An international team of chemists have come up with a new way of mechanically linking two large molecules. This forms part of the discipline known as supramolecular chemistry, where large molecules can be assembled in unusual ways to make ‘molecular machines’.
A chemist can use various methods to link two or more molecules together. One method is through covalent or ionic bonds, which require one or more atoms from each molecule to bind directly to each other. This can be a very strong interaction, but it fundamentally alters the structure of the molecules involved. For example, bonding a carbon nanotube to another molecule leaves a hole in the nanotube.
There are other types of interactions available, involving the electron clouds surrounding each molecule, but these are normally quite weak.
This is why a lot of research is now focused on mechanical bonds – knotting, tying or threading molecules together, in much the same way as you would two pieces of string. A mechanical bond is extremely strong, but it doesn’t affect the individual structure of each molecule in any way
In their new paper, Emilio Pérez and his team at Fundacion IMDEA Nanociencia in Spain, along with Dirk Guldi from Friedrich-Alexander Universität Erlangen-Nürnberg, Germany, have described a way of linking porphyrinoids and carbon nanotubes by braiding them together.
Porphyrinoids are a class of molecule that are often found in nature. Large, flat and with a space in the middle, they can be used to bind other atoms, which is helpful for various applications. For example haemoglobin, the protein responsible for transporting oxygen around the body contains porphyrinoids. Haemoglobin is large and complex, but central to its structure is a heme group – a porphyrin that has an iron cation (an iron atom with a positive charge) bound in its centre. The iron atom can in turn bind to an oxygen molecule. The oxygen binds reversibly, meaning that the heme can pick it up in the lungs and drop it off at other locations in the body.
Meanwhile, single walled carbon nanotubes are a synthetic material that has been attracting increasing interest among scientists. Long narrow tubes made from a single rolled-up layer of carbon atoms, they are stronger than steel and can be semiconductors (materials that can conduct electricity under particular conditions).
In combination, these two types of molecules could be used in everything from sensing contaminants in the environment to diagnosing diseases.
"The challenge of assembling organic rings around carbon nanotubes (and proving it!) has been lots of fun", say Emilio and Dirk. "Surely the most exciting thing is that, once we have developed chemical tools to combine molecular fragments and carbon nanotubes through mechanical links, we (and others) can address many different interesting questions: from very fundamental problems, like controlled (sub)molecular motion across very long distances, to more applied issues like making chemical sensors."
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