INTERACTION OF COLLAGEN MAC ROMOLEC U L ES 39 



usually yield single segments (whole monomelic type) together 

 with some polymeric forms of type A-B. 



With longer times of irradiation, scission of the macromolecules 

 becomes evident in SLS-type precipitates (Figs. 14-17). Fragmen- 

 tation appears to occur initially near the middle of the TC macro- 

 molecule, yielding fragments A'C and B'C about 55 per cent and 

 45 per cent, respectively, of the original TC length. These are ca- 

 pable of forming monomeric (Fig. 14) and dimeric (Fig. 15) SLS- 

 type aggregates, and of isomorphous growth on whole polymeric 

 SLS (Figs. 16, 17). However, dimerization or polymerization of 

 the scission products apparently never involves new ends resulting 

 from fragmentation; i.e., end-to-end linkages of the type C-C have 

 not so far been observed. It would appear, therefore, that the 

 original ends of the TC macromolecules have properties which are 

 not duplicated in the new ends formed by scission. Extensive 

 irradiation results in still smaller fragments of the type indicated by 

 breakage at points D and E in Fig. 12. Of the four scission products 

 presumably yielded by this process, only those which include A' or 

 B' ends have been observed to form ordered SLS-type structures. 



Detailed comparison of the band pattern of single segments 

 (whole monomeric SLS form) with that found in the whole poly- 

 meric form resulting from sonic irradiation has led to a hypothesis 

 concerning the mechanism by which TC macromolecules engage 

 in end-to-end linkage to form protofibrils (Hodge and Schmitt, 



Fig. 19. At top is depicted a model of the TC macromolecule in solution, 

 the three polypeptide chains being represented by wires (block, gray, and 

 white, respectively). Within the body of the macromolecule, the three wires 

 are coiled about one another in on orderly helical pattern, representing the 

 type of structure deduced from large-angle x-ray diffraction studies of collagen. 

 At the ends, the postulated end-chains are shown as single, randomly coiled, 

 polypeptide chains. Note that parameters, such as the length and diameter of 

 the macromolecule, the pitch of the helix, and the lengths of the end-chains, 

 are not to scale. In the center is illustrated the heterologous type of end-to-end 

 coiling envisaged in the formation of protofibrils during normal polymerization 

 (end-to-end linkage of type A-B), while at the bottom is shown the homologous 

 type of end-chain coiling presumed to occur during formation of whole poly- 

 meric SLS-type aggregates, such as those shown in Figs. 13, 16, 17. (From 

 Hodge, 1959b.) 



Fig. 20. The same model of the TC macromolecule as shown in Fig. 19, 

 illustrating the fragmentation by sonic irradiation (center), and the formation 

 of fragment dimeric forms CA -A'C and CB'-B'C (at bottom) on the addition 

 of ATP. Dimeric and polymeric forms involving junctions of type C— C have not 

 been observed. (From Hodge, 1959b.) 



