Formation of fibrin represents an essential physiologic mechanism in the arrest of bleeding and in pathologic conditions results in thromboembolism. This proposal aims to study the functional domains of fibrinogen involved in fibrin polymer information. Using well-defined fragments of fibrinogen and its chains, as well as a family of synthetic peptides that have been designed according to the sequences of naturally-derived chain fragments, I will probe two main domains (centers) involved in polymerization of fibrin, amino-terminal domain(s) encompassing Alpha and Beta chains of fibrin monomer, and carboxy-terminal domain encompassing Gamma chain of fragment D. Synthetic peptides modified and labeled with fluorescent and photocrosslinking groups will be used as functional probes for complementary domains. The hypothesis that the amino-terminal polymerization center is a heterodimer composed of Alpha and Beta chain sequences will be tested using synthetic peptide methodology. Direct interaction between synthetic peptides corresponding to the complementary amino- and carboxy-terminal polymerization centers will be examined using a number of physicochemical techniques. Coupling of synthetic peptides to each other and the effect of this complex on fibrin polymerization will be examined. The effect of calcium on the fibrin polymerization inhibitory activity of synthetic peptides will be studied. The calcium binding site in the carboxy-terminal part of the Gamma chain will be localized by generation of various chemical (CNBr) and proteolytic degradation products of the Gamma chain derived from fragment D followed by synthesis of corresponding peptides. This research will help to pinpoint the functionally important domains involved in fibrin polymerization and calcium binding, as well as in the design of synthetic peptides which will possess more potent and specific inhibitory activity toward the formation of fibrin than currently available inhibitors.