Influence of cation size and charge on the extrusion of a salt-dependent cruciform

We have made a comparative study of the kinetics of cruciform extrusion by a salt-dependent (S-type) cruciform in the presence of a variety of metal ions and related species. We find that the nature of the cation present has a marked effect on the observed kinetics, and that different cations differ greatly in the efficiency with which they promote the extrusion process. We can divide the ions into four classes according to the optimal ionic concentration required for maximal extrusion rate. Group Ia cations and tetramethyl ammonium are most effective in promoting extrusion at 50 to 60 mM. Group IIa and selected transition metal ions (notably manganese) are effective over a wide range, extending down to 200 microM. Hexaminecobalt(III) and the polyamines promote extrusion at concentrations as low as 15 to 40 microM. Most remaining ions examined, including trivalent ions such as Al(III) and many transition metal ions are totally ineffective. Within the first two groups, we observe a marked correlation between the rate of cruciform extrusion promoted and ionic radius, the larger ions giving faster extrusion rates. We interpret this to indicate specific ion binding occurring in the transition state of the extrusion process. We may rationalize all the data in terms of a model for salt-dependent cruciform extrusion, in which the transition state resembles a partially extruded protocruciform. This creates an anionic "cavity" with selective ion binding properties, and its stabilization is therefore ion size-dependent.