DISCUSSION 67 



is tli;il ol an auramiii-DNA complex. II wc compare ilic lluorescence excita- 

 tion spcctriun of the aiiramin-DNA complex at wavelengths under 300 m^u, 

 with that spectrum which one would expect if auramin would emit its own 

 absorbed quanta, one observes that the complex has a higher fluorescence. 

 The difference, of course, is due to the (juanta absorbed by the DNA which 

 are being transferred to and emitted by the auramin. From the data one 

 can calcidate the niuuber of purine and pyrimidine residues which are 

 feeding into one auramin molecule. If one assimies a cpiantiuii efficiency 

 of transfer of one, then the calculated number is 4 to 6. The number, of 

 course, would be higher if the quantum efficiency of transfer were less than 

 one. Therefore, it seems likely that we have proof from this experiment 

 that the singlet state can be transferred from 1 pyrimidine or purine residue 

 to the next, since it is luilikely that four pyrimidine or purine bases are 

 so situated that they can transfer to auramin with the quantum efficiency 

 of one. 



Dr. Bannister: Dr. Kasha, is the activation of fluorescence of chlorophyll 

 in hydrocarbon solvents to be given an explanation similar to the one you 

 gave for quinolines? 



Dr. Kasha: Yes, Piatt has described this in his paper, in the book Radia- 

 tion Biology, and Becker and I have discussed this as well (see text) . There 

 are three questions involved. (1) Are the solvent changes really associated 

 with the /? -^ TT* blue shift or not? They may be only partially associated 

 with the n — > tt* blue shift. There is a new peak and a tail. We think only 

 the tail is associated with the /; -^ it* region. (2) A further problem which 

 arises is this. When the non-fluorescent dry chlorophyll is studied, where is 

 the phosphorescence? Fernandez and Becker, working in the University of 

 Houston, have found that there is a unique emission in dry chlorophylls 

 which is absolutely missing when the material is hydrated. The great curiosity, 

 which I didn't have time to discuss, is why the intermediate triplet emits, 

 not the lowest triplet. The lowest triplet emits in hydroxylic solvents, but 

 in dry solvents the intermediate triplet emits. These are so close together, 

 we normally would have expected a radiationless transition leading only to 

 the lowest emission. However, I believe that the novel behavior comes be- 

 cause porphyrin is such a large molecule compared with the localized n -^ it* 

 excitation region, where, as a sort of contradictory answer to Calvin's ques- 

 tion, here is a case where apparently the internal conversion process is 

 restricted by the spatial characteristics of the orbitals with reference to the 

 whole molecular system. If this observation is correct, I think it extremely 

 important to study single lone pairs in other large molecules like formyl 

 porphyrin and dyestuffs with one nitrogen lone pair, etc. to see whether it 

 is characteristic that the rate constant for the radiationless process gets so 

 limited that one cannot really get to the lowest tt, tt* triplet state from 

 some upper n —^ tt* triplet. This will have some very important consequences 

 on the photochemistry and is something which should be investigated. 



