Preincubation of fibrinogen with polyP and Ca²⁺ influences final clot turbidity. (A) Final turbidities of clots formed with fibrinogen mixed with thrombin were quantified as described in “Measurements of clot turbidity.” Reactions contained 2.6 mg/mL fibrinogen preincubated for the indicated times in the presence of 2.5 mM CaCl₂ with 1 mM polyP (▼) or without polyP (○), after which clotting was initiated with 8 nM thrombin. Data are mean plus or minus SE (n = 3). (B) Final turbidities of clots formed with prothrombin-deficient plasma mixed with thrombin were quantified as described in “Measurements of clot turbidity.” Reactions contained citrated plasma (80%) with added CaCl₂ (13.7 mM) and were preincubated for 15 minutes with polyP concentrations as indicated on the x-axis. Clotting was then initiated with 10 nM thrombin. Data are mean plus or minus SE (n = 4).