Effect of Aspect Ratio on Protein Diffusion in Hydrogels

To elucidate the effect of the aspect ratio of protein on the diffusion in gels, the diffusion of a rodlike protein, tropomyosin, in two kinds of polysaccharide gels, agarose and carrageenan, has been investigated by electronic speckle pattern interferometry method and compared with that of a globular protein, myoglobin. In an agarose gel that has a comparatively large pore size, the diffusion coefficient D of the tropomyosin (aspect ratio R = 26) shows a behavior similar to that of the globular myoglobin ( R = 1.6), decreasing with the fiber volume fraction φ with a scaling exponent α = −0.3 to −0.4, which is close to the Rouse model of α = −0.5. Whereas in the λ-carrageenan gel that has a smaller pore size, the diffusion of tropomyosin is quite different from that of myoglobin and consists of two regions: a region at a low fiber volume fraction D ≈ φ -0.3, close to the Rouse model and a region at a high fiber volume fraction D ≈ φ -1.8, close to the reptation model of α = −1.75. As far as the authors know, this is the first reptationlike behavior observed for the diffusion of rodlike protein in a hydrogel.