34 RADIATION BIOLOGY 



hydrated protein molecules. These reatrtions have been studied exten- 

 sively in recent years (McLaren, 1!)M)), and some empirical generaliza- 

 tions can lie deduced from the results of these studies. The (juantum 

 yields of the reactions an^ in the range from 1()~- to 10~'. Radiation of 

 wave lengths shorter than 3100 A is reciuircd to produce the reactions. 

 Photochemical denaturation is irreversible. The primary jjliotochemical 

 product remains in solution at low temperatures (e.g., 4°Cj, but a precipi- 

 tate forms rapidly when the previously irradiated solution is heated to 

 40°C. The (juantum yield of the primary process is practically inde- 

 pendent of temperature over the narrow range availal)le. Photochemical 

 denaturation results from the irradiation of "dry" proteins as well as 

 proteins in dilute aqueous solution. The (juantum yield is not a function 

 of the intensity of the absorbed light. 



If the quantum yield is strictly independent of the intensity of the light 

 absorbed by the native protein, the primary act involves the interaction of 

 one photon with each molecule; i.e., it is a "single-hit" process, in which 

 there is no cooperative action between two or more photons either suc- 

 cessively or simultaneously. It should be realized, however, that no 

 simple mechanism predicts that the yield is independent of the product 

 of incident intensity and time of irradiation since the (dissolved) dena- 

 tured protein must act as an efficient internal filter. The "one-hit" 

 kinetics have been interpreted in terms of a primary act in which one 

 peptide linkage is broken by a single absorl)ed photon. The low (juantum 

 yields indicate that this primary process is very inefficient; most of the 

 absorbed quanta are degraded to heat. One plausible explanation for 

 the observed inefficiency is that the light, which is absorbed by aromatic 

 nuclei in the molecule, becomes available for chemical action by an act 

 of internal conversion. A few facts which support this tentative explana- 

 tion are (1) the yield increases with increasing frequency (i.e., energy) of 

 the photon, (2) the yield decreases with increasing size of the molecule, 

 and (3) the yield is greater for adsorbed films of proteins than it is for 

 solutions. 



SENSITIZED REACTIONS 



In sensitized reactions the substance which absorbs the light does not 

 undergo any permanent chemical change. This absorbing substance, 

 called the "sensitizer," catalyzes the photochemical reaction. The 

 simplest known example of this type is the xenon-sensitized photochemical 

 dissociation of hydrogen (Calvert, 1932). The resonance radiation of 

 xenon has a wave length of 1409 A, which corresponds to an energy of 

 193 kcal/mole. Molecular hydrogen, whose dissociation energy is 103 

 kcal/mole, does not absorb radiation of wa\'e length longer than 849 A. 

 If a mixture of xenon and hydrogen is illuminated with a xenon arc, hydro- 

 gen atoms are formed as was demonstrated by their color reaction with 



