Comparison of mammalian collagen and elasmobranch elastoidin fiber structures, based on electron density profiles.

Standard difference-Fourier methods of crystallography, applied to the axial small-angle X-ray diffraction of elastoidin, develop a four-strip model for the distribution of electron density along shark-fin ceratotrichial axes. This result is obtained directly from X-ray data. The model consists of three major strips with centers separated approximately by $\text{d}\text{3}$ ($\text{d = 670}\text{A}\text{?}$, the familiar collagen macroperiod), superimposed upon a wide strip extending over 0·49 d, representing the overlap-hole zone background, also found in mammalian collagens. The three narrower strips correspond to cross-sections which resist negative staining or diametral contraction on drying, reported to be characteristic of elastoidin from electron microscopy. Chemical evidence suggests that these unique cross-sections (“superbands”) occur at axial locations where a tyrosine-rich matrix, intimately associated with very thin collagen units, produces paracrystalline order with good axial registration but poor transverse order, across an entire ceratotrichium (diam. ~ 1 to 2 mm). The analogous but quite different condition in mammalian collagen fibers involves an intra-fibrillar paracrystalline order (diam. approx. 1000 Å), with inter-fibrillar stabilization by a mucopolysaccharide matrix. Stiffening of the elastoidin by means of the tyrosine-rich matrix exemplifies a way, alternative to mineralization processes (e.g. in bone), of reducing collagenous fiber flexibility.