EFFECT OF RADIATION ON PROTEINS 305 



change which the natural protein is capable of undergoing and is a definite 

 step in the transition of the natural, crystallizable protein molecules to 

 the amorphous state represented by flocculation. This change may be 

 brought about by various agencies, one of which is heat. Heat denatura- 

 tion is a change in the protein molecule brought about only in the presence 

 of water. It is unimolecular in nature and appears to be associated 

 with some internal physical or chemical change within the structure of 

 each protein unit and is generally regarded as being of an irreversible 

 nature, although recent work (1, 2, 37, 38) states that it may be reversible 

 under certain conditions. The denaturation produced by radiation is 

 apparently different from heat denaturation in some respects, but all 

 types of denaturation produce similar changes in the solubility of proteins. 

 A denatured albumin has the solubility characteristic of a globulin, as 

 it no longer stays in solution at the isoelectric point and precipitates 

 more readily with salts. Flocculation is the precipitation of protein in 

 the neighborhood of the isoelectric point in an amorphous form. A 

 distinction should be made between true flocculation (chain formation 

 produced by addition of successive protein units) at or near the isoelectric 

 point and the precipitation of a protein salt in amorphous form at a pH 

 removed from the isoelectric point. In an albumin, flocculation is possi- 

 ble only provided the original protein has suffered the change known as 

 denaturation. The two consecutive changes, denaturation plus floccula- 

 tion, are referred to as coagulation. 



ABSORPTION OF RADIANT ENERGY BY PROTEINS 



In considering the effect of radiation on proteins it must be remem- 

 bered that radiant energy produces changes only when energy is absorbed. 



Ultra-violet Radiation. — The absorption of ultra-violet radiation by 

 protein solutions has been studied by many investigators. Lewis (34, 35) 

 found the absorption of blood serum in the ultra-violet region to be due 

 to the proteins. Serum albumin, pseudoglobulin, and euglobulin from 

 horse and human serum gave similar curves, although the absorption of 

 globulins was greater than that of albumins. They all begin to absorb 

 at approximately 3100 A. The absorption increases to a maximum at 

 2800 A, falls to a minimum at 2500 A, and then rises sharply for shorter 

 wave-lengths. Smith (45) gave similar results, which are shown in Fig. 

 1. He found the extinction coefficient of globulin to be twice as great 

 as that of albumin at X 2800 A and the values for horse and human serum 

 proteins to be the same. He also examined the absorption of a number 

 of the constituent amino acids of the serum proteins and found the same 

 absorption maximum and minimum in tyrosin and tryptophan, a point 

 previously noted by Dhere (13). He therefore attributed the absorption 

 of serum proteins largely to these amino acids. The exact values given 

 by different authors for the extinction coefficients of serum albumin and 



