Chemistry and Biology of Proteins - F. Haurowitz 1953

Size and Shape of Protein Molecules
Streaming Birefringence

If a solution of a macromolecular substance, such as a protein, is placed between two crossed Nicol prisms, the field of view remains dark. If, however, such a solution is set in motion, the field becomes illuminated in protein solutions containing rod-shaped or filamentous molecules, whereas it remains dark in solutions of Globular Proteins. The appearance of light in the first case against a dark field with crossed Nicol prisms is caused by a phenomenon similar to the birefringence of crystals or other anisotropic structures. It is believed that rod-shaped protein molecules become oriented in a single direction within a flowing solution (Fig. 6). Quantitative measurements of Flow Birefringence are carried out using an apparatus in which the protein solution is placed between two concentric cylinders. Since one cylinder remains stationary while the second rotates, the rod-shaped protein molecules tend to align tangentially to the direction of flow. The angle between the long axis of the rods and the direction of flow decreases, on the one hand, with an increase in the rotation speed of the cylinder and, on the other hand, with an increase in the degree of molecular Asymmetry. Consequently, the axial ratio of a rod-shaped molecule can be calculated from the orientation angle [35, 36]. Under the conditions of a moving system, a dark cross—the isoclinic cross—is observed in the field of view, similar to the cross seen when examining starch grains under a polarizing Microscope. The average orientation angle of the rods is equal to 90° minus the isoclinic angle. Using this device, it has been found that Myosin molecules from Muscle [36], fibrinogen from Blood Plasma [37], and tobacco mosaic virus [38] are elongated into filamentous structures, $\gamma$-globulin molecules are less asymmetric [37], and $\beta_1$-globulin possesses a highly symmetric molecule (see Table 11).

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Fig. 6. Flow birefringence.

A — particles at rest; B — particles oriented by flow.