Electric birefringence studies of rabbit skeletal tropomyosin.

Abstract

AbstractThe electric birefringence of rabbit skeletal tropomyosin and its nonpolymerizable derivative was studied as a function of protein concentration, pulse length, and electric field strength. Analyses of the zero field birefringence decay curves show that nonpolymerizable tropomyosin, which has had on average six C‐terminal residues removed, and tropomyosin are both well approximated by rigid cylinders in solution at low salt concentrations at 20°C. The measured relaxation times for the monomers of polymerizable and nonpolymerizable tropomyosin are 1.5 ± 0.4 μs and 1.30 ± 0.2 μs, respectively, in good agreement with the values calculated from the known dimensions. For tropomyosin the electrical pulse induces the formation of linear dimers. Orientation occurs primarily by a permanent dipole mechanism. Permanent and induced dipole moments were calculated from reversing field experiments and from the saturation of the birefringence. Removal of the six C‐terminal residues decreases the measured permanent dipole moment by 9.5%, from 6300 to 5700 Debyes, which is in good agreement with the 7% decrease calculated for permanent dipole contributions arising from the peptide dipoles and from the asymmetric distribution of the formal charges. This change is due primarily to the removal of Asp 280.