176 



THE CELL AND PROTOPLASM 



Lomax 1935), giving them a uniformity of 

 position from which a diffraction pattern 

 may be obtained with X-rays, somewhat 

 similar to that from a fiber "where the long 

 chains are extended full length and ar- 

 ranged in parallel fashion, as has been 

 shown to be the case in silk. In silk they 

 are separated from one another laterally 

 at uniform distances of 4.3 A in one direc- 

 tion and about 7.0 A in another. These di- 

 mensions are slightly smaller in silk than 

 the corresponding dimensions of most pro- 

 teins, where they vary over only a narrow 

 range — from 4.4 A to 4.6 A for the thick- 

 ness of the chain, as illustrated in Fig. 4, 

 and from 9 A to 12 A as extremes for the 

 width. The width is obviously an average 

 depending upon the proportion of long and 

 short side chains, as may be seen in Fig. 3. 

 Averages taken for a protein chain for 

 general use are about 4.5 A for thickness 

 and 10.5 A for width. See Table VI in 

 which these dimensions are given for many 

 proteins. 



There are three ways of arriving at these 

 two dimensions ; one is by actual measure- 

 ments of width and thickness, on a model 

 made to scale from dimensions of the com- 

 ponent atoms ; a second way is from X-ray 



diffraction patterns ; and a third is from 

 computations involving density and molec- 



TABLE V 



Comparison op Width of Various Protein Mole- 

 cules AS Obtained by Different Methods 



Protein 



I 



Avei'. 

 mol. 

 wt. of 

 resi- 

 dues 



Keratin 



(wool) 

 Edestin 



(hemp) 

 Lactal- 



bnmin 

 Zein 



(corn) 

 Egg 



albumin 

 Casein 

 Gliadin 



(wheat) 

 Gelatin 

 Silk 



fibroin 



II 



Aver. 



length 

 of 

 resi- 

 dues 



III 



Width of i^rotein 

 molecule 



From 



mol. 

 wt. and 



spec, 

 gravity 



IV 



From 

 residue 

 length 



V 



From 

 X-ray 

 data 



VI 



9.8 a 

 11.0 



9.8 



10.3 

 10.2 



9.5 

 6.5 



Fig. 4. An end view of a protein nuidi 



The verticnl tliicl<:n.'.ss 

 and is about 4.5 A. 



in the middle is ihie to the backbone 



