828 LIGHT AND LIFE 



a shorter wavelength in water, and merges into the tt -> tt* transition 

 bands at 2300-2700 A. 



The molar absorption coefficient of deoxyribonucleic acid (DNA) 

 at 2600 A undergoes a reduction of 30-50 per cent, with little shift 

 or change in shape of the band, when the DNA goes from a randomly 

 coiled to a helically coiled state. A tt -> tt* excitation leads to the 

 expectation of parallel, side-by-side stacking of the dipoles, whereas 

 the presence of an n -» 77* transition producing a long-wavelength 

 tail in the purine and pyrimidine spectra would lead to the expecta- 

 tion of a head-to-tail stacking of the transition dipoles, since the 

 n -^ TT* transitions in the bases will be polarized along the helical 

 axis and perpendicular to the planes of the base pairs. One would 

 therefore expect a small hyperchxomisra during the transformation 

 from random coiling to helical coiling, instead of the /?/ypochromism 

 to be expected from the tt ^ tt* transitions. The hyperchromic 

 effect has been observed by Rich and Kasha. Other evidence of the 

 n -^ 77* transition in DNA is obtainable from study of the rotatory 

 dispersion curve; a Cotton effect centers on the region around 2900 

 A. From the agreement of the observations just summarized with ex- 

 pectations on the basis of the exciton model, energy transfer down 

 the DNA helix should be efficient. The occurrence of lowest level 

 n -^ 77-* transitions in the purine and pyrimidine bases of DNA im- 

 plies that photochemical processes in DNA will involve excited n,TT* 



states. 



The chlorophylls, as represented by chlorophyll b, exhibit in 

 anhydrous benzene a long-wavelength shoulder at 6700 A which dis- 

 appears when water or alcohol is added. At the same time, fluores- 

 cence appears. Kasha regards this shoulder as being probably attri- 

 butable to an n -^ tt* transition, and its disappearance, like the ac- 

 tivation of the fluorescence, as representing (in agreement with Piatt) 

 the interchange of n,7T* and tt.tt* electronic states. Supported by 

 observations made by Becker of the magnitude of the spectral shift, 

 which accords with that to be expected from an n -^ it* blue-shift, 

 and of the spectral emission at low temperatures, these conclusions 

 reinforce Franck's view that in photosynthesis the lowest n,TT* excited 

 states are truly important. In particular, it may turn out that xoet 

 chlorojihyll (in an aqueous phase) and dry chlorophyll (in a lipid 

 phase) may carry out complementary functions in photosynthesis. 



In a paper which was unfortunately not submitted for publication, 

 John R. Piatt discussed qualitative methods of assigning electron 

 density distributions to ground and excited states of simple organic 



