Structure and packing of dry elastoidin: a collagen phase change.

Wide angle X-ray patterns of wet shark fin elastoidin always show a very intense meridional streak arising from the small angle diffraction pattern. When the fin ray spicules (ceratotrichia) are permitted to air dry, the pattern departs from the meridian. Studies show that the dry spicules are, in fact, radially anisotropic. Diffraction patterns taken with the X-ray beam perpendicular to the edge of the ribbon-shaped spicules have two distinct arms (with an interarm angle of as much as 70 degrees), while the small angle streak taken with the beam perpendicular to the ribbon face is generally either on the meridian, or is so weak that it cannot be resolved from the background. An analysis of the intensities of the small angle diffraction pattern of the dry form indicate that it has essentially a normal dry collagen structure. Addition of methanol serves only to expand the dry structure, without reversal of the phase transition. These data can be interpreted by postulating that as the wet structure collapses during drying, the collagen molecules tilt in a direct perpendicular to the face of the ribbon, forming domains of molecules with similar axial orientation. The implication is that the original interactions are lost, and the new structure is formed by overlaps between formerly non-interacting molecules.