Anomalous diffusion and FRAP dynamics in the random comb model

TL;DR

This paper investigates anomalous diffusion in a comb model with teeth lengths following a power-law distribution, incorporating binding effects, and evaluates the applicability of scaled Brownian motion models to FRAP recovery curves.

Contribution

It introduces a mean-field CTRW approach for variable-length teeth in the comb model, derives analytical diffusion coefficients, and assesses the fit of scaled Brownian motion to FRAP data in complex geometries.

Findings

Analytical diffusion coefficient derived and confirmed numerically.

Scaling law established for retardation effects on diffusion.

Scaled Brownian motion models approximate FRAP curves with small errors.

Abstract

We address the problem of diffusion on a comb whose teeth display a varying length. Specifically, the length $\ell$ of each tooth is drawn from a probability distribution displaying the large-$\ell$ behavior $P(\ell) \sim \ell^{-(1+\alpha)}$ ($\alpha>0$). Our method is based on the mean-field description provided by the well-tested CTRW approach for the random comb model, and the obtained analytical result for the diffusion coefficient is confirmed by numerical simulations. We subsequently incorporate retardation effects arising from binding/unbinding kinetics into our model and obtain a scaling law characterizing the corresponding change in the diffusion coefficient. Finally, our results for the diffusion coefficient are used as an input to compute concentration recovery curves mimicking FRAP experiments in comb-like geometries such as spiny dendrites. We show that such curves cannot be fitted perfectly by a model based on scaled Brownian motion, i.e., a standard diffusion equation with a time-dependent diffusion coefficient. However, differences between the exact curves and such fits are small, thereby providing justification for the practical use of models relying on scaled Brownian motion as a fitting procedure for recovery curves arising from particle diffusion in comb-like systems.