SOME PHOTOCHEMICAL CONSIDERATIONS 21 



close together (not more than 1000 A apart). These conditions are fulfilled 

 in protein molecules. From a theoretical point of view, Karreman and Steele 

 (29) investigated whether such resonance energy transfer could be possible 

 in the respiration chain. They found that the fluorescence spectra wholly or 

 partly overlap the absorption spectra of the next steps in the chain 



DPNH — ^ oxidized FAD — > oxidized cytochrome c 



so that energy transfer by resonance is highly probable. 



At the beginning of this paragraph it was pointed out that, as a result of 

 theoretical considerations, free radicals are formed as intermediates in 

 biological processes concomitant with the transfer of single electrons. How- 

 ever, until the participation of unpaired electrons was experimentally estab- 

 lished, the compulsory, univalent oxidation-reduction was still unproved. 

 It has been stated in § 8 that an unpaired electron must be influenced by an 

 external magnetic field. Zavoisky (69) discovered that at a definite value 

 H of an external magnetic field a resonance phenomenon — the so-called 

 electron spin resonance — can be observed. This type of resonance is due 

 to the interaction between the external magnetic field and the magnetic 

 momentum of the spinning, unpaired electron. With the aid of special 

 spectrometers, the absorption of short wave radiation energy under the 

 influence of an appropriate external magnetic field is recorded. At a given 

 frequency v the value of H is changed and at a certain value of H the curve 

 shows a deflection from the base line indicating the electron spin resonance 

 and thus the presence of an unpaired electron.* 



This technique was first applied by Commoner el al. (14, 15, 16) to biological 

 systems such as enzymatic processes, chloroplasts and living cells. The 

 oxidation-reduction reactions of the type substrate-dehydrogenase-DPN are 

 among the simplest biochemical processes in which unpaired electrons occur. 

 Commoner et al. studied the alcohol dehydrogenase system 



. 1 , T^T^TVTj alcohol dehydropenase ,, , , , ^^^^^^ , ^^ , 



Alcohol + DPN+ i t > acetaldehyde + DPNH + H + 



They found that none of the participants in the reaction show electron spin 

 resonance at any value of H. However, when the mixture of alcohol, 

 alcohol dehydrogenase and DPN+ is examined, the curve shows deflection 

 at a definite value of H, indicating electron spin resonance and thus the 

 occurrence of unpaired electrons. The same results were obtained with 

 mixtures of acetaldehyde, alcohol dehydrogenase and DPNH. These and 

 further experiments on other enzyme systems have established that in oxida- 

 tion-reduction reactions unpaired electrons, i.e., free radicals, occur and thus 

 prove the correctness of Michaelis' hypothesis. 



§ 10 Free Radicals as Intermediates in Photosynthesis 



The work done by Uri (55) seems to confirm that free radicals occur as 

 intermediates in photosynthesis. His experiments are based on the fact 



* For literature on electron spin resonance, see Wertz (66) . 



