Assistant Professor NAKAMURA Takashi, Institute of Pure and Applied Sciences

Have you ever heard of the term "supramolecule"? A supramolecule is an aggregate of multiple molecules pulled together by gentle forces, and it acquires a specific function or structure. In fact, the proteins that make up our bodies, the hemoglobin that carries oxygen in our blood, and the DNA that carries our genetic information are all supramolecules. Dr. Jean-Marie Lehn, the French chemist who proposed the concept of supramolecules, was awarded the Nobel Prize in Chemistry in 1987.

Phosphate-ion capture by an artificial supramolecule developed by Professor NAKAMURA

"As a chemist, I would eventually like to artificially create complex protein-like substances that work in living organisms from scratch," says Professor NAKAMURA, who is currently developing new functional materials based on supramolecules. Last year, his team developed an artificial supramolecule based on a cyclic organic compound called cyclodextrin. This supramolecule selectively captures phosphate ions in water. Since water is ever present in living organisms and the environment, technologies that recognize and capture specific molecules in water have played essential roles in drug development and environmental science.

The forces that link molecules into supramolecules are called intermolecular forces. The most representative example is hydrogen bonding. For example, hydrogen bonds entangle the two long macromolecules of DNA into its double helix. To understand the mechanism of hydrogen bonding, let us look at a water molecule (H₂O).

A water molecule is formed via covalent bonds between two hydrogen atoms and one oxygen atom. These bonds are strong sustained because the hydrogen and oxygen atoms share electrons, which are drawn toward the oxygen atom, thereby imparting a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. This phenomenon, called polarization, generates an electrostatic force that attracts the hydrogen atom in one water molecule to the oxygen atom in a neighboring water molecule, forming a hydrogen bond. Similar hydrogen bonds form between ammonia (NH₃) molecules, in which hydrogen is covalently bonded to nitrogen. As hydrogen bonds are only approximately one-tenth as strong as covalent bonds, the double helix of DNA can easily attach and detach.

Professor NAKAMURA's Laboratory: A collaborative space with advanced analytical instruments

Earlier, I mentioned that Professor NAKAMURA and his group developed a technology that recognizes and captures specific molecules in water. This process, which exploits hydrogen bonding, was a groundbreaking achievement because overcoming the competition between the hydrogen-bonding sites of the molecule to be captured and those of water molecules is extremely difficult. To solve this problem, Professor NAKAMURA's group added a number of chemical structures called amide groups (–CO–NH–) to cyclodextrins.

Amide groups contain hydrogen, nitrogen, and oxygen atoms that enable hydrogen bonding. By forming hydrogen bonds with water molecules and phosphate ions, they allow the selective capture of phosphate ions. The phosphate structure is contained in adenosine triphosphate (ATP), a major molecule related to biological activities that resides in DNA and supplies energy to cells. Therefore, this technology can play an active role in diverse fields.

Professor NAKAMURA and his group have also worked on metal-containing cyclic supramolecules. Although cyclic molecules can incorporate molecules into their pores, large rings are difficult to create, and their pore size is limited. NAKAMURA's group developed a giant hexagonally shaped organic compound called hexapap. They plan to integrate these macrocycles with multiple metal ions to selectively trap specific organic compounds. This approach could serve as a catalyst for the direct conversion of biological biomass resources into fine chemicals (chemical products with complex structures, such as pharmaceuticals).

"Whether the experiment succeeds or fails, the first person in the world to know the result is the person who set up the experiment. I want to experience that excitement with the students in my laboratory every day. I also want more people to know about supramolecules," says Professor NAKAMURA, whose laboratory is abuzz with positive energy.

Article by Science Communicator at the Bureau of Public Relations