PHOTOCHEMICAL OXIDATION-REDUCTIONS 503 



chlorophyll-protein complexes (c/. Fig. 58, page 530) shows that in these 

 complexes no long-lived active states occur at all. 



Lipophilic substances also have a protective effect on chlorophyll. 

 Wiesner noted, as early as 1874, that the same degree of bleaching of 

 chlorophyll could be achieved in 3 minutes in 75% alcohol, in 7 minutes 

 in benzene, and in 12 minutes in ether; while a solution in olive oil 

 required 3.5 hours for the same result. Chautard (1874) observed that 

 chlorophyll-colored oils keep their color unchanged for months while 

 exposed to light and air. According to Stern (1920, 1921), lipide- 

 protected aqueous chlorophyll colloids also are photostable. Since 

 lipophilic substances protect rather than quench the fluorescence of 

 chlorophyll (c/. Vol. II, Chapter 23), they apparently do not interfere 

 with the short-lived fluorescent state of the pigment. However, they 

 seem to afTect the long-lived activated state. If the transition into this 

 state is initiated by tautomerization, lipoid solvents may make it less 

 probable (if the tautomer is an enol, as suggested on page 444, it is likely 

 to be less stable in a lipophilic medium than in a polar solvent). If 

 the long-lived state is brought about by a reaction of excited chlorophyll 

 with the solvent, this reaction, too, may be less probable in a nonpolar, 

 lipophilic solvent than in a solvent of the type of alcohol or acetone. 



The stability of chlorophyll in vivo may be attributed to its association 

 with protein (as suggested by Reinke as early as 1885), or lipides, or both 

 (c/. page 393). The high partial pressure of oxygen required to bring 

 about photoxidations in the living cell {cf. Fig. 58) proves that in this 

 case, too, no long-hved active products are formed in light. (For a 

 more detailed discussion of the mechanism of sensitization by chlorophyll 

 in vivo, see Chapter 19, pages 544 et seq.) 



Kautsky and Hormuth (1937) described experiments on the oxygen consumption 

 by grana sediments (obtained by the centrifugation of leaf press juices). Suspensions, 

 prepared from two grams of leaves, absorbed, in two hours of illumination, up to 0.16 

 ml. oxygen, without showing signs of saturation (which is not astonishing, since not 

 more than 0.02 mole of oxygen was absorbed up to this point by one mole of chlorophyll). 

 The velocity of autoxidation increased with increasing pH. The oxygen consumption 

 was strongly reduced by narcotics (e. g., phenylurethan). 



4. Photochemical Oxidation-Reduction Reactions of Chlorophyll in vitro 



Photochemical reactions of chlorophyll with azo dyes, in which it 

 plays the part of a reductant, were described by Bohi (1929). The azo 

 dyes are reducible (first to hydrazo compounds, then to amines), and in 

 contrast to typical "leuco dyes," the reduction products are not re- 

 oxidizable by oxygen, so that the reaction: 



(18.28) Chi* + D > oChI -|- rD 



(D = dye) can be observed without excluding air. The result is the 



