42 



£*, but may also be able to take over that of other molecules and 

 do with it likewise. In this case, this molecule will be a quenclier. 

 In this way we can hope to arrive at molecules which may have not 

 only the desired quenching activity but may have also selective 

 affinities. 



To quote an example: benzene is known to be weakly fluores- 

 cent but if NO2 is introduced into its molecule the resulting nitro- 

 benzene will be fluorescent no more, though it is even more easily 

 excitable than the benzene itself, as witnessed by its strong ab- 

 sorption in the UV and the blue. The NO2, as a "chromophore" 

 group, enables the molecule to pick up energy still easier and to 

 act with its conjugated double bond system, so to speak, as an 

 antenna, which picks up energies from its surroundings by electro- 

 dynamic coupling, taking E* over from other molecules, acting 

 thus as their quencher, while the NO2 dissipates the JE* thus 

 acquired. 



By introducing additional NO2 groups at the points most fit for 

 resonance, or by introducing electron donor groups, as OH, or by 

 extending the resonating system by adding one or more benzene 

 rings to it, we can further improve the quenching abilities as well 

 as the specificity of our molecule. Such ideas lead us into fascinat- 

 ing fields of biology. When building such quenchers, we might be 

 following the same route which nature followed when she intro- 

 duced iodine into extensive resonating aromatic structures creating 

 thyroxine and related substances. 



These considerations also lead us to one of the most fascinating 

 puzzles of contemporary biochemistry: 2,4-dinitrophenol. This 

 substance (Fig. 7a) uncouples oxidative phosphorylation, the 

 transmission of the energy from foodstuffs to ATP. Mitochondria, 

 in its presence, go on oxidizing foodstuflFs, may do so even at an 

 increased rate, but waste the energy thus liberated as heat. This 

 caused dinitrophenol, a few decades ago, to become a most fash- 

 ionable drug for reducing body weight. Unfortunately, its use in 

 medicine had to be discontinued because in some cases, mysteri- 



