440 



H. Gaffron, W. Wiessner and P. Homann 



manganese in some manner as Gaffron had assumed, Homann dis- 

 covered that it was the excited flavin molecules whose sensi- 

 tizing actions were strangely altered. A short description of 

 these experiments has recently appeared in "Science". Let us 

 point out only the far-reaching implications which would arise 

 if indeed these in vitro experiments had anything in common with 

 the oxygen liberating reactions in green plants. No other sen- 

 sitizing dyes showed remotely the affinity for the substituted 

 ureas which flavins display in aerobic as well as in anaerobic 

 light reactions. In several enzymatic dark reactions in which 

 flavin is required for the activity of the enzyme, DCMU had no 

 detectable effect. It is the light-excited flavin which falls 

 prey to the specific affinity of the substituted ureas. This affi- 

 nity results in an interaction with the electron transfer from the 

 reducing substrate to the dye or in a protection against the 

 quenching of the triplet state by oxygen or iodide, which may go 

 together with an influence on the catalytic effectiveness of the 

 manganous ions. 



We would like to explain, of course, the parallelism we have 

 found between the action of DCMU in our in vitro experiments 

 and its well-established effects in vivo. 



The problem we are up against is to find out how possibly an 

 electronically activated flavin could be so intimately connected 

 with that specific reaction sequence which leads to the release 

 of free oxygen and which seems to be the exclusive prerogative of 

 system II, if we accept the now prevailing ideas on the mechanism 

 of photosynthesis. 



