98 CYTOCHEMISTRY OF PROTEINS 



resolution is given by A/2, it is necessary to have a region at least 

 3 A. in diameter before quantitative studies of its absorption spec- 

 trum can be made. Thus, if one is employing light with a wave- 

 length of 4 fi 7 whilst the resolution limit is 2 p, the size of area 

 which is needed for studying the absorption spectrum for light of 

 wavelength 4 ju is at least 12 /x. Thus studies in this region of 

 the infrared would only be useful with very large cells. 



The characteristic absorbing unit of the nucleic acids is the 

 pyrimidine ring, found in both the pyrimidines and in the purines. 

 In nucleic acids these rings produce a very strong absorption of 

 light in the region of 2600 A.U. The exact position of the absorp- 

 tion band of individual purines and pyrimidines varies, according 

 to the side-groups which are present on the ring structure. Thus 

 a considerable change in position of the absorption bands may 

 occur when an NH 2 group is replaced by an OH group. Until 

 now observations on the nucleic acids have been limited to those 

 which can be made without modification of the purine and pyrim- 

 idine rings. But it may quite well be possible to modify the 

 absorption spectrum by carrying out reactions with these ring 

 structures. 



In the proteins the main absorbing components are trypto- 

 phane and tyrosine, which absorb strongly in the region of 2800 

 A.U. The absorption of light by tyrosine is sensitive to pH 

 changes. The remaining amino acids absorb light rather strongly 

 in the region of 2400 A.U. 



Generally speaking, it is probably true that one can determine 

 the amount of the nucleic acid and of protein by making observa- 

 tions at a number of different wavelengths in the ultraviolet. It 

 must, however, be remembered that the absorption of light is a 

 property not of a molecule as a whole, but of some of the smaller 

 constituent groups. It is not impossible for some of the purines 

 and pyrimidines occurring in nucleic acids to occur as constituent 

 groups of proteins, and for some of the amino acids to occur as 

 constituent groups of nucleic acids. Panijel has found a protein 

 in Ascaris sperm which contains purine but no phosphorus. Thus 

 although this protein has an absorption spectrum similar to 

 nucleo-protein since it lacks phosphorus it cannot contain nucleic 

 acid. 



Very few studies have been made in the visible region of the 

 spectrum, since most proteins do not absorb light to a significant 



