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. 2011 Jul 5;108(31):E365-E373. doi: 10.1073/pnas.1102255108

Fig. 2.

Fig. 2.

Normal free diffusion in the cytosol of E. coli. (A) A single experimentally obtained single-molecule mEos2 trajectory with a frame time of 4 ms and an exposure time of 1 ms. (B) Overlay of 1,355 single-molecule mEos2 trajectories obtained in an individual E. coli cell with a frame time of 4 ms and an exposure time of 1 ms. (C) Local apparent diffusion coefficients in the sample (x-y) plane. The apparent diffusion coefficients are evaluated every 20 by 20 nm in an x-y grid. Each point in the figure is false-colored according to the apparent diffusion coefficient calculated from the mean square displacement over 4 ms for experimental (Upper) and simulated (Lower) displacements originating within 200 nm of this point. The simulations assume normal diffusion at D = 12.5 μm2 s-1 in the volume defined by the geometry of this cell. There is good agreement between apparent experimental diffusion coefficients and those obtained from simulations. The apparent diffusion coefficients are higher in the middle of the cell, as the molecules are less confined along the long axis. Noise contributions make the apparent diffusion faster close to the cell wall. (D) Mean square displacements (MSDs) in the sample (x-y) plane for different time intervals. Experimental MSDs and error bars representing experimental standard errors of the means are displayed in red. The confidence intervals (blue) are obtained from simulations in the volumes defined by the cell geometry by calculating and sorting MSDs for trajectories using a diffusion coefficient of 13 μm2 s-1. The average MSDs (black, dashed) are also obtained from simulations. Here we vary the diffusion coefficient for each cell to obtain the closest match to the experimental curve. (E) Overlay of 500 positions of single-molecule mEos2 trajectories in one E. coli cell with a frame time of 4 ms and an exposure time of 1 ms. Each position is represented by a Gaussian with a standard deviation equal to the localization error. The mean localization error is 44 nm and the scale bar represents 500 nm.