Scientia Sinica

104 SCIENTIA. SINICA Vol, V

theory leads to higher values for molecular asymmetry. Unpublished results of Tsao seem to show that in concentrated urea solutions the osmotic pressure curves of proteins are usually very steep, and the straightforward application of Zimm and Onsager’s theory is obviously unwarranted. It is clear that other factors, possibly protein-urea or protein—ion interactions, may be involved. Viscosity measurements, however, give more consistent results.

The axial ratio of duck gizzard tropomyosin seems co be independent of pH, whereas prawn tropomyosin is 70% more asymmetric in dilute acid than in relatively concentrated neutral salt solutions. It may be an indication that this protein is, to a small extent, reversibly flexible. Electrostatic repulsion at pH 2.1 tends to keep the molecules in a more stretched condition.

Just as in the case of the molecular weight, the molecular shape of tropomyosin shows considerable species or functional difference.

4. Electrophoretic mobility of tropomyosin

In order to compare the distribution of charges in the different tropomyosin molecules, we have undertaken to measure the electrophoretic mobilities of the tropomyosins in a phosphate buffer of ionic strength 0.1, pH 65 at 05°C. The results are given in Table 4.

Table 4. Electrophoretic Mobilities of Tropomyosins.

Electrophoretic Mobility = Muscle Type Animal or Tissue 10-* cm? /V/ sec Ascending ‘Descending Striated beef —10 — 8 rabbit —11 —10 Tat —11 a} carp — 9 —8 shell-carp —13 -—9 crab —n9: —9 prawn —10 — 8 Smooth bovine uterus =11 —9 duck gizzard —12 —11 bivalve foot —14 —11 Mixed striated bivalve adductor and smooth muscle —11 = Cardiac pig heart —12 = 8

It is seen from Table 4 that the different tropomyosins move in the electric field at about the same speed, the ascending and descending mobilities