Date: 16:00-18:00, Friday, July 4, 2025
発表者：八木健彦
Speaker : Takehiko YAGI
タイトル : 日本では地震がなぜこんなによく起こるのか – プレートテクトニクスがもたらした地球科学のパラダイムシフト
Title : Why Do Earthquakes Occur So Frequently in Japan ? – The Paradigm Shift in Earth Science Brought by Plate Tectonics –
要旨：
1960年代から1970年代にかけて構築されたプレートテクトニクス理論は、それまでの混沌とした静的な地球観を動的地球観に一新させる地球科学のパラダイムシフトであった。この理論によって、日本近辺で頻発する大地震の原因が明瞭に理解されるようになった。本セミナーでは、プレートテクトニクス理論がどのようにして生み出されたのかをふり返り、我々が学べることを考えてみたい。また日本ではまた必ず大地震が起きることが予測され、我々としてはそれにどう対処すべきかも考察する。
発表者：吉岡 美香
Speaker : Mika YOSHIOKA
タイトル: 高圧下におけるアラニンの非生物的エナンチオマー濃縮：ホモキラリティーの起源に関する検討
Title: Abiotic Enantiomeric Enrichment of Alanine at High Pressure: Implications for the Origin of Homochirality
Abstract:
Life on the Earth is composed almost exclusively of L-amino acids and D-sugars, a phenomenon known as homochirality. However, under abiotic conditions, amino acids are typically formed as racemic mixtures, meaning that some asymmetric process must have occurred to achieve this molecular handedness. Although small enantiomeric excesses are proposed to exist in outer space, these slight imbalances are insufficient to fully explain the origin of homochirality. Therefore, it is necessary to consider mechanisms by which such tiny asymmetries could be amplified to reach L-amino acid dominance. One hypothesis involves the separation of enantiomers and racemic compounds by differences in solubility. To explore this possibility, alanine, which is the simplest chiral amino acid, was examined. L-alanine and DL-alanine differ in crystal structures and the solubility of L-alanine is 0.7% higher than that of DL-alanine at ambient pressure.In this study, the solubilities of L-alanine and DL-alanine were investigated under high-pressure conditions (0–0.8 GPa), which is a regime where water remains in the liquid phase. The volumes of single crystals of L-alanine or DL-alanine immersed in their respective saturated aqueous solutions were determined from depth-dependent optical microscopic images obtained at several pressure pointsusing a diamond anvil cell. At 0.8 GPa, L-alanine crystals exhibited approximately 45% volume reduction caused by the dissolution, while DL-alanine crystals showed only about 10% reduction. The actual solubility of L-alanine is estimated to have increased by approximately 20%, while that of DL-alanine exhibited only a slight increase of about 2%. These results indicate that L-alanine is more soluble than DL-alanine under high pressure.To investigate the precipitation behavior of alanine from an aqueous solution, four types of alanine aqueous solutions with different L:D ratios were compressed to about 2.5 GPa. In all compressions, ice VI appeared in the first place and then transformed into ice VII. The crystallization behavior of alanine was found to depend strongly on the enantiomeric composition of the solution. In the L:D=7:3 solution, only DL-alanine was precipitated. This DL-alanine precipitation behavior reflects the lower solubility of DL-alanine under high pressure. The results of solutions with other ratios(L:D=10:0, 9:1 and 5:5) will be discussed in the presentation.These findings suggest a separation mechanism accompanied by precipitation of high-pressure ice: when L-alanine is in slight excess in aqueous solution, DL-alanine crystallizes and excessive L-alanine remains in solution. Such selective crystallization and subsequent segregation could operate in icy extraterrestrial environments where high-pressure ice phases and liquid water coexist. This process may explain how a slight enantiomeric imbalance could be amplified under prebiotic conditions, offering a potential pathway toward the origin of biological homochirality.