DISCUSSION 65 



one loses nitrogen base pairs and gains carbonyl base pairs and vice versa. 

 The real problcMii. I think, is to find which lone pairs are the ones which 

 are optically and photochemically active, and 1 think, that sui)stitution at 

 the 2,6 positions only suggests that that conjugated substitution may cause 

 spectral ellects which might reflect themsehes in the tliymine spectrum. 



Dr. Kalckar: Since you brought up tautomerism and dipoles, I may refer 

 to some recent optical studies in tlic infrared region by Todd Miles* on 

 nucleic acids which, as I understand, might be quite pertinent to some of the 

 problems you raised in connection with DNA. 



Dr. Kasha: I know the work of Mason on the use of infrared spectroscopy 

 to establish tautomeric forms of the bases. 1 don't know any work on DNA 

 itself. 



Dr. Kalckar: These studies were not done on DNA but on the enzy- 

 matically synthesized polynucleotide double strands of Warner and Ochoa 

 which according to Rich and Davis show some crystallographic resemblance 

 with DN.\. Todd Miles {Xature 183: 1814. 1959) found striking spectral 

 changes in the infrared region upon the formation of double helixes. These 

 changes he ascribed for various reasons to changes in dielectric constants as 

 well as to hydrogen bonding. 



Dr. Kasha: I am unfamiliar with those infrared studies. 



Dr. Calvin: I would like to ask Dr. Kasha for further elaboration on 

 item 4, that is, fluorescence versus phosphorescence comparing the n — > tt* 

 and TT -» TT* transitions. First of all, as I understand it, the excited - state 

 to which vou o-o in both cases is the same and therefore the difference be- 

 tween the two spectra lies in the difference between the energy levels of 

 the highest occupied - state and the n state from which it starts. Now what 

 effect does that have on item 4? 



Dr. Kasha: I gave a paper at the Brookhaven Bioenergetics Conference 

 on this point. Your statement is one that people commonly make but there 

 are many subtleties behind this. Consider a system in which the first 

 three tt orbitals are filled and the 'tt orbitals 4, 5 and 6 are empty. You are 

 then presuming that the final energy level is the same whether ';7 4 is 

 reached through an n -^ -tt* transition or a -tt -> -77* transition. This is the 

 simplest way of describing it in the LC.\OMO molecular orbital description. 

 But actually what happens is that the orbital order may not be the same 

 because when you take an electron from a lone pair such as from the nitrogen 

 in pyridine and put it into the ring, you leave a positive nitrogen. This has 

 a tremendous affect on the -tt orbital energy as has been pointed out by a 

 number of people in the literature and it is even possible that the charge 

 rearrangement may even upset the order of orbitals 4 and 5. 



Dr. Calvin: Can you conceive of a transition from the excited tt level to 

 which you have gone by the - -» 77* transition to the tt level to which you 

 would normally go from /?? 



Dr. Kasha: They are formally and symmetry-wise alike, but energetically 

 they can never be the same. 



