87 



improbable quenching transitions can be effective. In the pure solvent you have 

 the self-quenching to a very high degree, and for that reason it is hard to say 

 just what the "inherent time" for radiative transition or quenching transitions 

 means. 



o 



POLLARD: Are those pretty big dipoles which are effect at 50 and 60 A? 



o 



LINSCHITZ: Yes, these are very strong. The 60 A value is actually obtained 

 from experiments, and calculations on dye solutions. The mechanism for ener- 

 gy migration may involve molecule to molecule jumps, but there is also a possi- 

 bility that it may move a great distance in a single jump. 



At any rate, in liquid scintillation counters, I think the relevant point to make 

 is that energy can migrate through at least 1000 molecules in times of the order 

 of 10~9 seconds or possibly smaller. 



KAMEN: Is this the mechanism that is used for the transfer of energy in 

 the chlorophyll case? 



LINSCHITZ: Probably so. 



POLLARD: It is phrased a little differently by Arnold and Oppenheimer (15), 

 but it amounts to this: it is a dipole-dipole interaction. 



KASHA: In the chlorophyll system you never do the experiments under the 

 same conditions. You never have a homogeneous solution and you never can tell 

 whether co-adsorption is required or not. 



LINSCHITZ: Energy migration is, of course, very intriguing from the stand- 

 point of biochemistry, and I should like to point out that certain cell systems 

 seem uniquely well fitted for such transfer. Evidence, which is, of course, still 

 very incomplete, indicates that the DNA molecule is a kind of long thread with 

 the aromatic bases oriented parallel to each other along the thread, and further- 

 more that the distance between the purines and pyrimidines is extremely small, 

 of the order of 3.5 A. At such small distances one can certainly expect that 

 these aromatic rings will be very strongly coupled to each other. If you assume, 

 say, 3000 such rings, roughly, for a molecular weight of 1 million for a DNA 

 molecule, this is within the range for which energy can be transferred through 

 the whole DNA chain from a single activating step. 



KAMEN: Is there any recipe for specifying the kind of a system in which you 

 expect this to happen? 



LINSCHITZ: The systems that favor this most are those with intense transi- 

 tions, which overlap each other in fluorescence and absorption bands, and speak- 

 ing generally, which have absorptions which lie as far out to the red as possible. 



KAMEN: In other words, almost any peptide? 



LINSCHITZ: Essentially anything biological except water. 



ALLEN: With scintillation counters do you always use aromatic materials? 



LINSCHITZ: You can use a hexane solvent or dioxane, with an efficiency of 

 about half what you get with aromatic materials. 



FANO: While it is true that anything biological has aromatic rings, still the 



