REACTIONS OF FIXATIVES WITH PROTEINS. 2 45 



over and over again, like vertebrae; they articulate through the 



O 



II 

 peptide link, — C — NH — . The backbone is folded in various 



ways; this folding will not be represented in the structural 

 formulae used. The side-groups are those parts of the amino-acids 

 that distinguish one from another. These project laterally from the 

 backbone, like ribs. They alternate in direction, but in this book 

 they will all be represented as projecting to the right. This con- 

 vention will be adopted partly for economy of space, partly because 

 the different side-groups are more easily recognized if always 

 written in the same sequence (just as words are more easily read 

 if the sequence of the letters is not reversed). 



A sequence of linked amino-acids (backbone and side-groups) 

 will be called a chain. The word molecule will not be used loosely. 

 It seems inapplicable when protein chains are linked together by 

 chemical bonding through their side-groups to form net-like struc- 

 tures of indefinite size, perhaps extending from one end of a cell 

 to the other. 



A substance cannot act as a fixative if it attacks the peptide link. 

 Far from being proteolytic, fixatives tend to link protein chains 

 together and thus give mechanical stability. 



An important feature of the structure of proteins is that a 

 particular side-group may react chemically without necessarily 

 changing the nature of the chain as a whole. Suppose, for instance, 

 that a particular protein containing very few cysteine side-groups 

 is exposed to a substance that reacts with nothing in the chain 

 except -SH groups. Obviously the chain as a whole will be scarcely 

 affected, and no fundamental change in the nature of the protein 

 will have occurred. If a particular side-group is capable of reacting 

 with a particular dye, it will retain this property when a fixative 

 has only blocked some other side-groups. 



Despite these facts, the appearance and 'nature' of a protein, 

 especially its solubility in water, are often markedly changed by 

 fixation, and the substance is often said to be 'denatured'. Un- 

 fortunately this word has been used vaguely, and with different 

 meanings by different authors. So long as it referred to a loss of 

 solubility, one knew what it meant ; but loss of solubility is accom- 

 panied by increase of reactivity, and this fact has altered the mean- 

 ing entirely: for if the increase in reactivity occurs, the process is 

 often called denaturation, even though solubility is actually 



