Models of fibrin.

High-symmetry models of the fibrin fiber are proposed that reproduce the experimentally observed high specific volume of the fiber. The models meet the following criteria: fibrin monomers have the three-domain Hall and Slayter structure; the monomers are arranged lengthwise into strands (protofibrils) in which successive monomers half overlap; the monomers' alignment is nearly parallel to the fiber axis; and the monomers make adequate longitudinal and lateral contacts, as required by observed fiber properties and the high affinity of monomers for one another. All the models contain helical protofibrils related to each other by rotation axes parallel to the fiber axis; as a consequence the protofibrils are in register in the fiber direction. The protofibrils may contain two, three, or four monomers per helical turn and can be packed in four different symmetries (space groups). A large specific volume is achieved only if the D-domain (which are presumed to contain the lateral polymerization sites) are somewhat displaced from the helical axes of the protofibrils. This displacement may involve either a lateral shift of the monomers away from the helix axis or a tilt of the monomers, which swings the D-domains away from the helix axis. It is shown that the fiber containing tilted monomers is more highly interconnected; the two D-domains of each tilted monomer form lateral contacts with different adjacent protofibrils, whereas the two D-domains of each nontilted monomer contact the same adjacent protofibril(s).