The temperature dependence of the kinetics of the binding of ATP to myosin subfragment−1 was studied by an ATP chase technique in a rapid−flow—quench apparatus: A temperature range of 30°C to −15°C was obtained with ethylene glycol as antifreeze. The Arrhenius plot of k 2 is discontinuous with a jump at 12°C. Above the jump ΔH ≠ = 9.5 kcal/mol, below ΔH ≠ = 28.5 kcal/mol. Few such Arrhenius plots are recorded in the literature but they are predicted from theory. Thus, we explain our results as a phase change of the subfragment 1−ATP system at 12°C. This is in agreement with certain structural studies.
