SUPRAMOLECULAR: THE BOND BEYOND

Supramolecular chemistry is defined as chemistry "beyond the molecule". Just as ordinary molecules are aggregates of atoms, 'supramolecules' are aggregates of molecules. The bonds holding together atoms in an ordinary molecule are strong, covalent bonds. The bonds holding molecules together in a supramolecule are 'weak' bonds such as hydrogen bonds; bonds formed because of the attraction of a hydrogen bonded to an electronegative element like nitrogen or oxygen, to other oxygen or nitrogen atoms.

Supramolecular chemistry is immensely important for understanding life. That's because all the action in living organisms is mediated by such molecular assemblies connected by weak hydrogen bonds. Even though they are weak, just like many individual strands in a rope, they add up to a very strong force when they are present in large numbers. Also, in biological systems, strength is not as important as timing. If a bond is too strong, it may be useless for biological interactions because once formed, it will be very difficult to break in order to proceed to the next step. Molecular interactions in biological systems are multiple-step, dynamic interactions, and it's only the right strength and combination of weak and strong bonds that optimizes the function of biological systems.

If the molecules in biology are human beings, then weak bonds such as hydrogen bonds are the relationships and interactions that human beings have with each other and with the outside world in general. As with the human world, biology would not function without these relationships. Hence their pivotal importance for actually making things work in living organisms. And hence the importance of supramolecular chemistry for understanding, and hopefully learning to modulate, these interactions for practical purposes.

Supramolecular chemistry is also very important for materials science. It is obvious that every material around us consists of molecules which interact with each other. Understanding these interactions through supramolecular chemistry is helping us devise new materials for electronics, plastics, and biopolymers which may one day replace vital elements in our body. Because weak bonds are not as strong as covalent bonds, they enable us to have subtle and fine control over the synthesis of new materials which wil help us to sensitively manipulate their properties. Think of biosensors which will detect negligible concentrations of pesticides in our food, or toxins in the soil, or chemical warfare agents. Think of 'smart' materials inside the body which will respond to temperature and the presence of nutrients, enabling us to selectively and periodically release drugs in the body only under certain conditions and only in certain cells or organs. All these applications and many more will require the understanding of the the weak interactions that confer such exquisite sensitivity in biological systems.

Last but not the least, supramolecular assemblies look pretty, and provide an artistic edge to science. For example, here is a supramolecular shape I made from just two molecules, cyanuric acid and melamine, both of which are useful molecules in their own right. Melamine might be familiar as a material used in plastic kitchen utensils. Modifications of cyanuric acid are used as disinfectants.



The purple atoms are carbon, the red and blue ones are oxygen and nitrogen respectively, and the white atoms are hydrogens. The yellow hydrogen bonds are clearly seen. The beauty and symmetry of such structures is striking, and countless geometrical possibilities abound for combining any number of such molecules into symmetrical shapes. The economy of construction- just two molecules in this case- is also striking.

And if one is bored (as I was) one can also create whatever shapes he or she wants to. For instance, here's a "supramolecular dog" that I made. In the first view, every atom is represented as a big sphere, which reflects its actual size.



And here's the real character of the supramolecule, a hydrogen bonded assembly like that pictured above. It's clear that even though every hydrogen bond may be weak, many such bonds are quite enough to preserve the shape of the dog.



As in many other matters, don't underestimate the strength of the 'weak', and the strength of numbers.