2 PROTEINS 347 



welatin filaments are positively uniaxial as referred to the fibre 

 direction, i.e., the same as silk and hairs. Rodlet birefringence is also 

 evident if the tendons are tanned before imbibition (Kuntzel, 1929). 

 The tendons are very liable to swell in the presence of most imbibition 

 liquids, or to shrink (e.g., with xylene). Collagen behaves peculiarly 

 on tanning; for whereas the optical character of the tendons is 

 retained with mineral tanning materials (chromic salts) and formol, 

 it is reversed and becomes negative with pyrogallic tanning agents 

 (tannin, sumach) and other phenols (trinitrophenol) and aldehydes 

 (eugenol, cinnamic aldehyde). Schmidt (1934) imagines that the 

 optical negative reaction is brought about by orientated adsorption, 

 as the non-tanning univalent phenols and aldehydes may be washed 

 out again, whereupon the normal optically positive reaction returns. 

 Personally, I am inclined to believe that it is rather a matter of chemi- 

 cal- changes in the side chains. Tanning depends upon the permanent 

 connecting of one polypeptide chain molecule to another by strong 

 side-group linkages. Moreover, the pyrogallic tanning agents must 

 thereby change the polarity of the side groups in a manner similar 

 to what takes place in the nitration or acetylation of cellulose. In view 

 of the lability of many side chain reactions of the polypeptide chains, 

 it is not surprising that washing out of the non-tanning phenols should 

 easily upset the chemical changes brought about by trinitrophenol, 

 eugenol, etc. Rodlet birefringence and X-ray analysis thus provide 

 evidence for the submicroscopic fibrous structure of tendons. 



It is not only the strange optical behaviour of tendons which has 

 for long attracted attention (v. Ebxer, 1894), but also their re- 

 markable swelling power. In water they swell by 50% in thickness, 

 which, as X-ray evidence shows, involves expansion up to 35 % of 

 the crystal lattice (Kuntzel and Prakke, 1933), while the fibre period 

 remains unchanged. Hence the swelling is not intermicellar as in 

 cellulose, but intramicellar, inasmusch as the individual primary 

 valence chains are pushed apart. This explains why the swelling of 

 tendons may assume unprecedented dimensions. In dilute acids and 

 alkalies, which obviously completely hydrolyze the side-chain bonds, 

 they are liable to swell 550*^0 in thickness, though admittedly they 

 shorten at the same time by 30%. Despite this shortening, the increase 

 in volume due to the infiltration of fluid may amount to as much as 

 4500% (Kuntzel and Prakke, 1933). 



