REFRACTIVE INDICES 359 



ing to Soret it is unaffected by acids, but on adding NaOH or 

 ammonia to the protein solution the region of transparency in 

 the ultraviolet disappears completely and the band of absorption 

 is displaced towards the less refrangible end of the spectrum. On 

 neutralizing the solution with acid the original absorption-spectrum 

 is restored. A similar absorption-spectrum is yielded by solutions 

 of tyrosin and it is affected in a similar manner by alkalies. Soret 

 therefore attributes the absorption-band in the absorption spectra 

 of protein solutions to the tyrosin radical which the proteins 

 contain. 



Kober (23) has recently carried out a spectrographic examina- 

 tion of solutions of a number of amino-acids and polyamino-acids. 

 He finds that the aliphatic amino-acids display only general 

 absorption in the extreme ultraviolet. The aromatic acids, 

 tyrosin and phenylalanin do, however, show definite absorption- 

 bands in the ultra-violet spectrum. The absorption-band in the 

 ultraviolet which is displayed by proteins in solution is therefore 

 attributable not only to tyrosin radicals, as Soret supposed, but 

 also in some measure to phenylalanin radicals. 



Since the occurrence of any photochemical reaction is dependent 

 upon the absorption of the chemically active rays by some con- 

 stituent or constituents of the reacting system it would appear 

 possible that the markedly toxic action of ultraviolet light upon 

 many unicellular organisms may be dependent upon the absorption 

 of the ultraviolet light-rays by the proteins of the organisms 

 affected. Strong confirmation of this view is afforded by the 

 experiments of Harris and Hoyt (12) who have shown that if ultra- 

 violet light be passed through a thin layer of gelatin or peptone 

 solution its toxicity for paramoecia is diminished to a much 

 greater extent than by passage through a similar layer of distilled 

 water. Solutions of urea, sugar or alanin do not confer this pro- 

 tection, while solutions of the aromatic amino-acids protect the 

 organisms very markedly. Leucin likewise confers a very marked 

 protection, but this amino-acid is rapidly decomposed by ultra- 

 violet light, yielding colored products, and the decomposition is ac- 

 companied by an increase in the protective power of the solution. 



3. The Refractive Indices of Protein Solutions. — The in- 

 fluence of dissolved proteins upon the refractive indices of various 

 solvents has been especially studied by Reiss (41) (43), Herlitzka 

 (13). Schmidt (56) and myself (43-54). 



