INFLUENCE OF LIGHT UPON LIFE-PROCESSES 433 



colorless organisms, and since this toxicity presumably depends 

 upon and is attributable to photochemical reactions, the question 

 presents itself: To which constituent of the protoplasm are we to 

 attribute the selective absorption of these rays which we may presume 

 to be the necessary precedent to their photochemical activity? 



It was pointed out over forty years ago by Soret that the majority 

 of proteins exhibit a well-marked absorption-band in the ultraviolet 

 part of the spectrum. In seeking for the origin of this absorption- 

 band Soret found that it is especially well exhibited by solutions of 

 Tyrosine and therefore referred it to the tyrosine radical in the protein 

 molecule. These observations have been extended by Kober, who has 

 carried out a spectrographic examination of solutions of the various 

 Amino-acids which are the end-results of protein hydrolysis and of 

 certain Polypeptides. Kober has confirmed the existence of an absorp- 

 tion-band in the ultra-violet in solutions of tyrosine, and finds that a 

 similar band is exhibited by solutions of Phenylalanine. The other 

 amino-acid constitutents of the protein molecule exhibit only general, 

 i. e., non-selective absorption in the ultraviolet spectrum. 



The possibility is thus indicated that the tyrosine and phenylalanine 

 radicals of the proteins constitute the optical sensitizers which render 

 living cells susceptible to the toxic action of ultraviolet light. If this 

 were the case, then passage of the light through solutions of proteins or 

 the aromatic amino-acids should, by absorption of the toxic ray, to a 

 greater or less extent, deprive the light of its toxicity for protoplasm. 

 This possibility has been investigated by Harris and Hoyt, who have 

 found that the passage of ultraviolet light through protein or peptone 

 solutions partially detoxicates it, while passage through solutions of 

 Cystine, Tyrosine or Amino -benzole Acid has a remarkable effect in 

 shielding the organisms from injury. Other dissolved substances such 

 as sugar, urea, alanine, glycocoll, etc., were found to be devoid of pro- 

 tective power. Leucine undergoes decomposition when exposed to 

 ultraviolet light and it exerts a certain measure of protection. The 

 following are illustrative results, the light from a Cooper-Hewitt ultra- 

 violet light being passed through the solution contained in a quartz 

 beaker before reaching the organisms (Paramoecia) suspended in dis- 

 tilled water below the beaker : 



Average determination- 

 Solution, period, seconds. 



Water 130 



1 . per cent, al anine 130 



1 . per cent, glycocoll . 130 



1 . per cent, aspartic aci d 130 



1 . per cent, glutamic acid 135 



1 . per cent, leucine . 250 



. 5 per cent, tyrosine 420 



1 . per cent, amino-benz-oic acid 2400 



. 5 per cent. NaOH 150 



1 . per cent. NaOH 170 



1 . per cent, glutamic acid in 1 per cent. NaOH 200 



1.0 per cent, cystine in 0.5 per cent. NaOH 1200 



1 . per cent, tyrosine in 0. 2 per cent. NaOH . . . Unaffected after 40 



minutes' exposure, 

 28 



