THE NATURE AND SIGNIFICANCE OF n -^ tt* 



TRANSITIONS 



Michael Kasha^ 



Departments of Chemistry 

 Harvard University, Cambridge, Massachusetts 



and 

 Florida Stale University, Tallahassee, Florida' 



I. Biological Function and Electronic Structure 



Biological function as related to electronic structure of molecules, 

 and optical phenomena manifested as a consequence of electronic 

 structure, should be carefully distinguished in such a symposium as 

 this one on "Light and Life." It is the purpose of this paper to 

 present a comprehensive though qualitative treatment of a topic 

 in molecular electronic spectroscopy which has broad biological ap- 

 plications: the nature and significance of n -^ n* transitions in 

 polyatomic molecules. Special reference will be made to the proper- 

 ties of deoxyribonucleic acids (DNA) and the chlorophylls. In the 

 latter case, the optical phenomena and biological function are in- 

 tricately interwoven; in DNA, the optical phenomena bear most di- 

 rectly on the structural problem, and on the effects of radiation on the 

 DNA molecule. 



The treatment given will be phenomenological. However, since 

 the experimental work described is based on deductions derived from 

 spectroscopic theory, pertinent aspects of the latter will be surveyed. 

 Although use is made of one application of Symmetry Groups in 

 classifying electronic states and optical transitions, the presentation 

 presupposes no previous knowledge of this subject. 



' On leave from Florida State University, academic year lOoO-lOfiO. 

 = This work was supported by a grant from the Petroleum Researcii Fund. 

 American Chemical Society. 



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