Description:
<jats:p><jats:bold>A<jats:sc>bstract</jats:sc>: </jats:bold> The early events in the thrombin‐induced formation of fibrin have been studied by the use of stopped‐flow multiangle laser light scattering (SF‐MALLS). This technological advancement has allowed the recovering, as a function of time with a resolution of about 0.5 sec, of the mean square radius of gyration <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/NYAS_167_mu1.gif" xlink:title="inline image" /> and of the molecular weight <jats:italic>M<jats:sub>w</jats:sub></jats:italic>, and to place an upper bound to the values of the mass/unit length <jats:italic>M<jats:sub>L</jats:sub></jats:italic>. The ionic strength, pH and salt type conditions investigated were all close to physiological, starting with a 50 mM Tris, 104 mM NaCl, pH 7.4 buffer (TBS), to which either 1 mM EDTA‐Na<jats:sub>2</jats:sub> or 2.5 mM CaCl<jats:sub>2</jats:sub> were also added. Fibrinogen was 0.2–0.3 mg/ml and rate‐limiting concentrations of thrombin were used (0.05–0.25 NIH units/mg fibrinogen). By plotting <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/NYAS_167_mu2.gif" xlink:title="inline image" /> and <jats:italic>M<jats:sub>L</jats:sub></jats:italic> versus <jats:italic>M<jats:sub>w</jats:sub></jats:italic> on log‐log scales, runs proceeding at different velocities and under different solvent conditions could be compared and confronted with model curves. It was found that: (1) within this thrombin range, the mechanism of association does not depend on its concentration, nor on the buffers employed; (2) the <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/NYAS_167_mu3.gif" xlink:title="inline image" /> versus <jats:italic>M<jats:sub>w</jats:sub></jats:italic> curves could all be reasonably fitted with a bifunctional polycondensation scheme involving semiflexible worm‐like, double‐stranded, half‐staggered polymers with persistence length between 200–600 nm, provided that a ratio <jats:styled-content><jats:italic>Q</jats:italic>= 16</jats:styled-content> between the rate of release of the two fibrinopeptides A was employed; (3) the <jats:italic>M<jats:sub>L</jats:sub></jats:italic> versus <jats:italic>M<jats:sub>w</jats:sub></jats:italic> data seemed more compatible with lower <jats:italic>Q</jats:italic> values (4 < <jats:italic>Q</jats:italic> < 8), but their uncertainty prevented a better assessment of this issue; the formation of fibrinogen‐fibrin monomer complexes may also play a role in the polymer distributions; (4) in the very early stages (e.g., when <jats:italic>M<jats:sub>w</jats:sub></jats:italic> < 7 × 10<jats:sup>5</jats:sup>), the <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/NYAS_167_mu4.gif" xlink:title="inline image" /> versus <jats:italic>M<jats:sub>w</jats:sub></jats:italic> data were fitted well only in TBS and at the lowest thrombin concentration, suggesting that a transient, either sequential or concurrent fast second mechanism, involving longer and thinner polymers, may be at work.</jats:p>