MICHAEL KASHA 61 



per cent in its molar absorption coefficient at 2()()() A. There is 

 little spectral shiit or change in shape of the band. Tinoco (39) has 

 applied the exciton model (16a) to this problem, taking into account 

 the interaction between dipoles excited in the N-heterocyclic base pairs. 

 Since the base pairs have an essentially parallel stacking in native, or 

 helical-coil, DNA, the dipoles for tt -^ it* excitation will be essentially 

 in a parallel side-by-side stacking, and hypochromism is anticipated 

 by Tinoco's treatment, and was considered by him. 



However, il there is an n -^ tt* long-wavelength tail in the purine 

 and pyrimidine spectra, a head-to-tail stacking of the transition dipoles 

 would be expected, since the // -> 77* transitions in the N-heterocyclic 

 bases will be polarized along the helical axis, or perpendicular to the 

 planes of the base i)airs. Consequently, a small long-wavelength 

 //)'pe»-chromism would be expected in the random-coil to helix 

 transformation according to Tinoco's interpretation. A. Rich and 

 the author (33) have examined denaturation curves of various syn- 

 thetic polynucleotides and nucleic acids and have observed the ex- 

 pected hyperchromic effect for the helix-form. 



B. Rotatory Dispersion oj DNA 



The presence of a long-wavelength n -^ tt* tail in the DNA ab- 

 sorption spectrum should be detectable by the observation of the 

 rotatory dispersion curve. In particular, a Cotton effect might be 

 anticipated corresponding to this band. Fresco and Corn (9) have 

 made a careful study of the optical rotatory dispersion of native calf 

 thymus DNA and have found a Cotton effect centering on about 

 2900 A, confirming a characteristic absorption band in this region. 

 Murakami (27a) had previously suggested that pyrimidines might 

 show this effect, but it appears from Mason's work (25) that purines 

 should possess a long-wavelength n ^^ tt* region as well, even in 

 aqueous solution. 



The fact that the exciton model seems to apply to the case of DNA 

 implies that energy transfer down the DNA helix should be quite 

 efficient. The application of this process is mainly to radiation effects, 

 in which electronic excitation can result. In addition, the occurrence 

 of n -^ 77-* lowest transitions in the pyrimidines and purines suggests 

 that photochemical processes in DNA will involve 71,77* excited states 

 with their characteristic features. 



C. The Chlorophylls 



Livingston, Watson, and McArdle (19) reported on the remarkable 



