LUMINESCENCE OF PHOTOSYNTHETIC ORGANISMS 123 



Bassham : I would like to ask a couple of questions. First, what was the effect of 

 ox^-gen, if any, on the chemihiminescence? Second, does the heat of activation 

 refer to the long component or to the short component or to both? Third, can the 

 short component possibl}- be, rather than chemiluminescence, a long-lived 

 metastable state of some kind in the pigment? 



Strehler : I am not sure it is possible to give a definite answer to any one of these 

 (luestions. Arnold and I did not find an appreciable effect of anaerobiosis on 

 luminescence. After incubation of the algae for some time under extreme anaerobic 

 conditions, however, interesting effects have been observed, as shown bj- Brugger 

 and Franck. We measured the heat of activation for the long component. Cer- 

 tainly the short component could be due to a long-lived metastable component. 



Linschitz: Two comments. First, if this luminescence is actually due to some 

 metastable product, I claim the product can not be a molecule in the triplet state, 

 since the radiative lifetime of the triplet would not be of the proper order of 

 magnitude. Second, there may also be an activation energy for the formation of a 

 metastable product. This activation energy may possiblj^ be associated with the 

 falling off of the quantum yield on the long-wave side of the absorption. 



Strehler : But, as 3'ou pointed out at one time, the sharpness indicates that there 

 is not too much activation energy involved. 



Linschitz: That is right. 



Lumry: I wonder if there is any forward reaction or any other reaction by 

 which you could lose high-energy forms. 



Strehler: Inasmuch as the yield of fluorescence is of the order of 1% of the light 

 absorbed, excited singlet states derived by any other mechanism would probably 

 have the same probability therefore of emitting light, namely 1 in 100. The rate 

 of the reaction that we are studying is probably of the order of 100 times greater 

 than what we deduce directly from the chemiluminescence intensity. 



Linschitz: In other words, you are saying that if there are any other reactions 

 they are small compared to the reaction — 



Strehler : No, I w-ould not say that. I think that all of the reactions involved in 

 photosynthesis which make use of the same intermediates would have an influence 

 on the concentration of luminescent intermediates and therefore on the 3-ield of 

 luminescence. There may also be recombinations of primary oxidant and re- 

 ductant at other sites that do not lead to luminescence. 



Linschitz: T was trying to find out what the activation energy means here. If 

 you have other non-luminescence-producing processes leading to a destruction of 

 your high-energy substances, then the activation energy becomes related to the 

 luminescence in a comphcated way. 



Strehler: You have emphasized a serious objection to glib interpretations of 

 activation energy calculations. 



Rabinowitch : 1 wonder how long the fluorescence due to return from the triplet 

 to the singlet state survives. If the activation energy — that is, the difference in 

 energy between triplet and excited singlet — is about 10 kcal., then one can calcu- 

 late how much emission there should be when molecules in the phosphorescent 

 state return to the fluorescent state. Is this "delayed emission" large enough to 

 actually be a comjjonent of luminescence? Of course, the luminescence can bo 

 reabsorbed by the chlorophyll and thus tend to maintain the concentration of 



