ABSORPTION OF ULTRAVIOLET RADIATION BY PROTEINS 159 



Notice the striking resemblance between the absorption of serum 

 albumin and egg albumin. An analogous absorption curve was obtained 

 by Gates [1934] for pepsin of pH 2.06. These data place the maximum 

 absorption at 2775 A. 



The energy required to inactivate 50 per cent of pepsin, for the wave- 

 length band between 2640 and 2820 A, is 230,000 ergs/mm 2 , and between 

 2425 and 2570 A is 305,000 ergs/mm 2 ; between 2300 and 2400 A it 

 falls to 77,500 ergs/mm 2 . 



The most striking and important change produced by absorbed ultra- 

 violet light in all proteins, whether globulins or albumins, and whether 

 positively or negatively charged, is a change in solubility or denaturation. 



v o 



It should follow that the wavelengths from 2650 to 2900 A, which in- 



o 



elude the absorption maximum at 2790 A, are highly efficient denaturat- 

 ing agents. 



o 



The absorption band of blood serum around 2800 A is apparently due 

 to the proteins present, and the tyrosin and tryptophan constituents 

 of the proteins are mainly responsible for this band. 



In conclusion some relations between the absorption of proteins and 

 some of the amino acids should be pointed out which support the theory 

 that the ultraviolet absorptions of the proteins in the 2800 A region are 

 due to the aromatic acids. 



An extensive analysis by Coulter, Stone, and Kabat [1935] shows 



o 



that all the narrow absorption bands between 2530 and 2690 A found in 

 all protein may be assigned to phenylalanine, while the bands at 2700, 

 and 2850 to 2900 A, may be attributed to tryptophan and tyrosin, 

 respectively. Their conclusions were drawn from an examination of 

 serum albumin, egg albumin, thyroglobulin, englobulin, pseudoglobulin, 

 pneumococci antibody, gelatin, insulin, tyrosin, tryptophan, and 

 phenylalanine. 



The general evidence suggests that ultraviolet radiation and soft 

 x-rays cause liberation of material of low molecular weight which with 

 albumin residues undergo photo-oxidation reactions, due to the absorp- 

 tion of ultraviolet energy. 



In order to illustrate what a powerful tool the spectroscopist has 

 made available to the biochemist attention is called to the work of 

 Wald [1934], who demonstrated the presence of vitamin A in ox and 

 pig retina, in sheep pigment-choroid, and in sheep retina by means of 

 the ultraviolet absorption band of vitamin A with maximum at 3280 A. 

 The standard method of determining the vitamin-A content of cod- 

 liver oil is by the extinction coefficient of its absorption spectrum band 

 at 3280 A. 



