30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 41 



Energy is absorbed by molecules in discrete units or quanta and, accord- 

 ing to the Einstein law of photochemical equivalence, each molecule which 

 reacts as a result of exposure to light absorbs one quantum of the radiation 

 provoking the reaction. This does not mean, however, that every quantum 

 absorbed results in a reaction. 



If the frequency of the absorbed radiation is v = c/X where v is in vibra- 

 tions per second, X is the wavelength in centimeters, and c is the velocity 

 of light, 2.9977 X 10 10 cm. per second then the energy of a quantum is 

 E = hv where h is Planck's constant, 6.62 X 10~ 27 erg seconds. The energy 

 per mole of quanta, defined as an einstein, is therefore 



1 einstein = NE = Nhv = Nh - 



X 



where N is Avogadro's number, 6.023 X 10 23 . Substituting, and expressing 

 X either in A. (10~ 8 cm.) or nu* (10~ 7 cm.), we have 



'. . . 2.859 X 10 5 . , , . 2.859 X 10 4 . . , . 



1 einstein = 5 kcal./mole = kcal./mole 



X(in A.) X(in imz) 



An einstein of green light, such as the 546.1 m/z mercury arc line, there- 

 fore corresponds to an energy of 52 kcal./mole. At 253.7 m/z, the wavelength 

 most commonly used in photobiological work, the corresponding energy is 

 113 kcal., sufficient to break most chemical bonds. It does not necessarily 

 follow that bond rupture will result from its absorption since the energy 

 may not be localized at the required bond, but distributed over several 

 modes of vibration. 



The law of photochemical equivalence is usually expressed quantitatively 

 in terms of the quantum efficiency or quantum yield 



no. of molecules reacting no. of moles reacting 



no. of quanta absorbed no. of einsteins absorbed 



which theoretically should equal unity. With a few exceptions 1 this is rarely 

 so, owing to the fact that the photochemical equivalence law applies only 

 to the primary photochemical reaction, that in which the radiation is ab- 

 sorbed. Deviations from quantum yields of unity are due to secondary 

 reactions and it is the products of these secondary "dark" reactions which 

 are normally followed experimentally. 



A good illustration of a photochemical reaction for which the quantum 

 yield is practically unity is the dissociation of the carbon monoxymyoglobin 

 complex by either visible or ultraviolet light. 2, 3 



1 S. Gladstone, "Textbook of Physical Chemistry." Macmillan, London, 1948. 



2 T. Biieher and J. Kaspers, Biochim. et Biophys. Acta 1, 21 (1947). 



3 O. Warburg, "Heavy Metal Prosthetic Groups and Enzyme Action." Oxford 

 Univ. Press, London and New York, 1949. 



