8 PHOTOCHEMICAL PRINCIPLES 



the required energy of 120 kcal would be accounted for (cf. Chap. 8 

 of Hill and Whittingham, 1955). 



Although some dyes such as the acridines are difficult to reduce by 

 ordinary chemical means, they are readily photoreduced in the pres- 

 ence of suitable electron donors. The reduced dye is a powerful reduc- 

 ing agent, its extra energy having been obtained from the hght. For 

 example, reduced acriflavin will reduce nitrobenzene to aniline. All 

 photoreducible dyes in their reduced form will convert tetrazolium 

 salts to their corresponding reduced formazans (highly colored water- 

 insoluble species). I somehow feel that reactivation by blue light of 

 organisms which had been inactivated with far ultraviolet light (Kell- 

 ner, 1949) involves a flavin-sensitized photoreduction of nucleotides 

 which had been photooxidized (although no O2 is involved) by far 

 (254 m/i) ultraviolet light. 



The reduced form of a dye will react with oxygen to regenerate the 

 normal dye. This dark reaction is accompanied by the production of 

 free radicals (probably hydroxyl radicals). Hence, if a vinyl monomer 

 is present, considerable polymerization ensues via a chain reaction 

 (Oster, 1954). Overall quantum yields as high as one billion are possi- 

 ble by this technique (Oster, Oster, and Prati, 1957). 



In the total absence of oxygen the dye may not be regenerated in the 

 dark if the reduced dye is too feeble a reducing agent. This appears to 

 be the case for leuco methylene blue, for example, which was made by 

 photoreduction. With acriflavin, on the other hand, the leuco dye can 

 reduce the oxidized reducing agent (e.g., dehydroascorbic acid) so 

 that the system is regenerated in the dark (Oster and Millich, to be 

 published). Even with weak leuco dyes, however, the dye can be re- 

 generated if near ultraviolet light is employed (Oster and Wotherspoon, 

 1954). Thus we have the completely reversible system 



red 



D -f AHo ^ DHo + A 



uv 



where D and DHo are the dye and reduced (leuco) dye, respectively, 

 and AH- and A are the reducing agent and its oxidized form, respec- 

 tively. This reaction, which proceeds best below pH 7 (Oster and 

 Wotherspoon, 1957), has been proposed as a model for photoperiodic 



