INTERACTION OF COLLAGEN MACROMOLEC U LES 21 



naturation, the original macromolecules, of weight ca. 360,000, yield 

 one chain of weight ca. 120,000 and another of weight ca. 240,000 

 (see also Orekovitch and Shpikiter, 1958). According to Doty and 

 Nishihara, the larger of these components in calfskin collagen may 

 comprise two chains linked by an alkali-labile ester bond. 



The physical properties of collagen solutions and the other ob- 

 servations already cited leave little doubt that the tropocollagen 

 macromolecule, ca. 14 A X 2800 A in dimensions, is indeed the 

 monomeric unit of the various forms of ordered aggregates and must 

 possess a high degree of configurational order over at least the major 

 part of its length. A striking property of native collagen macro- 

 molecules is their capacity to aggregate under appropriate condi- 

 tions to yield a number of highly characteristic ordered structures, 

 the type of pattern obtained depending on the environmental con- 

 ditions. These various ordered aggregation states are characterized 

 by the highly specific band patterns they exhibit when observed in 

 the electron microscope, especially if the contrast is enhanced by 

 treatment with an "electron stain" such as phosphotungstic acid 

 (PTA). The ability to form such ordered structures is lost if the 

 macromolecules are denatured by thermal or other means. It seems, 

 therefore, that the polar side-chains responsible for the banding 

 seen in the electron microscope must be in a rather precise stereo- 

 chemical array, which is maintained in the native macromolecule 

 by hydrogen-bonding between the constituent polypeptide chains, 

 and which is lost when these bonds are ruptured during denatura- 

 tion. As we shall see, the polar groups (both basic and acidic) are 

 located in clusters at discrete loci along the length of the TC macro- 

 molecule and are separated by regions apparently poor in or lacking 

 polar groups. It is likely that these non-polar regions are mainly 

 responsible for the characteristic large-angle x-ray diffraction pat- 

 tern of collagens. On the other hand, there seems to be no doubt 

 that the small-angle pattern is the direct result of the particular 

 distribution of the polar side-chain clusters (see Bear and Morgan, 

 1957). Chemical analytical and electron microscope studies on 

 collagen fibrils by Kiihn et al. (1957) and Kiihn (1958) indicate 

 that the basic and acidic residues are in the same loci (bands) in 

 the native collagen structure. Their work shows that, while the 

 lysine-bound PTA is easily washed out, that held by arginine is 

 firmly bound and is responsible for the characteristic band pattern 

 and, further, that the rather large phosphotungstate ion is bound 



