| Abstract: | Equilibria and rates were observed over the ranges 1-1600 atm, 3-10 mM Mg2+, at 60 mM NH4Cl, pH 7.5, 20 degrees C, by light scattering. The main reaction is accurately represented at all conditions by the following phenomenological equations. 30 S + 50 S = 70 S, KA70 = ka/kd = 70 S]/30 S]50 S] The equilibrium constants obey simple rules: the volume of association, delta VA0, has the constant value 242 +/- 9 ml/mol, independent of pressure, at all Mg2+ concentrations; the derived values of log KA70 at 1 atm increase linearly with log Mg2+] at a slope of 7.5. In contrast, the rate constants show a clear break at 6 mM Mg2+: below 6 mM, log ka decreases with pressure with a delta Va of 105 +/- 9 ml/mol and increases with log Mg2+] at a slope of 4.9; above 6 mM, these values are halved; a split can actually be seen at 6 mM Mg2+, near 500 atm. The usual two-step mechanism for second order reactions in solution, which would insert a 70 S' species, either an encounter complex or a true low concentration steady state intermediate, into the above equation can accommodate these results: as Mg2+] increases, the rate of transformation of 70 S' into 70 S finally predominates over the rate of dissociation of 70 S' into subunits. The bulk of the pressure effects and all of the Mg2+] dependence arise from the progressive increase in delta GA0 (electrostatic) that occurs when 30, 50, and 70 S particles all lose equivalent fractions of their internal Mg2+ in response to increases in pressure or decreases in Mg2+]. |
