Cyclic Polyether-Protonated Organic Amine Binding: Significance in Enzymatic and Ion Transport Processes

The cyclic polyether, 18-crown-6, reacts with protonated amines in methanol to form complexes whose formation constants (log K) decrease in the order NH$_{4}{}^{+}$, RNH$_{3}{}^{+}$ > R$_{2}$NH$_{2}{}^{+}$ > R$_{3}$NH$^{+}$. In the case of the organic amines, this stability order is identical to the earlier observed permeability sequence for protonated organic amines in glyceryl dioleate bilayers treated with valinomycin, nonactin, or gramicidin, and in bullfrog and rabbit gallbladder membranes. The decrease in log K values in the above series is primarily a result of decreased enthalpy change (ΔH) values, the entropy change (TΔ S) term being essentially constant for the systems studied.