REVERSIBLE PHOTOCHEMICAL PROPERTIES OF DYES 7 



indicate the possible role of specific high polymeric substrates to which 

 the pigments of the photoperiodic system may be bound. 



PHOTOREDUCTION 



In none of the photoreactions described above is a chemical change 

 in the surrounding medium indicated. If the mystery of photoperiodism 

 is ever to be solved, it must be determined what chemical transforma- 

 tions have taken place in the cell as a result of the illumination with 

 red or with far-red light. Lacking this essential information, I venture 

 to sugsest that the photochemical act involves an oxidation-reduction 

 reaction. If that is the case, then some of the photochemical reactions 

 studied by me and my students may provide suggestive models for 

 photoperiodic reactions. First let us consider the photoreduction of 

 dyes. 



Many colored substances (azo dyes and carotenes seem to be nota- 

 ble exceptions) undergo photoreduction when irradiated in the pres- 

 ence of mild reducing agents. Such mild reducing agents include, 

 among other things, ascorbic acid and glutathione, two naturally oc- 

 curring reducing agents. Riboflavin is an exceptional dye in that it has 

 its own reducing agent built in, so to speak. Ethylenediaminetetraacetic 

 acid and other chelating agents containing secondary and tertiary 

 nitrogens serve as electron donors for the light-excited dyes (Oster and 

 Wotherspoon, 1957). These chelating agents are not reducing agents 

 in the ordinary sense since they are not oxidized by oxygen even when 

 flushed with the gas for 24 hours. 



The photoreduction of dyes generally results in colorless species, 

 since the conjugation and hence color-imparting structure is inter- 

 rupted by addition of hydrogens. With the porphyrins, however, 

 photoreduction can take place in stages. The red form of chlorophyll 

 obtained by photoreduction of the pigment in pyridine in the presence 

 of ascorbic acid (Krasnovsky, 1948) is, in my opinion, only one of the 

 many reduction states of chlorophyll, and intermediate colored forms 

 of photoreduced porphyrins are obtainable. It is possible that chloro- 

 phyll in the plant is photoreduced in stages, each stage being a colored 

 species. If three or four such stages could take place successively and 

 if the energy thus accumulated could be used for chemical reaction, 



