Thermodynamics of the slow–fast transition in bacteriophage T2L

We have measured the thermodynamic parameters of the slow‐fast tail‐fiber reorientation transition on T2L bateriophage. Proportions of the virus in each form were determined from peak‐height measurements in sedimention‐velocity runs and from average diffusion coefficients obtained by quasielastic laser light scattering. Computer simulation of sedimentation confirmed that there were no undetected intermediates in the transition, which was analyzed as a two‐state process. Van't Hoff‐type plots of the apparent equilibrium constant and of the pH midpoint of the transition as function of reciprocal temperature led to the following estimates of the thermodynamic parameters for the transition at pH 6.0 and 20°C: Δ H ° = −139 ± 18Kcal mol −1 , ΔS° = −247 ± 46 cal K −1 mol −1 , and Δ G ° = −66 ± 22 kcal mol −1 . Per mole of protons taken up in the transition, the analogous quantities were −15.9 ± 1.7 kcal mol −1 , −26.3 ± 2.2 cal K −1 mol −1 , and −8.22 ± 1.8 kcal mol −1 . The net number of protons taken up was about 8.5 ± 1.5. The large values of the thermodynamic functions are consistent with a highly cooperative reaction and with multiple interactions between the fibres and the remainder of the phage. The negative entropy of the transition is probably due to immobilization of the fibres.