Characterizing anomalous diffusion in crowded polymer solutions and gels over five decades in time with...

Area of Science:

• Biophysics
• Physical Chemistry
• Materials Science

Background:

• Macromolecule diffusion in cellular environments and complex fluids often deviates from Fickian behavior.
• Molecular crowding is hypothesized to cause anomalous diffusion, characterized by mean-squared displacement scaling as 〈r(2)〉∝t(α) with α < 1.
• Conventional methods like fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP) are limited to narrow lengthscales, hindering comprehensive diffusion analysis.

Purpose of the Study:

• To introduce and validate variable-lengthscale FCS (VLS-FCS) as a method to retrieve mean-squared displacement (MSD) over extended time ranges.
• To assess the Gaussian or non-Gaussian nature of diffusion propagators using VLS-FCS.
• To investigate anomalous diffusion in agarose gels and crowded dextran solutions.

Main Methods:

• Variable-lengthscale FCS (VLS-FCS) was employed, varying the observation volume over several orders of magnitude.
• A numerical inversion procedure was applied to correlation data to extract MSD.
• VLS-FCS data was analyzed to determine the characteristics of diffusion propagators.

Main Results:

• VLS-FCS successfully retrieved MSD over five decades in time, bridging different lengthscales.
• In agarose gels, anomalous diffusion was confirmed at short lengthscales with a crossover to simple diffusion around 1 μm, consistent with caged diffusion.
• Crowded dextran solutions exhibited a linear MSD but non-Gaussian propagators at short lengthscales, suggesting a 'diffusing diffusivity' model.

Conclusions:

• VLS-FCS is a powerful technique for characterizing anomalous diffusion across multiple lengthscales and time regimes.
• The study provides insights into diffusion mechanisms in complex systems like gels and crowded solutions.
• Findings in dextran solutions highlight complex diffusion behaviors relevant to biological systems.