Supramolecules and molecular recognition

Chemistry used to be the science of how to make or break covalent bonds i.e. firm bonds between atoms which share electron pairs. The concept of molecules is defined by the criterion that their atoms are held together by these chemical bonds. Biochemists, however have found that once a biological macromolecule is built (which indeed requires a large number of covalent bonds to be formed) a different, weaker kind of binding becomes more important for its everyday life. When proteins fold up to their native structures, when DNA forms double helices, or when molecules bind to each other in molecular recognition or signalling events, a whole set of non-covalent interactions is involved  

The major novelty of supramolecular chemistry is that it used this kind of non-covalent interactions to build up complex structures. Thus it borrows a concept which is extremely common in the cell, without, however, making use of the actual biomolecules. 

This transfer rate which is extremely high for a biological system raised suspicions with some researchers. However, research performed in 1995 by Thomas Meade and Jon Kayyem at Caltech confirmed the observation in principle. Meade and Kayyem used rather complex organometallic compounds with ruthenium as the electroactive part (see Plate II). They needed almost a microsecond to get the electron from one ruthenium atom to the other. However, the difference may be rationalized with the fact that in the more recent study the metal centers were not lined up with the stacked 7r-systems. Hence a substantial fraction of the time may be spent on the way to and from the DNA-highway. 

Obviously, if the goal is to transport an electron from A to B, DNA is a material too expensive to be competitive. Applications of the DNA conductivity are rather expected in the field of biosensors. If the presence of a known DNA sequence (e.g. a virus in the blood) is to be assayed, the complementary sequence with the electron-donor and acceptor group could be used as a probe. Only if a double helix is formed (i.e. only if the sequence of interest is present), will the fast electron transfer be observed.. 

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