Fig. 3. Stern-Volmer plots of pyrene-actin (A) and fluorescein-actin (B) quenching by nitromethane.

A, pyrene-modified F-actin (4.0 μm) was titrated with nitromethane in the presence of (•) and absence (○) of caldesmon (1.5 μm). Fluorescence changes are expressed as a ratio of initial fluorescence (F₀) to the fluorescence after addition of nitromethane (F). Table I lists Stern-Volmer constants for all quenching experiments. Solvent conditions were identical to those described in the legend to Fig. 2. B, actin modified with 5-(iodoacetamido)-fluorescein at Cys-374 was polymerized with 2 mm MgCl₂ and 40 mm NaCl in G buffer. The polymerized fluorescein-actin (4.0 μm) was titrated with nitromethane in the presence (•) and absence (○) of caldesmon (1.5 μm). Solvent conditions were the same as described in the legend to Fig. 2.