REACTIONS OF FIXATIVES WITH PROTEINS. 2 63 



are often temporary (see p. 33). This may be connected with the 

 fact that osmium tends to free itself from combination with ring- 

 substances, while transforming these into diols. 



Many amino-acids react with osmium tetroxide; this can be 

 seen by the development of a dark colloidal reaction product, or 

 sometimes a dark precipitate. It is doubtful, however, whether all 

 amino-acids that react when free necessarily do so when they are 

 constituents of a protein. Thus, the basic amino-acids, lysine and 

 arginine, are reactive when present in proteins ; but the protamines, 

 which contain a high proportion of basic amino-acids, scarcely 

 react with osmium tetroxide at room-temperature.^^ Tryptophane, 

 histidine, and cysteine react especially strongly when free, forming 

 dark precipitates.^^ Histidine, like tryptophane, possesses a double 

 bond between carbon atoms; the nature of the reaction with 

 cysteine has not been worked out. Among the various proteins, the 

 most reactive are those that contain a high proportion of these 

 three amino-acids.^^ The higher the amount of tryptophane in a 

 soluble protein, the lower the concentration at which that protein 

 can be gelled by osmium tetroxide. *^^ 



The anomalous non-coagulant fixatives, potassium dichromate 

 and acetic acid, remain to be discussed. They are anomalous be- 

 cause they appear not to be fixatives of ordinary proteins. 



At the end of the last century Fischer ^^^ showed that potassium 

 dichromate did not coagulate albumin or globulin. We saw in 

 chapter 2 that it does not render gelatine gels insoluble in warm 

 water nor coagulate nucleoprotein. All this is radically altered if 

 the solution be acidified: the reactions are then the same as though 

 we had used chromium trioxide (see p. 106). We are concerned 

 here with unacidified dichromate. The reaction with protein is 

 slow at room-temperature. The unacidified salt is sometimes used 

 in preparing wool for subsequent dyeing, but the process is carried 

 out at a high temperature and the chemistry of it is unlikely to 

 throw much light on fixation. In ordinary microtechnical use un- 

 acidified potassium dichromate seems to do little to proteins be- 

 yond making them somewhat more basic: that is to say, more 

 capable of being coloured by acid dyes.^^^ There is no good evi- 

 dence that it has a definitely fixative effect on simple proteins, 

 and nucleoproteins it positively dissolves. It is, however, one of 

 the few fixatives that make certain lipids capable of resisting 

 solution during embedding in paraffin. This is almost certainly the 



