508 



PHOTOCHEMISTRY OF PIGMENTS IN VITRO 



CHAP. 18 



distinction merely indicates that the photodynamic actions are autoxidations (while the 

 photographic process is an oxidation-reduction, which does not require oxygen). The 

 connection between fluorescence of dyestuflfs and their photodynamic activity, which 

 has assumed in the eyes of some authors an almost mystic character (it has even been 

 suggested that a fluorescent dye can be recognized as such by its photodynamic action, 

 even if no fluorescence is visible!), is a simple consequence of the proportionaUty between 

 mean Ufe time of the excited molecule and yield of fluorescence (Vol. II, Chapter 23). 

 This often makes sensitization by a fluorescent dye more probable; but under certain 

 cu-cumstances, for example, if the absence of fluorescence is caused by tautomerization, 

 or if a permanent association exists between sensitizer and substrate (a condition 

 realized in the sensitized photographic plate), the life span of the activated nonfluores- 

 cent molecule is sufficient to bring about sensitization. 



Both autoxidations and oxidation-reductions can be sensitized by 

 chlorophyll. Sensitized autoxidations can be of three kinds: reversible, 

 leading to unstable " moloxides " ; "half-reversible," leading to peroxides 

 from which one-half of the absorbed oxygen can be recovered again; or 



irreversible, leading to stable oxi- 

 dation products. It is not always 

 known to which type a given autox- 

 idation belongs, since very often the 

 process was followed only by observ- 

 ing the absorption of oxygen. To 

 determine the amount of peroxide 

 formation, Gaffron (1927, 1933) 

 treated the reaction products with 

 manganese peroxide and often a 

 considerable part of absorbed oxygen 

 was liberated again. Meyer (1935) 

 found that oleic acid, citronellal, 

 pulegone, etc., when subjected to 

 sensitized photoxidation and then 

 tested by means of bromine or per- 

 manganate, seemed to have their 

 double bonds intact. In other cases, 

 e. g., that of benzidine (Noack 1925, 

 1926) or pyruvic acid (Meyer 1935), 

 the photosensitized reaction was a 

 true oxidation. Probably, the pri- 

 marily formed moloxides or perox- 

 ides later are converted into stable 

 oxidation products. This may explain why Windaus and Bruncken 

 (1928) have observed, in studying the sensitized photoxidation of 

 ergosterol, the absorption of one mole of oxygen and the formation of a 

 crystalHzable ergosterol peroxide without antirachitic properties, whereas 



20 40 60 



Tim«, minutes 



80 



Fig. 56. — Oxygen uptake by oleic 

 acid and ohve oil in fight with chloro- 

 phyfi as sensitizer (after Meyer 1935). 

 Horizontal sections correspond to inter- 

 ruptions of iUumination. 



