Electrooptical properties of reduced protein-sodium dodecyl sulfate complexes

Two independent electrooptical properties, the specific Kerr constants and the electric birefringence relaxation times, of the saturated sodium dodecyl sulfate complexes of a series of reduced polypeptides of known molecular weight are reported. Both the Kerr constants and the relaxation times are unique functions of the molecular weight of the polypeptide chain. The specific Kerr constants depend upon the square of the polypeptide molecular weight. The relaxation times of the complexes, which are proportional to the rotational diffusion constants, are dependent on the molecular weight to approximately the first power. The latter finding is inconsistent with the compact prolate ellipsoid model for sodium dodecyl sulfate-protein complexes proposed by Reynolds and Tanford ((1970) J. Biol. Chem. 245, 5161) in which the cross section is constant and the length depends on linearly on molecular weight; for this model the relaxation times would depend on approximately the 2.5 power of the molecular weight in the range of sizes investigated. Combination of the present results with other properties in the literature rules out a number of other models characterized by compactness and near inflexibility. No firm conclusions can be drawn as to the suitability of the free-draining flexible model of Shirahama, Tsujii, and Takagi ((1974) J. Biochem. (Tokyo) 75, 309).