Protein Diffusion and Stokes-Einstein Deviation in Supercooled Cryoprotectant Solutions

TL;DR

This study investigates protein diffusion in supercooled cryoprotectant solutions, revealing deviations from classical models and linking nanoscale heterogeneity to solvent dynamics during cryopreservation.

Contribution

It provides the first detailed measurement of protein diffusion at supercooled temperatures and introduces a fluctuating-friction model to explain Stokes-Einstein deviations.

Findings

Ferritin diffusion exceeds Stokes-Einstein predictions below 230 K.

Nanoscale friction heterogeneity increases with cooling, reaching 80% of mean friction.

Arrest temperature for ferritin is significantly lower than for larger nanoparticles.

Abstract

Vitrification during cryopreservation requires a detailed understanding of the dynamic behavior of biological solutions. We investigate ferritin diffusion in glycerol-water mixtures at supercooled temperatures using X-ray Photon Correlation Spectroscopy (XPCS). Diffusion coefficients were measured from ambient conditions to $T = 210$ K and analyzed using the Vogel-Fulcher-Tammann (VFT) relation, yielding an arrest temperature of $T_0 = 85 \pm 11$ K for ferritin ($R_{\rm h} = 7.3$ nm), markedly lower than $T_0 = 122 \pm 4$ K for larger nanoparticles ($R_{\rm h} = 50$ nm). Below $T \approx 230$ K, ferritin diffusion exceeds the Stokes-Einstein prediction by up to a factor of 2.7, revealing nanoscale deviations from bulk viscosity. A fluctuating-friction model quantitatively links this enhancement to local friction heterogeneity, with fluctuations increasing upon cooling and reaching $\sim 80\%$ of the mean friction at $T=210$ K. These results establish a molecular-scale connection between protein diffusion and solvent dynamical heterogeneity in cryoprotected solutions.