Rapid helix--coil transitions in the S-2 region of myosin.

Temperature-jump studies on the long S-2 fragment (100,000 daltons) isolated from myosin show that this structure can undergo alpha-helix--random coil transitions in a time range approximating the cycle time of a crossbridge. Two relaxation times are observed after temperature jumps of 5 degrees C over the range 35--55 degrees C, one in the submillisecond (tau f) and the other in the millisecond (tau s) time ranges. Both processes exhibit maxima near the midpoint of the helix--coil transition (tm = 45 +/- 2 degrees C) as determined by optical rotation melt experiments. Similar results were observed for the low temperature transition (tm = 45 degrees C) of the myosin rod. Viscosity studies reveal that the S-2 particles has significant flexibility at physiological temperature. Results are considered in terms of the Huxley--Simmons and helix--coil transition models for force generation in muscle.