TRYPTOPHAN FLUORESCENCE QUENCHING IN RABBIT SKELETAL MYOSIN ROD

The solvent accessibility of the four tryptophans of rabbit skeletal m uscle myosin rod was investigated using steady-state and time-resolved fluorescence quenching by iodide, acrylamide, and cesium. The quenchi ng by iodide and acrylamide was biphasic; the discrete, long lifetime component was quenched with bimolecular collision constants (k(q)) of 1 X 10(9) M(-1) s(-1) and 1.6 x 10(9) M(-1) s(-1), respectively, while the Gaussian distributed, short lifetime component was quenched with a k(q) value of 0.3 x 10(9) M(-1) s(-1) and 0.04 x 10(9) 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% fbr the short lifetime component. Cesium q uenching was monophasic and provided evidence of an excess of positive charge around these tryptophans. Our findings cast doubt on the gener al application of the simple coiled-coil model to describe coiled-coil interactions in this protein in solution.