40 MACROMOLECULAR COMPLEXES 



1958). In the A -A' junction (Fig. 18) of the polymeric form, the 

 two bands corresponding to the A-terminal band of the monomeric 

 SLS form appear as a doublet separated by an interband region 

 about 100 A wide. Similarly, in the B'-B' junction, the two B-ter- 

 minal bands are spaced about 180 A apart, and a new band ( arrow 

 1 in Fig. 18), not observed in monomeric SLS, is seen in the middle 

 of the junctional regions. If it is assumed that well-defined bands, 

 as discussed earlier, occur only in those regions where the poly- 

 peptide chains are in an orderly helical configuration, thus provid- 

 ing the specific array of polar side-chains necessary for the coopera- 

 tive binding of ions such as PTA, these observations suggest the 

 following conclusions (see Figs. 19, 20): 



1. The TC macromolecule, the bulk of which is presumablv in 

 the characteristic three-strand helical configuration deduced from 

 x-ray diffraction studies, possesses speciahzed "end structures" 

 which are most probably in the form of short dangling chain ap- 

 pendages with maximum lengths of about 100 A and 180 A for the 

 A and B ends, respectively. Such terminal peptide chains would 

 most likely adopt a random coil configuration in solution, as indi- 

 cated diagrammatically in Figs. 19 and 20. 



2. The appearance of a new band in the center of the B'-B' junc- 

 tions of whole polymeric SLS forms suggests that end-to-end poly- 

 merization of TC involves an orderly coiling of these terminal chains 

 about one another to form a junctional region with an ordered heli- 

 cal structure, stabilized perhaps bv hvdrogen bonds. It should be 

 noted that this is the simplest hypothesis. However, the possibility 

 is not excluded that at one end there may be two polypeptide chains, 

 in which case the formation of polymers involving linkages of the 

 type A-B would result in the formation of a three-stranded struc- 

 ture in the junctional regions. Such a structure would pose a prob- 

 lem in finding a structural basis for end-to-end interactions of 

 homologous type ( A'-A' or B'-B' ) . 



On the basis of the picture just outlined, the presence of a new 

 band in the B'-B' junction and the absence of such a band in the 

 A'-A' junction are indicative of differences in amino acid composi- 

 tion, particularly of polar groups, between the A and B end-chains. 

 Indeed, it seems likely that the specificities of the terminal chains 

 reside in just such differences of amino acid composition and per- 

 haps to some extent depend on the presence of other components 

 such as carbohydrate. The marked changes in the end-to-end inter- 



