PROTEINS 



349 



These authors think that the third cross-band is due to remains of 

 a second transverse system of fibrils which bind the separate collagen 

 fibres into a fabric-like system. It is likely that it is the pairs of cross 

 striae which adsorbs more easily silver than the rest of the segment 

 (Dettmer, Neckel and Ruska, 195 i). 



0^ 



^'. - 



Fig. 174. Striation of collagen fibrils, 34,000: i (from Pratt and Wyckoff, 1950). 



The reason why collagen fibrils display a submicroscopic seg- 

 mentation is obscure. Possibly there is some relation to the globular 

 state of proteins in solution. Bahr (1950) and Noda and Wyckoff 

 (195 1) succeeded in reconstituting tendons dissolved in dilute acetic 

 acid into collagen fibrils by precipitation of a dilute collagen solution 

 of 0.75 to 0.05% with salts, (0.7 to 1.5% at p^ 3.8 to 7.0). This 

 reconstitution furnishes segmented collagen fibrils with a period of 

 635 A or 650 A which can be dissolved again by dialysis against water 

 and acetic acid. Since dissolved collagen represents a globular protein, 

 as known from gelatin (see p. 93), precipitation may join these 

 spheres in a linear way causing beaded chains ; if such chains associate 

 laterally to form fibrils, denser and less dense cross-bands are likely 

 to be produced. 



In the work of Schmitt, Hall and Jakus (1942, 1943, 1945) the 

 dense segments are marked A, the more transparent, B. Upon arti- 

 ficial elongation the B segments increase in length at the expense of 

 the A segments ; the period can be raised to as much as 6000 A. It is 

 :Supposed that the polypeptide chains are more tightly folded in the 



