Wanted: Scalable Tracers for Diffusion Measurements

Abstract

Scalable tracers are potentially a useful tool to examine diffusion mechanisms and to predict diffusion coefficients, particularly for hindered diffusion in complex, heterogeneous, or crowded systems. Scalable tracers are defined as a series of tracers varying in size but with the same shape, structure, surface chemistry, deformability, and diffusion mechanism. Both chemical homology and constant dynamics are required. In particular, branching must not vary with size, and there must be no transition between ordinary diffusion and reptation. Measurements using scalable tracers yield the mean diffusion coefficient as a function of size alone; measurements using nonscalable tracers yield the variation due to differences in the other properties. Candidate scalable tracers are discussed for two-dimensional (2D) diffusion in membranes and three-dimensional diffusion in aqueous solutions. Correlations to predict the mean diffusion coefficient of globular biomolecules from molecular mass are reviewed briefly. Specific suggestions for the 3D case include the use of synthetic dendrimers or random hyperbranched polymers instead of dextran and the use of core–shell quantum dots. Another useful tool would be a series of scalable tracers varying in deformability alone, prepared by varying the density of crosslinking in a polymer to make say “reinforced Ficoll” or “reinforced hyperbranched polyglycerol.”

Probing cytoskeletal modulation of passive and active intracellular dynamics using nanobody-functionalized quantum dots

“…Objects of 300–500 nm in size would be mostly stuck inside the filament network and move together with it. Further development of scalable tracers should allow precise mapping of those boundaries and a thorough description of the non-equilibrium mechanical environment of the cell22. Importantly, apart from the actin meshwork, other intracellular structures, such as intermediate filaments1423 and membrane organelles2124 are also likely to affect transport processes inside cells.…”

Title: Single-molecule diffusion-based estimation of ligand effects on G protein-coupled receptors

“…Currently, it is unknown why G protein binding sometimes accelerates the diffusion of mGluR3 ( Fig. 4G and Movie2), even though simple physical models predict slower diffusion of particles of larger volume (40). We speculate that the major determinant of the diffusion coefficient of mGluR3 is the viscosity of the membrane surrounding the receptor molecule, which is dependent on the lipid composition.…”

Interpretation of fluorescence correlation spectra of biopolymer solutions

“…How might nature generate subdiffusive or non‐Gaussian behavior in real systems? For related discussions of the same topic, note in particular Saxton . Three alternatives are apparent: First, the diffusing particles may be polydisperse or elsewise non‐uniform.…”

“…For related discussions of the same topic, note in particular Saxton. [34][35][36][37] Three alternatives are apparent:…”