28 MACROMOLECULAR COMPLEXES 



A < ^B 



0»S/M 



Fig. 9. Dimorphic ordered aggregate of TC, produced by exposing native- 

 type fibrils to a solution of TC containing ATP, under closely controlled condi- 

 tions of pH and ionic strength. Note that all of the segments so formed are 

 similarly polarized, as indicated by the labeled arrows, and are located 

 identically with respect to the intraperiod band structure of the native-type 

 fibril, which is itself polarized. Stained with PTA. X70,000. (From Hodge and 

 Schmitt, 1960.) 



very fine filaments with no striation visible in the electron micro- 

 scope. The viscosity of the collagen solutions during such a dialysis 

 rises steadily as the pH rises (Hodge and Schmitt, 1960; Hodge 

 et al., 1960), indicating that end-to-end polymerization of the TC 

 is giving rise to linear polymers or protofibrils. However, Glimcher 

 and Bonar (personal communication) have shown that the small- 

 angle x-ray diffraction pattern of fibers drawn from such gels is 

 characteristic of the native (700 A-repeat) type of packing of the 

 TC. This means that the protofibrils are packing in staggered 

 array (see Fig. 7) to form the very thin fibrils of the gel. If the 

 dialysis is carried out in the presence of suitable concentrations of 

 certain substances such as serum glycoprotein (see Schmitt et al., 

 1955), the TC comes out of solution in a second ordered fibrous 

 form, the so-called fibrous long-spacing (FLS) form (Fig. 5). 

 Since the axial period of FLS is about equal to the length of the 

 TC macromolecule and the distribution of bands within each period 

 is centrosymmetric, it seems likely that this type of structure arises 

 by side-to-side aggregation of protofibrils in antiparallel array, i.e., 

 with no preferred "polarity" (Figs. 1, 5). This change in the inter- 



