Probes in gels and biological systems

Schmidt, et al. conclude that alternative physical systems, such as fd-virus solutions, are far superior to F-actin solutions as models for testing theories of polymer dynamics. 

used holographic relaxation spectroscopy to measure Dp of a dye and a labeled protein through polyacrylamide gels over distances orders of magnitude larger than any structure in the gel(105). On comparison with literature data on other small diffusants in the same medium. Park, et al. found 

Newman, et al. used QELSS to moiutor the diffusion of polystyrene latex spheres through F-actin solutions(106). Four probe species with radii of 55, 110, 270, and 470 nm were employed. Actins were polymerized with MgCh, KCl, or CaCl2 however, all samples studied were liquids, not gels. Prior to polymerization. Dp in the actin solutions was not sigiuficantly different from Dp of the same probe in pure water. On addition of salt to start the polymerization process. Dp of the 

Luby-Phelps and collaborators used FRAP to study the diffusion of fractionated ficolls in cellular cytoplasm and in concentrated protein solutions(108). Ficoll fractions had hydrodynamic diameters 60-250 A. The Dp/Dpo of ficolls in protein solutions was independent of the size of the ficoll molecules. That is, protein solutions are not size filters they retard to approximately the same fractional extent the diffusion of small and large probe particles. In contrast, cell cytoplasm is a size filter. The Dp/Dpo of ficolls in cytoplasm falls sixfold as the ficoll diameter is increased from 60 to 500 A. For the largest ficoll, FRAP shows that a third of the ficoll particles are trapped only 2/3 of the particles can diffuse through substantial distances. Observations that cellular interiors are size filters were also made by Arrio-Dupont et a/.(109,110). 

(lll) compared their results with findings of Luby-Phelps, et a/.(108) on probes in vivo. In living cells, Dp/Dpo depends strongly on probe radius Rh 

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