The size, shape and aggregation of tropomyosin particles

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Abstract
Rabbit tropomyosin was studied by osmotic pressure and viscometric methods under varying conditions. In salt solns. (pH 6.5) the avg. particle wt. falls as the ionic strength of the solute increases, and approaches the value found in a depolymerizing solvent such as 6.7 [image] urea. The particle wt. in acid (pH 2) is the same as in urea, i.e.,53,000; in alkali (pH 12) it is somewhat higher (61,000). Tropomyosin recovered from salt solns., acid or alkali behaves in every way like the original material and can be crystallized. This is true even when the protein is stored at pH 2 or 12 for several weeks at 0[degree]. End-group assay shows that no peptide bonds are split. Tropomyosin recovered from urea no longer crystallizes, and after removal of urea will aggregate into fibrils of indefinite length. At extremely low shear rates, the vicosity of tropomyosin is anomalous. This is due, not to orientation, but to structural factors, which are evident also in strong protein-free solns. of urea and of guanidine HC1. The intrinsic viscosity of tropomyosin was obtained from values of relative viscosity determined at moderate shear rates under conditions of Newtonian flow. The asymmetry of the particles in various solvents was calculated from viscosity data by means of the Simha equation. In its monomeric form, tropomyosin has a particle weight of 53,000 and consists of a cyclic peptide chain. The calculated asymmetry for fully extended loop in the alpha-configuration agrees with that obtained from viscosity data. The approx. dimensions of the monomer are as follows: length 385 A, mean width 14.5 A., axial ratio 25. Whether the double chain is held together through the side chains or the backbone linkage is unknown.