Solution conformations of valinomycin-divalent cation complexes.

The solution conformations of complexes of valinomycin with magnesium and strontium were investigated by circular dichroism, nuclear magnetic resonance and infrared techniques. The results were compared with our earlier results on lithium, calcium, manganese and barium complexes. All these cations, except lithium, form 2:1 ion sandwich and 1:1 carrier-cation complexes with valinomycin. The 1:1 complex has a conformation different from that of the valinomycin-potassium complex. Lithium forms only the 1:1 complex. Strontium and barium form a large number of 1:2 complexes with open conformations rapidly interconverting in solution in addition to the above 2:1 and 1:1 complexes. These observations are rationalized taking into account the ionic radii and coordination numbers of the cations and the conformational restraints of valinomycin molecules. It is suggested that cations with co-ordination numbers of about six (magnesium and calcium) form the 2:1 and 1:1 complexes whereas those with higher co-ordination numbers (strontium and barium) form 1:2 complexes also.