For eons, marine organisms have taken in carbon dioxide dissolved in water and turned it into protective calcium carbonate layers, or seashells with the assistance of proteins. Inspired by these creatures and looking to reduce atmospheric carbon dioxide levels, a team led by scientists at Pacific Northwest National Laboratory sought to mimic this process by a rational design of highly stable protein-like molecules (called peptoids). The team discovered that they could speed or slow calcium carbonate or calcite formation by changing the peptoid's structure and tuning its electric charge and affinity for water.
"Because peptoids are highly stable and can be easily and cheaply synthesized, peptoid-controlled carbonate precipitation offers a broad range of potential applications, such as geologic carbon dioxide sequestration," said Dr. Chun-Long Chen, a materials chemist, now at PNNL, and a co-author of the study.
Rising carbon dioxide levels are linked to decreased nutritional quality of food crops, ocean acidification, and other forms of environmental change. Peptoids could help sequester carbon dioxide emitted by concentrated, high-volume sources, such as coal or cement plants. Carbon dioxide could be captured, turned into a supercritical fluid, and injected into underground reservoirs for geologic sequestration. Peptoids may be able to accelerate the conversion of carbon dioxide into minerals, sealing the injection site, and becoming part of the millennia-old rocks. Further, the basic principles elucidated in the study could be used in manufacturing and refinement processes.
"Fundamentally, this research provides the principles for designing synthetic polymers as crystallization promoters, and not just for calcium carbonate," said Dr. Jim De Yoreo, a co-author of the study and leader of PNNL's Materials Synthesis and Simulation Across Scales Initiative.
The team from PNNL, Lawrence Berkeley National Laboratory, and Imperial College London designed and synthesized different peptoids that mimic proteins that crystallize calcium carbonate in nature. They varied the peptoids by changing the molecules' hydrophobicity, or "water fearing" nature. They did this by altering the structure of the peptoids. For other molecules, they altered how well the molecule bound to calcium carbonate, by varying the number of carboxylic acid (-COOH) groups. In the end, they had 28 different peptoids. They characterized the resulting crystal growth, or lack thereof.
The team found that two aspects of crystal growth could be controlled by tuning the peptoid chemical structure: crystal shapes and growth speed. Tuning is done by balancing the electrostatic and hydrophobic interactions. The hydrophobic interactions played the dominant role in determining the speed of formation and the resulting shapes.
Through PNNL's Materials Synthesis and Simulation Across Scales Initiative, the team is pushing peptoids to do more. They are studying the ability of peptoids to assemble into porous materials and mimic protein membranes in filtering out desired targets, such as carbon dioxide.
Explore further: Study shows calcium carbonate takes multiple, simultaneous roads to different minerals
Chen CL, J Qi, J Tao, RN Zuckermann, and JJ DeYoreo. 2014. "Tuning Calcite Morphology and Growth Acceleration by a Rational Design of Highly Stable Protein-Mimetics." Scientific Reports 4, article number 6266. DOI: 10.1038/srep06266