Scientia Sinica

94 SCIENTIA SINICA Vol. V

previously been correlated in the form of a standard curve. 4. Osmotic pressure measurements

These were carried out at 1°C in the toluene osmometers of Adair!"®!, Details were described in a previous paper (Chi and Tsao"). For the calculation of the particle weight, the ratio of pressure P (in mm Hg) and concentration C, P/C or log (P/C)"!, was plotted against C and the limiting value at zero concentration, obtained by the method of least squares, was employed. :

5. Viscosity measurements

These were made in the Tsuda! horizontal capillary viscometer as described in another paper (Tan and Tsao'*!), The axial ratic of each tropomyosin was assessed by the use of Simha’s equation!** *! from the value of intrinsic viscosity [7].

6. Flow birefringence This was observed with the aid of polaroid sheets. 7. Determination of electrophoretic homogeneity and mobility

The purity of protein preparations was examined in the Antweiler microelectrophoresis apparatus. The electrophoretic mobility was measured at 05°C in a L.K.B. Tiselius-Svensson apparatus model 3021 B. The conductivity of protein and buffer solutions was measured with the aid of a Dr. Lange conductometer.

Ill. Resurts

1. Polymerizability of tropomyosin

In neutral, salt-free solutions, tropomyosin sols have an exceedingly high viscosity which decreases in a dramatic way as small amounts of salts are added. Simultaneously flow birefringence disappears. Physico-chemical investigations, such as electron optical observations! and measurements of viscosity, osmotic pressure! and light scattering™), have demonstrated that rabbit tropomyosin molecules aggregate linearly to form fibrils in neutral, salt-free solutions and salt ions bring about depolymerization. We have examined the relative viscosity and the flow birefringence of solutions of duck gizzard, pig heart, prawn and sepia tropomyosins as a function of ionic strength, and have chosen the same protein concentration, 0.85%, in order to facilitate a direct comparison of the polymerizability. The results are shown in Figs. 1 and 2.. The scale of the abcissa in Fig. 2 is four times that of Fig. 1 so as to make the differences in viscosity behaviour easily discernible. In both figures, the curve for rabbit tropomyosin is included for comparison ; the portion <0.1 is taken from Bailey’s paper®" and the portion > 0.1 com-