6o FIXATION 



reactive, for polyglycine and poly glutamic acid bind very little 

 formaldehyde. Nylon also binds very little formaldehyde, ^^^ 

 though the -NH.CO- group occurs repeatedly in the mole- 

 cule. 



' \C(CH2)2C] 



f 



HC(CH2)4N— CHa— N^ 



I H H C=0 



? 



A methylene bridge (bold type) linking tzvo protein chains through lysine (left) 



and ghitamine (right) 



Bridges of these kinds would tend towards gel-formation in 

 proteins, or to the strengthening of pre-existent gels; coagulation 

 does not result. In general, cross-linkages of any kind between 

 protein chains result in brittleness, loss of elasticity, reduction in 

 the ability to bind water, and, in the case of soluble proteins, 

 lessened solubility."^^ 



Zein is interesting in this connexion. The mechanical strength 

 of fibres and films made of this protein is considerably increased 

 by the action of formaldehyde, yet it contains no lysine (and very 

 little arginine). It must be presumed that links are formed between 

 the nitrogen of an amide (-CO.NHg) group in one protein chain 

 with a similar group in another, or possibly with the nitrogen of a 

 peptide link.^'^ 



The reaction between proteins and formaldehyde is remarkably 

 slow and contrasts strongly in this respect with the quick ionic 

 reactions of some fixatives. In experiments with soluble proteins 

 and polypeptides, carried out at 70° C, only one-half of the total 

 amount of formaldehyde that eventually combined was taken up 

 in 8 hours ; after the lapse of 24 hours one-tenth of the amount still 

 remained unbound. ^^* In practice, fixation by formaldehyde is 

 presumably nearly always incomplete, except when tissues are 

 not only fixed but stored in a solution of formaldehyde. That is 

 why tissues remain capable of taking up a limited amount of acid 

 dyes. 



The eff'ect of pH on the reaction between proteins and formalde- 

 hyde is complex. The reaction wdth amide groups is said to be 

 promoted by acidity, ^^^ but that with simple amino-groups is 

 antagonized ; the total effect is a slowing down, and the reaction is 



