and ice VII obtained from the 7:3 solution were notably smaller than those from the other solutions. This implies that uncrystallized L-alanine molecules may have some effects on the ice structure. Possibly, three states of existence of the L-alanine molecule are expected; concentrated solution, amorphous, and incorporation into the ice.
Furthermore, the solubility of L-alanine was compared with that of DL-alanine under pressures(0.0–0.8 GPa) where water remains liquid at room temperature. L-alanine and DL-alanine crystals were placed in their respective saturated aqueous solutions, and their crystals were observed while pressure was increased in steps of about 0.2 GPa. As a result, the size of the L-alanine crystals decreased by approximately 40%, indicating that dissolution occurred with increasing pressure. In contrast, DL-alanine crystals showed no change in size, suggesting that dissolution did not proceed under high pressure. Given that the solubilities of L-alanine and DL-alanine are nearly identical at ambient pressure, it was revealed that only L-alanine exhibited an increase in solubility under high pressure. This result supports the observation that DL-alanine precipitates more readily than L-alanine under high pressure.
This study suggests that the selective precipitation of ice and DL-alanine leads to the enantiomeric enrichment in an L-excess alanine aqueous solution.
Various hypotheses have been proposed to explain the origin of homochirality. One leading hypothesis is that circularly polarized ultraviolet light in space selectively induced the formation or decomposition of one enantiomer. However, the enantiomeric excess of L-amino acids generated in space is estimated to be only about 1%, and even the highest L-excess detected in meteorites is only 10–20%. So, these hypotheses are insufficient to fully explain the origin of homochirality. To solve the mystery of the origin of life, it is essential to elucidate how L-excess amino acids underwent enantiomericenrichment and ultimately led to homochirality after being incorporated into meteoritic parent bodies or comets.
In this study, we investigated the enantiomeric enrichment of L-amino acid under high-pressure conditions, simulating environments where water and amino acids coexist, such as inside icy planets.
Saturated aqueous solutions of L-alanine and D-alanine were mixed at volume ratios of 10:0, 9:1, 7:3, and 5:5. Each sample solution was pressurized up to about 3 GPa using the gas-loading diamond anvil cells. The pressure was determined by the ruby fluorescence method. In-situ X-ray diffraction measurements were performed at KEK-PF BL18C to observe the crystallization of alanine and high-pressure ice phases (ice VI, VII).
In all the four solutions, ice VI began to precipitate at 1.0–1.5 GPa and transformed into ice VII at 2.0–2.5 GPa. In contrast, the precipitation behavior of alanine crystals varied depending on the D/L ratio of the solutions. Only L-alanine crystals precipitated from the solutions with L:D = 10:0 and 9:1, while only DL-alanine precipitated from the solutions with L:D = 7:3 and 5:5. In the 7:3 solution, no diffraction peaks corresponding to excess L-alanine were observed, despite the excess L-alanine molecules. This suggests that L-alanine that did not crystallize under high pressure may exist in a non-crystalline form. Additionally, the unit cell volumes of ice VI and ice VII obtained from the 7:3 solution were notably smaller than those from the other solutions. This implies that uncrystallized L-alanine molecules may have some effects on the ice structure. Possibly, three states of existence of the L-alanine molecule are expected; concentrated solution, amorphous, and incorporation into the ice.
Furthermore, the solubility of L-alanine was compared with that of DL-alanine under pressures(0.0–0.8 GPa) where water remains liquid at room temperature. L-alanine and DL-alanine crystals were placed in their respective saturated aqueous solutions, and their crystals were observed while pressure was increased in steps of about 0.2 GPa. As a result, the size of the L-alanine crystals decreased by approximately 40%, indicating that dissolution occurred with increasing pressure. In contrast, DL-alanine crystals showed no change in size, suggesting that dissolution did not proceed under high pressure. Given that the solubilities of L-alanine and DL-alanine are nearly identical at ambient pressure, it was revealed that only L-alanine exhibited an increase in solubility under high pressure. This result supports the observation that DL-alanine precipitates more readily than L-alanine under high pressure.
This study suggests that the selective precipitation of ice and DL-alanine leads to the enantiomeric enrichment in an L-excess alanine aqueous solution.