46 CYTOLOGICAL TECHNIQUE 



circumstances, of simultaneous reaction at both ends of the 

 double bond present in many organic substances. ^^' ^' The 

 process is additive. 



There are not very many suitable double bonds in proteins, and 

 indeed we have no actual proof that osmium tetroxide reacts with 

 proteins in this way. The evidence does point, however, in this 

 direction. 



Most compounds of osmium, other than the tetroxide, are dark 

 or even black, and one can use this fact to find which substances 

 react with the tetroxide and which do not. Of the amino-acids, 

 tryptophane and histidine react strongly, forming dark precipi- 

 tates.^ These substances contain double bonds in their five- 



NH, NH, 



HC.CHa— C- 



HCn 



HC.CH>— C CH 



C=0 N NH 



C=0 N I \ / 



I H OH C 



OH H 



Tnyptophane Histidine 



membered rings. Among the various proteins, the ones that 

 contain a high proportion of tryptophane and/or histidine are 

 particularly reactive with osmium tetroxide. There is a positive 

 correlation between the tryptophane-content of a protein and the 

 capacity of that protein, in the form of an aqueous sol, to be gelled 

 by osmium tetroxide. ^^"^ 



The capacity of osmium tetroxide to gel protein sols and to 

 harden and stabilize protein gels (p. 45) would be more compre- 

 hensible if it could be shown that separate protein chains were 



\ O / 



HC— O ! O— CH 



I Ml/ 



I Os 



I / \ 

 HC— O O— CH 



/ \ 



Osmium acting as a bridge between two ring-compounds 

 {The latter are shown only in part.) 



joined together by this substance. Of this we have no positiva 



