In Escherichia coli, the behavior of the flagellar rotary motor near zero load can be studied by scattering light from nanogold spheres attached to proximal hooks of cells lacking flagellar filaments. We used this method to monitor changes in speed when cells were subjected to changes in temperature or shifted from a medium made with H2O to one made with D2O. In H2O, the speed increased with temperature in a near-exponential manner, with an activation enthalpy of 52 ± 4 kJ/mol (12.0 ± 1.0 kcal/mol). In D2O, the speed increased in a similar manner, with an activation enthalpy of 50 ± 4 kJ/mol. The speed in H2O was higher than that in D2O by a factor of 1.53 ± 0.14. We performed comparison studies of variations in temperature and solvent isotope, using motors operating at high loads. The variations were small, consistent with previous observations. The implications of these results for proton translocation are discussed.
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