| Abstract: | Electric birefringence measurements of suspensions of T3 and T7 bacteriophages in 10?2 M phosphate buffer, pH 6.9, show that there is a difference in their rotational diffusion coefficient. The values corrected to 25°C and water viscosity are D25,w = 4630 ± 130 sec?1 and D25,w = 5290 ± 260 sec?1 for T3 and T7, respectively. The value obtained from shell model calculations (according to Filson and Bloomfield) is D25,w = 4500 ± 600 sec?1. The apparent permanent dipole moments are 4.5 × 10?26 C·m and 1.7 × 10?26 C·m for T3 and T7, respectively. For both phage particles the intrinsic optical anisotropy is +7.2 × 10?3. It is shown that this anisotropy is mainly due to the DNA molecule inside the head of the phage. Its positive value means that there exists an excess orientation of the DNA helix perpendicular to the symmetry axis of the particle. For T7 an unexpectedly large increase of Δns and Ksp occurs at a glycerol concentration of about 30% (v/v). This increase is interpreted as being caused by a change of the shape of the particle and/or a change in the secondary structure of the DNA inside the head of the bacteriophage. |
