September 28, 1917] 



SCIENCE 



319 



and therefore referred it to the tyrosin radical 

 in the protein molecule. These observations 

 have recently been greatly extended by Kober,* 

 who has carried out a spectrographic examina- 

 tion of solutions of the various amino-acids 

 which are the end-results of protein hydrolysis 

 and of certain polypeptida. Kober has con- 

 firmed the existence of an absorption-band in 

 the ultra-violet in solutions of tyrosin and also 

 finds that a similar band is exhibited by solu- 

 tions of phenylalanin. The other amino-acid 

 constituents of the protein molecule exhibit 

 only general (i. e., non-selective) absorption in 

 the ultra-violet spectrum. 



The possibility is thus indicated that the 

 tyrosin and phenylalanin radicals of the pro- 

 teins constitute the optical sensitizers which 

 render living cells susceptible to the toxic ac- 

 tion of ultra-violet 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 rays, to a greater or 

 less extent deprive the light of its toxicity for 

 protoplasm. With this possibility in view the 

 following experiments were undertaken: 



Definite volumes of a densely inhabited cul- 

 ture of paramecia were washed by suspending 

 the organisms in tap-water and concentrating 

 them by moderate centrifugalizations until a 

 thick suspension of uninjured organisms in a 

 colorless liquid was obtained. All of the sus- 

 pensions used were prepared in exactly the 

 same manner and were derived from the same 

 culture. 



Our first step was to determine what we have 

 called the "normal extermination period," 

 that is to say the duration of time in seconds 

 of exposure to the direct rays of a Cooper-Hew- 

 itt Ultra-violet Light Type Z at a distance of 

 12 cm. from the quartz tube. For this pur- 

 pose 0.5 c.c. of Paramecium suspension was 

 placed in a flat-bottomed (Syracuse) watch- 

 glass and 0.5 c.c. of tap-water was added. 

 Trials were made with varying times of ex- 

 posure and the percentage of organisms killed 

 was estimated by counting the individuals of 

 which the cilia had ceased moving. The nor- 



*F. A. Kober, Journ. Biol. Ch^m., 22 (1915) 

 p. 433. 



mal extermination-period was found, under 

 these conditions, to be about 100 seconds. To 

 determine whether the gases formed during the 

 exposure to the ultra-violet light (ozone and 

 nitric-oxide) hastened the killing of the organ- 

 isms appreciably, a trial was made with a sus- 

 pension protected from the ultra-violet rays by 

 a thick glass plate, but still exposed to the 

 gases. In this way it was determined that this 

 factor could be overlooked, since after 900 sec- 

 onds exposure no noticeable effect was ob- 

 served. 



After determining the normal extermina- 

 tion-period with the above procedure, trials 

 were made with similar suspensions in solu- 

 tios of Witte-peptone, gelatin, amino-acids, 

 etc., the results of 160 such trials being sum- 

 marized in the table below. Thus a 1 per cent, 

 alanin suspension of paramoecia was prepared 

 by adding 0.5 c.c. of a 2 per cent, solution of 

 alanin to 0.5 c.c. of washed Paramecium sus- 

 pension. 



The extermination-periods enumerated in 

 the tables are meant to indicate that immedi- 

 ately after the stated period of exposure 100 

 per cent, of the organisms were dead. For it 

 was found that even after an exposure as brief 

 as 40 seconds in a water-suspension the organ- 

 isms were affected and ultimately all died. 



AVXKAGE EXTERMINATION PERIODS 



(Paramecia immersed in Test Solution) 



Water suspension 100 sees, 



1 per cent, cane sugar suspension 110 



1 per cent, urea suspension 110 



1 per cent, alanin suspension 110 



1 per cent, leucin suspension 215 



1 per cent, gelatin suspension 220 



1 per cent, peptone suspension 300 



Glutamic acid, amino-benzoic acid and as- 

 partic acid all proved to be themselves toxic 

 for the organisms and could not therefore be 

 tested by this method. Tyrosin is very spar- 

 ingly soluble in cold neutral water. A satu- 

 rated solution, although exceedingly dilute, 

 conferred marked protection, the extermina- 

 tion-period being lengthened to 180 seconds. 

 An alkaline solution proved to be toxic and 

 therefore could not be employed in this way. 



