Studies of fibrin film. II. Small-angle x-ray scattering.

Abstract

AbstractMeasurements of small‐angle x‐ray scattering have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 12–15% fibrin plasticized with either aqueous buffer or glycerol. Most of the experiments were made with water‐plasticized films because of the better x‐ray contrast. Both unligated and ligated (i.e., with α‐α and λ‐λ ligation by fibrinoligase, factor XIIIa) films were studied. Theoretical angular scattering profiles were calculated for a simplified model of the fibrin protofibril as a function of stretch ratio in uniaxial elongation based on an orientation distribution derived in the preceding paper. Measurements were made on unstretched films both with slit geometry and with pinhole geometry and two‐dimensional position‐sensitive detection. From the former, which had better resolution, the fundamental peak corresponded to a repeat spacing of 230 ± 5 Å, in agreement with early results of Stryer et al. [(1963) Nature 197, 793–794] and electron micrographs of stained fibrin, and with the half‐staggered overlapping model of the protofibril. Measurements were made with pinhole geometry on film stretched up to a stretch ratio λ = 1.59 at different times after imposition of strain and after release and recovery. The changes in meridional and equatorial scattering profiles, including the more prominent appearance of a harmonic in the former, show the orientation of the protofibrils as previously deduced from birefringence and related quantitatively to stretch ratio. In addition, the original repeat spacing is gradually replaced by one of about 290 Å. The “internal” stretch ratio λx, averaging about 1.28, is independent of the macroscopic stretch ratio λ from 1.14 to 1.59. The change, which is more nearly complete for unligated film and is reversible on release and retraction of the stretched strips, is attributed to an internal transition in the fibrin monomer units, probably involving extension of either the helical connectors or the terminal nodules. The results of stress and birefringence relaxation reported in the preceding paper of this series are interpreted on this basis.