SOME PHOTOCHEMICAL CONSIDERATIONS 19 



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:0:0: or :0::0 



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It may be that the high reactivity of Oo in biological systems is due to its 

 free radical nature. 



§ 9 Migration of Energy 



According to Michaelis (36,37), the reversible oxidation-reduction ol 

 organic compounds proceeds in two successive, univalent steps. Instead ol 

 the simultaneous removal or uptake of two electrons, as previously assumed, 

 only one electron at a time is removed or taken up so that free radicals are 

 formed as intermediates (65). This is the principle of compulsory, univalent 

 oxidation-reduction. From many examples we may select the hydrogenation 

 of DPN+ or TPN+ which is nothing but the hydrogenation of nicotinic amide 



H H 



H 

 H 



-CONH.. H 



Jh . ' H 



— CONH.. 



+ H 

 Ho 



R R 



The relevant part of the nicotinic amide molecule is 



C— N+=CH 



R 



The change in the electronic formula shows that an intermediate free mono- 

 radical is formed 



C:N + ::C:H ^' , C:N:C:H ^' , C:N:C:H 

 R R * R H 



The positive charge of the N atom is neutralized by the uptake of the first 

 electron. The double bond N=C disappears and the C atom gets an un- 

 paired electron. After the uptake of the second electron the free monoradical 

 is changed into the reduced nicotinic amide (DPNH or TPNH). 



Transport of single electrons occurs also in semiconductors. In biological 

 systems reacting molecules are mostly bound to proteins and it is supposed 

 that electrons must migrate through the proteins. Such a transfer of electrons 

 shows some analogy to semiconductivity. 



Semiconductors have electrical properties intermediate between those of 

 metals which conduct electricity very well and those of insulators which 

 conduct electricity very poorly or not at all. C^rystals of germanium, silicon, 

 copper oxide, silver sulfide, lead sulfide etc. are important semiconductors 

 used as transistors in modern radiotechnique.* In the n type semiconductor 



* For further details on semiconductivity the reader is referred to the excellent paper by Shockley 



(47). 



