132 QUANTITATIVE STUDIES 



This latter problem is by no means simple. Suppose, for ex- 

 ample, that we consider a reaction such as that of dinitrofluoro- 

 benzene with tyrosine. With free tyrosine the reaction is rapid 

 and quantitative, but with a protein steric effects may be suffi- 

 cient to mask some reactive groups from the reagent. When 

 denaturation occurs, some masked groups may be exposed to 

 the reagent, and other hitherto exposed groups may be masked. 

 Thus the results obtained may be a function of the degree of 

 denaturation. Material which has been frozen-dried is prob- 

 ably little denatured but, on the other hand, can rarely be used 

 without further denaturation. And the greater the denaturation, 

 the greater is the deviation of masked groups from that prevail- 

 ing in the intact cell. Consequently, before commencing a study 

 of proteins it is necessary to decide first whether what is wanted 

 is a measure of the total concentration of a particular amino acid, 

 or whether it is the concentration of these groups which is steri- 

 cally unmasked in the native protein. And, when this decision 

 has been made, it still remains to design an effective way of 

 getting the required information. 



Damage Caused by Illumination 



When work is being done in the visible region of the spectrum, 

 damage caused to the specimen by the incident illumination is 

 unlikely to be serious, but as one moves into the ultraviolet the 

 hazard steadily becomes greater. First, it becomes impossible to 

 make continuous studies upon living cells, and then as the energy 

 of the incident radiation increases damage to the specimen on 

 the chemical level may become evident. This hazard can prob- 

 ably be adequately allowed for by measuring changes in extinc- 

 tion with time. 



With studies in the electron microscope damage again may be 

 caused by the electron beam. The magnitude of this damage 

 on the chemical level has yet to be explored. 



REFERENCES 



Caspersson. 1947. Symp. Soc. Exp. Biol, 1, 127. 



1950. Cell Growth and Cell Function (Norton, New York). 



Commoner. 1948. Ann. Mo. Bot. Gard., 85, 239. 



1949. Science, 110, 31. 



