EXCITED STATES OF PROTEINS 



G. Weber 



Department of Biochemistry 

 Sheffield University, Sheffield, England 



In this survey the concept of the "excited state" is restricted to 

 those states that resuk from the absorption, and are potentially capa- 

 ble of the emission, of radiation. Such states have a lifetime deter- 

 mined in principle by radiation theory and include both electronic 

 and vibrational states. Knowledge of the vibrational excited states 

 of proteins is at present virtually nil because of the experimental 

 difficulties attaining their investigation. In turn, the electronic ex- 

 cited states may be expected to be either singlet states with a radia- 

 tion lifetime of the order of millimicroseconds, or metastable states 

 with lifetimes of milliseconds to seconds, states that after the original 

 investigation of Lewis and Kasha (9) have been the object of con- 

 siderable investigation by Livingstone (12), Linschitz and Sarkanen 

 (10), and Porter (20). Interesting observations upon such metastable 

 states of amino acids and proteins have been made by Debye and 

 Edwards (4), Grossweiner (7), and Steele and Szent-Gyorgyi (21). 

 Knowledge about them is, however, much more restricted than is 

 that of the singlet states, which is already being used successfully in 

 many problems of protein structure and function. In this summary, 

 I shall restrict attention to those states that are brought about by 

 absorption of light of 200 m^u, or longer wavelength. These excited 

 states belong to one of three groups of absorbing species: (i) the 

 aromatic amino acids; (//) groups occurring naturally in certain pro- 

 teins (heme, flavin mononucleotide, flavin adenine dinucleotide, 

 carotenoids) or bound reversibly and specifically to certain proteins 

 (DPNH and its analogues) ; and (Hi) groups artificially attached to 

 the protein by chemical linkage such as the anthracyl isocyanate con- 

 jugates (3) fluorescein isocyanates (2) , or the dimethylaminonaph- 

 thalene conjugates (25) . 



Before attempting to discuss the phenomena observed in the pro- 

 teins, it will be useful to discuss the properties of the excited states 

 of the components just mentioned. 



82 



