Angular velocity of F1 with various sizes of rotary probes. (a) Time course of the stepping rotation of wild-type F1 in the presence of 1 mM ATP. We attached various sizes of rotary probes (ϕ = 40, 60, 80, 200, and 500 nm) to the γ portion of F1. Thin lines show 17 successive steps, with their average (heavy orange line). The heavy cyan line shows the linear fitting of the average step between 40° and 80°. The trajectories are aligned at 60°. (b) Average angular rate estimated from the slope of the fitted straight line (red circles). Each data point was determined from three to five molecules. Blue and light blue solid areas represent the viscous coefficients of doublet and singlet beads, respectively, calculated using fluid dynamics, as previously reported (15). To see this figure in color, go online.