| Abstract
| - Small proteins move in crowded cell compartments by anomalous diffusion. In many of them, e.g., theendoplasmic reticulum, the proteins move between lipid membranes in the aqueous lumen. Molecular crowdingin vitro offers a systematic way to study anomalous and normal diffusion in a well controlled environmentnot accessible in vivo. We prepared a crowded environment in vitro consisting of hexaethylene glycolmonododecyl ether (C12E6) nonionic surfactant and water and observed lysozyme diffusion between elongatedmicelles. We have fitted the data obtained in fluorescence correlation spectroscopy using an anomalous diffusionmodel and a two-component normal diffusion model. For a small concentration of surfactant (below 4 wt %)the data can be fitted by single-component normal diffusion. For larger concentrations the normal diffusionfit gave two components: one very slow and one fast. The amplitude of the slow component grows withC12E6 concentration. The ratio of diffusion coefficients (slow to fast) is on the order of 0.1 for all concentrationsof surfactant in the solution. The fast diffusion is due to free proteins while the slow one is due to the protein−micelle complexes. The protein−micelle interaction is weak since even in a highly concentrated solution(35% of C12E6) the amplitude of the slow mode is only 10%, despite the fact that the average distance betweenthe micelles is the same as the size of the protein. The anomalous diffusion model gave the anomality index(〈r2(t)〉 ∼ tα), α monotonically decreasing from α = 1 (at 4% surfactant) to α = 0.88 (at 37% surfactant).The fits for two-component normal diffusion and anomalous diffusion were of equally good quality, but thephysical interpretation was only straightforward for the former.
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