| Popis: |
The entropy-driven polymerization of tobacco mosaic virus protein is favored by an increase in ionic strength, μ, and by a decrease in pH. The effect of ionic strength is interpreted in terms of salting-out and electrical work, a function of charge and, therefore, of pH as well as of μ. The extent of polymerization is measured in terms of a characteristic temperature, T ∗ , corresponding to a characteristic value of the equilibrium constant, K c T∗ is measured at an early stage in the polymerization process where the optical density increment from light scatter is 0.01. The theory developed encompassing both salting-out and electrical work terms relates 1 T ∗ to μ approximately according to the equation, 1 T ∗ = C + Bμ − A μ 1 2 , where C is the ratio of entropy to enthalpy, B is proportional to the salting-out constant divided by enthalpy, and A μ 1 2 depends upon the square of the charge and is proportional to the electrical work contribution divided by the enthalpy. Data in which μ varied from 0.025 to 0.150 at three pH values, 5.95, 6.35, and 6.50, were fitted to this equation and the parameters C , B , and A were evaluated. Experiments were also carried out at a constant μ of 0.10 at pH values in increments of 0.1 between 5.9 and 6.8. The theory predicts that, at constant μ, 1 T ∗ , corrected for the electrical work contribution, is a linear function of pH with a negative slope proportional to the number of hydrogen ions bound per protein unit during polymerization, divided by the enthalpy. The data obtained fit two straight lines with different slopes above and below pH 6.3. Independent experiments carried out by the method of Stevens and Loga show that the number of hydrogen ions bound per protein unit also differs above and below pH 6.3 and the ratio of these is the same as the ratio of the above mentioned slopes. The data, therefore, make it possible to evaluate the enthalpy to be 24.8 kcal/mol of associating A protein and, with this value, the parameters C , B , and A can be interpreted. Standard entropies range from 86 e.u. at pH 6.5 to 88.5 at pH 5.95 and the salting-out constant, K S ′ , is 2.2 at all pH values studied. At μ = 0.10, the values of the electrical work contribution at pH 5.95, 6.35, and 6.50 are +0.298, +0.455, and +0.534 kcal/mol, respectively. Theoretical calculations from models predict values in agreement within a factor of less than two. |
