VIII 

 THE EFFECT OF RADIATION ON PROTEINS 



Janet Howell Clark 



School of Hygiene and Public Health of the Johns Hopkins University, 



Baltimore, Md. 



Introduction. Absorption of radiant energy by proteins. Effect of radiation on 

 suspe?isoids. Effect of radiation on emulsoids: Ultra-violet radiation— Sensitization to 

 visible light— Effect of X-rays and radium rays on proteins— General conclusions. 

 Primary and secondary light reactions. Relation of protein denaturation to general 

 physiological light reactions. References. 



INTRODUCTION 



A substance is said to exist in the colloidal state when, because of the 

 large size of the molecules, or aggregates of molecules, it is unable to 

 diffuse through certain membranes. When the internal or dispersed 

 phase is solid and the external or continuous phase is liquid, the colloid 

 is called a suspensoid. When the dispersed phase is liquid and the con- 

 tinuous phase liquid, it is an emulsoid. Generally speaking, most natural 

 colloids are emulsoids and most artificial colloids are suspensoids. 



The two great classes of colloids, emulsoids and suspensoids, differ 

 in the state of the internal or dispersed phase, liquid in the former and 

 sohd in the latter. They differ also in the affinity of the internal phase 

 for water, the external phase. Emulsoids are lyophile or hydrated 

 colloids, and suspensoids are, in general, lyophobe or nonhydrated. 

 Metallic hydroxides can combine more or less with water and, though 

 classed as suspensoids, are more like emulsoids in many respects. This 

 difference in hydration brings about differences in stability. Suspensoids 

 are precipitated by the addition of small amounts of electrolytes carrying 

 the opposite charge, whereas emulsoids are precipitated only by large 

 amounts of salts. Suspensoids are stable in solution only when charged 

 and fall out of solution when they lose their charge. Some emulsoids 

 (globulins) precipitate when uncharged and others (albumins) stay in 

 solution even when their charge is lost. Although it is possible to prepare 

 some suspensoids with either a positive or negative charge, suspensoids 

 in general carry a charge of definite sign depending on the material of the 

 dispersed phase. Emulsoids carry either a positive or negative charge 

 depending on the hydrogen ion concentration of the medium. 



Proteins are in many respects the most important members of the 

 emulsoid class. The chemical behavior of proteins in solution does not 



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