Page 61 - ILL Annual Report 2019

SCIENTIFIC HIGHLIGHTS
58-59
Figure 2
Simulated difference (agreement for R ≈ Reff) in particle diffusion between polydisperse and monodisperse crowding.
Main figure: normalised translational diffusion coefficient D/Dmono of spheres with radius R versus the rescaled radius R/Reff. Dmono is the diffusion coefficient in a monodisperse solution of spheres with radius
R at similar volume fraction φ. The effective radius Reff = Ri31/3 characterises the crowding in a mixture of spheres with radii Ri. The filled symbols depict the diffusion coefficients of the tracers at the ratios ytr = φtr/φ, with the tracer volume fraction φtr. In particular, the filled symbols in the grey rectangle refer to the experimental radius of Ig. In this case, the deviations of the diffusion in the polydisperse environment from that in a monodisperse system are less than 5 %. The empty symbols denote the diffusion of the crowders, plotted for Rtr = RIg.
Inset: reduced tracer diffusion Dtr/D0 with the dilute limit coefficient D0, for an Ig-sized tracer (R = RIg = 5.5 nm) versus the reduced diffusion coefficient Dmono/D0 in the monodisperse suspension.
The shaded areas (main figure and inset) depict a ±5 % deviation of Dtr from Dmono.
For the experiments, fully deuterated cellular lysate from Escherichia Coli cells was produced and mixed with natural, i.e. protonated polyclonal Immunoglobulin
(Ig), tracer proteins and deuterated water (D2O). The difference in the neutron scattering cross sections from the deuterated and protonated components of the samples allowed us to focus on the scattering signal from the Ig. Surprisingly, we found that the diffusion of Ig in both lysate and pure Ig solutions depends on the total
volume fraction of Ig and lysate only, i.e. no effect of polydispersity was observed in the experiments.
Subsequently, the experimental results were analysed using theoretical concepts from colloid physics, in particular Stokesian dynamics computer simulations (figure 2).
In this way, the protein diffusion was accessed on nanosecond time scales where hydrodynamic interactions dominate over negligible protein collisions. Combined with these coarse-grained simulations, the experimental results
for the complex, flexible molecules can be understood consistently using colloid theory. The simulations show that tracer proteins in polydisperse solutions close to an effective particle radius diffuse approximately, as in a monodisperse suspension. Furthermore, macromolecules of sizes that are different from the effective radius are slowed differently even on nanosecond time scales. This result is highly relevant for a quantitative understanding of cellular processes.
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