X-ray Diffraction Studies of Tendon and Intestinal Wall Collagen

Abstract

THE x-ray diffraction patterns obtained from the fibrous proteins usually are characteristic fiber patterns but differ widely from each other in degree of perfection, depending upon the nature and relative proportions of the different amino acids of which the protein is built up. In the case of silk fibroin and keratin, a satisfactory solution of the crystal structure has been worked out from the data of the x-ray diffraction pattern. On the other hand, collagen or gelatin, being made up of some fifteen or more different amino acids of widely differing functional groups and molecular weights, previously have not given patterns with sufficient orientation and number of interferences to lend themselves readily to accurate analysis, and as a result no entirely satisfactory solution of structures has been developed. The diffraction pattern of ordinary untreated collagen or gelatin indicates the existence of long chain-like molecules which are rather easily oriented but which lack sufficient organization of their component parts of amino acid residues to produce a well-defined crystal lattice. Indeed, the existence of a halo, which shows indication of partial orientation, superimposed upon a fairly sharp interference pattern actually indicates a structure which truly may be called “pseudo-crystalline.” The explanation of the lack of structural organization found in this protein is generally considered to be due to mutual attraction of the large number of polar groups which naturally must exist in such a structure. In a substance whose make-up is as complex as that of collagen in which highly polar groups must be distributed at unequal intervals in both the side chains and along the main chains, the structural design cannot be expected to conform to any definite geometric pattern but, instead, will be fashioned more likely according to the chemical nature of this material, occurring naturally in so many forms, and will yield readily to the unbalanced forces, resulting in a highly distorted structure. The interpretations of the diffraction pattern of this protein usually have been given with an effort to conform to conclusions already deduced from the mass of carefully compiled chemical data and as a result often were in direct contradiction to what few facts were actually indicated. The practical, as well as the theoretical and biological, significance of new information on the crystal structure of collagen is readily seen from its wide use in the manufacture of leather, gelatin, glue, surgical catgut, musical instrument strings, etc. It is the purpose of this paper to describe new results of the application of x-ray diffraction technic to the study of this protein, which should throw more light upon the nature of its structure. X-ray Spectra of Unmodified Collagen and Gelatin Although x-ray investigations of the structure of collagen and gelatin have not been so extensive in recent years, nevertheless a considerable amount of work has been done.