Tryptophan fluorescence quenching in rabbit skeletal myosin rod

The solvent accessibility of the four tryptophans of rabbit skeletal muscle myosin rod was investigated using steady-state and time-resolved fluorescence quenching by iodide, acrylamide, and cesium. The quenching by iodide and acrylamide was biphasic; the discrete, long lifetime component was quenched with bimolecular collision constants (k_q) of 1 × 10⁹ M^-1 s^-1 and 1.6 × 10⁹ M^-1 s^-1, respectively, while the Gaussian distributed, short lifetime component was quenched with a k_q value of 0.3 × 10⁹ M^-1 s^-1 and 0.04 × 10⁹ M^-1 s^-1, respectively. Comparison with k_q values for N-acetyl-tryptophanamide indicated that the fractional solvent accessibility was about 25% for the long and less than 10% for the short lifetime component. Cesium quenching was monophasic and provided evidence of an excess of positive charge around these tryptophans. Our findings cast doubt on the general application of the simple coiled-coil model to describe coiled-coil interactions in this protein in solution.