Thrombin effects on molecular structure of fibrin fibers in purified systems. Clots were made with human plasminogen-depleted IF-1 purified fibrinogen (1 mg/mL), thrombin (0.005-10 U/mL), and CaCl2 (2.5 mM) and analyzed by turbidimetry. (A) Fibrin fiber radius. (B) Number of protofibrils within fibrin fiber. (C) Protein density of fibrin fibers. (D) Distance between protofibrils inside of fibrin fibers. The results represent the mean values ± standard deviation (SD); n = 3. Statistical significance, using a 1-way ANOVA, is denoted with **P < .01, ***P < .005, and ****P < .001 for comparison between 0.005 U/mL and the remaining thrombin concentrations.