Electric birefringence of actin

The polymerization of actin was followed by the measurement of electric birefringence. Small elementary polymers of actin which appeared at very low salt concentrations or at the initial stage of polymerization showed a positive electric birefringence. They were oriented with the long axis parallel to the electric field. This was mainly due to a permanent dipole moment in the direction of the long axis, the magnitude of which was estimated from the Kerr constant to be about 75 Debye units per actin monomer. The elementary polymers grew into normal F-actin which showed a large negative electric birefringence. That is, F-actin filaments were oriented in the direction perpendicular to the field. Therefore, the electric structure of F-actin filaments was completely different from that of elementary small polymers. The negative electric birefringence of F-actin was cancelled by the addition of H-meromyosin.