24. 



MACROMOLECULAR COMPLEXES 



The simplest "crystalline" form of collagen is that which Schmitt 

 et al. (1953) termed segment long-spacing (SLS), and is readily 

 formed on addition of adenosine triphosphate (ATP) to a dilute 

 acid solution of TC, although segments can also be formed under 

 other conditions. In this aggregation state the macromolecules are 

 arranged in a parallel array with like ends and other features in 

 register (Figs. 1-3).-^' The asymmetric or "polarized" band pattern 

 observed when SLS is stained with PTA is thus, in effect, a "molec- 

 ular fingerprint," since it directly indicates the distribution and 

 concentration of basic side-chain groups ^ along the length of the 

 TC macromolecule. A comparable "fingerprint" of the acidic side- 

 chains (Fig. 2) may be obtained conveniently by staining the seg- 

 ments with cationic uranium under appropriate conditions (Hodge 

 and Schmitt, 1960). 



The end regions of individual segments are generally somewhat 

 disordered, and a detailed comparison of the staining properties of 



Fig. 4. Polymeric SLS-type aggregates obtained by addition of ATP to 

 collagen solutions subjected to sonic irradiation (see text). In these polymeric 

 forms, the distortions due to drying ore minimized, particularly for staining loci 

 near the ends of the TC macromolecules, and thus allow a better comparison 

 of the various band densities, (a) Stained with cationic uranium, (b) Stained 

 with PTA. XI 35,000. (From Hodge and Schmitt, 1960.) 



•"' Unless otherwise indicated, all the illustrations have reference to preparations 

 of calfskin collagen. 



" It should be understood that the actual band densities observed in the electron 

 micrographs represent both an "intrinsic electron density," resulting from the con- 

 centration of the bulky polar side-chains into clusters along the length of the TC 

 macromolecule, and an electron density contribution arising from the binding of 

 heavy metal ions. 



