28 The Structure of Protoplasm 



lysine is basic. These, and similar, side chains bestow upon proteins 

 their amphoteric properties. The presence of free lipophilic (hydro- 

 phobic) side chains in most fibrous proteins probably prevents them 

 from being dispersed by water. 



In the corpuscular proteins, however, it appears that the folding 

 of the backbone is more complete, with most of the nonpolar side 



R' 



C. HgN COOH 



/H\ 

 HoN COOH 



£ 



R* 



H R' H r'* H 

 N C C N C C, N 



/\h/\M/\h/\/h\/^ 



C N C C N C 

 R H R H 



Fig. 1. Back-bone structure produced by the union of amino acids through 

 the peptide linkage. 



chains held inside and the polar groups sticking out on the surface. 

 There is no unanimity of opinion as to the mechanism of the folding 

 process, but it is probable that weak bonds, such as hydrogen bonds 

 (35) , are formed and that these hold the structure together (26) . 

 This configuration of these proteins appears to confer upon them 

 their water solubility. Recent measurements of sedimentation veloc- 

 ity, diffusion, viscosity, and electrophoretic mobility (1, 20, 32) 

 have emphasized that few of the corpuscular proteins approach 

 sphericity although their axial ratios are not usually very high. 



It had been considered that the two groups were nearly inde- 

 pendent, but now it appears from investigations of denaturation and 

 X-ray structure that a corpuscular protein may be transformed into 

 a member of the fibrous group (3) . Denaturation produced by heat, 

 spreading on a surface, or by chemical treatment, apparently unlocks 

 the folded molecule so that it becomes extended (6, 25) . The slight 

 rise in the isoelectric point of egg albumin on denaturation (30) 

 indicates a change in the strength or number of the amino and 

 carboxyl groups, among other linkages (6, 26) . After extension has 



