A new fluorine-containing reagent has been synthesized and used to specifically label the reactive sulfhydryl [sulfhydryl-1 (SH1)] of myosin subfragment 1 (S-1). The labeled S-1 (S-1-CF3) demonstrates activated calcium and magnesium adenosinetriphosphatase (ATPase) activities relative to S-1 and a lower potassium ethylenediaminetetraacetate (EDTA) ATPase activity. Maximal effect is obtained with the modification of one thiol per S-1. The 19F NMR spectrum of S-1 CF3 contains only one resonance with a line width of 110 Hz, which implies a rotational correlation time of 2.3 X 10(-7) s. The chemical shift of this resonance is sensitive to temperature, PH, ionic strength, and nucleotides bound in the active site. The temperature dependence of the chemical shift clearly indicates two limiting states for the S-1-CF3 with a highly temperature-dependent equilibrium between 5 and 40 degrees C. The low-temperature state appears to be identical with the state resulting from the binding of Mg.ADP or Mg.AMPPNP at 25 degree C. The energetics of the conformational change have been studied under various conditions. At pH 7 in 25 mM cacodylate, 0.1 M KCl, and 1 mM EDTA, delta H degree = 30 kcal/mol and delta S degree = 105 cal deg-1 mol-1. A decrease in pH to 6.5 results in an increased population of the low-temperature state with delta H degree = 31 kcal/mol and delta S degree = 107 cal deg-1 mol-1. Similarly, the low-temperature state is favored by low ionic strength. In 5.8 mM piperazine-N,N'bis(2-ethanesulfonic acid) and 1 mM EDTA (pH 7), delta H degree = 8 kcal/mol and delta S degree = 27 cal deg-1 mol-1. We have also obtained 19F NMR spectra of S-1-CF3 in D2O solution with 30% ethylene glycol at pH 7.1. Increasing concentrations of ethylene glycol progressively stabilize the high-temperature states.