Self-assembly of fibrin monomer. A light scattering and electron microscopic investigation.

A light scattering method, together with complementary electron microscopy observations, was designed to investigate the self-assembly of fibrin. Calcium-free monomer was used, and clot reconstitution was carried out in solvents corresponding to limit interaction energies of the protein with the medium. The self-assembly process, under physiological conditions, conforms to the following sequence of events. 1) A fast polymerization step leading to linear aggregates. 2) Fiber growth; at this stage onset of the network occurs. 3) Gelation (clot formation). Bound calcium was found to be structurally required for gelation. Its removal results in the formation of thick fibers, which are unable to clot. Evidence is reported favouring our previous hypothesis (Conio et al., 1976) on the onset of the network: branching of linear aggregates is a prerequisite for clotting. The occurrence of a crystallization process, which overlaps to fiber growth, is demonstrated in this paper for the first time. Its dependence on solvent-protein interactions is analyzed. Our results suggest that fibrin monomer is to some extent a flexible molecule. Both flexibility and crystallization may play a functional role in the clotting process in vivo.