7. Quenching and Quenchers 



When studying a biological process, we mostly try to preserve 

 our material as well as possible. We can also do the opposite and 

 induce damage by poisons, drawing conclusions from the consecu- 

 tive changes in function. In an analogous fashion, we could also 

 try to find poisons for £*. 



Light emission, be it fluorescence or phosphorescence, indicates 

 electronic excitation and if a substance quenches it, it has to inter- 

 fere with the underlying £*. If this E* is involved in a biological 

 process then the quencher should poison it. So highly active and 

 specific quenchers could help us in our study and the question is 

 how to find them? 



It is known that certain atoms or atomic combinations have a 

 quenching action. Such an atom is the iodine and to a lesser de- 

 gree the bromine atom. SCN and NO3 are also known as quench- 

 ers, suggesting that the combination of atoms with nonbonded 

 "lone" pairs of electrons make quenchers; O, N, and S are such 

 atoms. The activity of these atoms and atom combinations declares 

 itself also by a strong absorption in the UV. 



These simple quenchers will not help us much, partly because 

 their activity is not high enough and partly because they have no 

 specific affinities to tissue elements which could support a specific 

 action. Such a specificity demands a more complex chemical struc- 

 ture. In our search for quenchers with such a structure we can 

 make use of the experience that the mentioned simple atoms or 

 atomic groups usually abolish the fluorescence if introduced into 

 a more complex fluorescent molecule. They make the molecule 

 dissipate its energy. Figuratively speaking, they act as a sink for 

 the jE*. The molecule may be able to dissipate not only its own 



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