How calcium may have guided early molecular directionality

Just as human hands are mirror images of one another, many organic molecules also appear in left- and right-handed forms. However, living systems display a pronounced bias: the sugars in DNA are uniformly right-handed, while amino acids used in proteins are exclusively left-handed. This phenomenon, known as homochirality, is fundamental to biology, but its emergence has remained one of science's deepest mysteries.

To explore possible prebiotic routes to homochirality, the researchers examined tartaric acid (TA), a small molecule with two chiral centers. They found that calcium significantly affects how TA molecules assemble into larger structures. When calcium is absent, solutions containing only one form of TA readily form polyester polymers. In contrast, mixtures containing equal parts of both forms resist polymerization. Curiously, the presence of calcium flips this behavior: it suppresses polymerization in pure TA solutions while facilitating it in racemic mixtures.

"This suggests that calcium availability could have created environments on early Earth where homochiral polymers were favoured or disfavoured," explained Chen Chen, a Special Postdoctoral Researcher at the RIKEN Center for Sustainable Resource Science (CSRS), and co-leader of the study. The team attributes this reversal to two key effects: calcium binds with TA to form crystals that remove equal amounts of both chiral forms, and it also alters the reaction pathways that govern how TA polymerizes. This combination could have enhanced slight chirality imbalances, eventually leading to the consistent handedness observed in today's biological molecules.

Importantly, the research suggests that simple polyesters formed from TA might have been among the first homochiral molecules, predating the rise of biomolecules like DNA, RNA, and proteins. "The origin of life is often discussed in terms of biomolecules like nucleic acids and amino acids," noted ELSI's Specially Appointed Associate Professor Tony Z. Jia, who also led the study. "However, our work introduces an alternative perspective: that 'non-biomolecules' like polyesters may have played a critical role in the earliest steps toward life."

The work also proposes that calcium levels in ancient environments could have influenced which polymers emerged. In areas low in calcium, such as certain lakes or ponds, conditions may have favored the development of homochiral polymers. In contrast, calcium-rich regions might have promoted polymers containing a mix of both chiralities.

Beyond offering insight into prebiotic chemistry, the study draws from and contributes to multiple disciplines, including geology, biophysics, and materials science. The findings represent the culmination of extensive global collaboration, uniting researchers from institutions across Asia, Europe, Australia, and North America.

"We faced significant challenges in integrating all of the complex chemical, biophysical, and physical analyses in a clear and logical way," said Ruiqin Yi, project co-leader from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. "But thanks to the hard work and dedication of our team, we've uncovered a compelling new piece of the origins of life puzzle."

This interdisciplinary approach not only enriches our view of how life began on Earth but also opens new possibilities for identifying similar chemical pathways on other planets.

Research Report:Primitive homochiral polyester formation driven by tartaric acid and calcium availability