Macrocyclic Polyethers and Their Complexes

Abstract

The most important, and almost unique, property of the macrocyclic polyethers ("crown compounds") is their tendency to form complexes with alkali metal salts and salts with similar cations. Such complexes are held together by electrostatic attraction between the cation and the negatice end of the C -0 dipoles. The stability of the polyether complexes depends primarily upon how well the cation fits into the polyether ring; other factors are the charge density ofthe cation andin solution-the solvating power of the medium. Cyclic polyethers have been successfully employed, inter alia, in experiments with ionic compounds in organic solvents and in studies of ion transport in biological systems.

Two mechanisms of ion selectivity in protein binding sites

“…For example, the determination of the three-dimensional structure of the KcsA K þ channel at atomic resolution using X-ray crystallography reveals a series of K þ binding sites along the narrow pore, that seem perfectly adapted to provide an optimal coordination for K þ but not for Na þ (3). This view, which relies on a precise control of the protein structure at the sub-ångström level, is in close correspondence with the classical concepts invoked in "host-guest" chemistry (13,14). In the field of permeation and ion channels, this structural explanation of selectivity has been traditionally called the "snug-fit" mechanism (15).…”

“…For Na þ and K þ , the difference in ion-ligand interaction energy, hU il Na i ðNaÞ − hU il K i ðKÞ , is always a large negative number (favorable), whereas the corresponding difference in ligandligand interaction, hU ll i ðNaÞ − hU ll i ðKÞ , is more typically a positive number (unfavorable) (32). Thermodynamically, the mean ionligand interaction does not give rise to selectivity in the confined microdroplet limit, in contrast with the host-guest mechanism with fixed cavity radius as described by [13]. The lack of selectivity from the mean ion-ligand interaction has been documented very clearly by Asthagiri et al (24).…”

“…The first corresponds to the situation where the geometric structural forces associated with ΔW g are very strong. As a consequence, the configuration of the binding site is very stiff, its geometry is strictly enforced, and the average ion-ligand interaction energy dominates the relative free-energy difference, lim λ g →∞ ΔG site ij ≈ hU il i i ðiÞ − hU il j i ðjÞ ; [13] where the brackets with subscripts represent thermal averages for ion i and j. This expression is consistent with the concept of a fixed cavity size as in host-guest chemistry (12), traditionally called the snug-fit mechanism of selectivity in the ion channel literature (15).…”

Mechanisms of catalyst action in the TiO2-mediated photocatalytic degradation and cis–trans isomerization of maleic and fumaric acid

“…Using the method of Valentine [51], 0.15 M solutions of KO 2 were prepared in the presence of 0.30 M 18-crown-6 [52]. Maintenance of the characteristic pale yellow color of these solutions was taken as evidence that the superoxide remained.…”

Reactivity of polyacrylamide-supported polyhalide anions: effect of macromolecular structure and cross-linking

“…These play a definite role in controlling the energetic interactions between the cross-linked resin and the substrate in solution [11]. The chemical reactivity of an immobilized specific active functional group is governed by its distribution and accessibility on the polymer backbone [12,13]. Polymeric reagents have the potential advantage of operational simplicity over the classical solution chemistry and result in a tremendous saving of time.…”