INTRODUCTION TO FIXATION 19 



NH2 

 HCR HC.CHa^ )OH tyrosine 



c=o 



I 

 OH 



General formula for HC(CH2)2C^ glutamic acid 



amino-acids \ \OH 



I 



Three amino-acids as part 

 of a protein chain 



each amino-acid are written on the right of the formula in each 

 case, although in fact they project in different directions; and the 

 repeated part or backbone of the formula is shown as straight, 

 though in nature it is folded, either to form a simple zigzag or in 

 more complex ways. These conventions will be found to simplify 

 the explanation of the reactions of proteins with fixatives and 

 dves. 



The first requirement of a fixative is that it should not be 

 proteolytic. Any substance that breaks the peptide links and sets 

 free soluble amino-acids is the opposite of a fixative. The changes 

 produced by a fixative must tend in the other direction, towards 

 stability. 



One may broadly distinguish two kinds of fixation of protein, 

 additive and non-additive. In the form_er, certain atoms of the 

 fiixative combine chemically with some part of the protein and 

 remain in combination. In the latter, no such obvious addition of 

 atoms occurs. Ethanol is a non-additive fixative in the sense that 

 none of its constituent atoms joins itself to protein, so far as is 

 known. The 'nature' of the protein is, however, profoundly 

 changed by its action, and the substance is therefore said to be 



