Macrocyclic ionophores

Eunctionalized supramolecular systems based on monolayers of bioactive compounds (macrocyclic ionophoric peptides, crown ethers, nucleoside derivatives) 98MI12. 

Crown ethers [364] have proved to be an excellent choice as ionophores for the fabrication of ion sensors because of their ability to complex selectively a particular ion. The cadmium selective sensors have been fabricated from poly(vinyl chloride) (PVC) matrix membranes containing macrocyclic ionophores benzo-15-crown-5 [365], monoaza-18-crown-6 [366], dibenzo-24-crown-8 [367], dicyclohexano-18-crown-6 [368], 3,4 ll,12-dibenzo-l,6,... 

Mohapatra, P.K. Pathak, P.N. Kelkar, A. Manchanda, V.K. Novel polymer inclusion membrane containing a macrocyclic ionophore for selective removal of strontium from nuclear waste solution, New J. Chem. 28 (2004) 1004-1009. 

Macrocyclic ionophores are molecules whose atoms are organized to form a cavity into which metal ions fit and bind with high affinity. Such compounds are also called polycyclic ethers, crown ethers, cryptands, or cryptahemispherands. Different macrocyclics can be made with cavities tailored to fit the ionic radii of different elements. When chromogenic properties are imparted to these ionophores, spectral shifts... 

Fig. 12. Model representation of the loading of a macrocyclic ionophore (monactin) with Na+. The solvated metal ion approaches the carrier, the conformation of which permits its polar groups to be in close contact with the solvent. The uptake of the metal ion then occurs under stepwise substitution of the solvent molecules by the polar groups of the ionophoric ligand. In its final conformational state the carrier completely encloses the metal ion with its non-polar groups being accumulated at the surface...

Moyer et al. [8] presented a novel synergistic study of the extraction of Cu(II) from sulfuric acid by using an impregnated resin prepared by adsorption of a tetradentate macrocyclic ionophore, tetrathia-l4-crown-4... 

Valinomycinis probably the best known and characterized of the natural ionophoretic, antibiotic compounds. It is a K -selective carrier molecule that is produeed by Streptomyces fulvissimusy It is a dodecadepsipeptide comprised of 12 alternating amino and hydroxy aeids to form a neutral, macrocyclic ionophore (Fig. 1). 

Because of the greater flexibility imparted them by the lack of head-to-tail covalent linkage, the carboxylic ionophores respond much more strongly to environmental forces such as local polarity than do the neutral macrocyclic ionophores. Upon leaving a membrane interface during the course of a catalytic transport cycle, an ionophore does not experience an abrupt change from a polar aqueous environment to an apolar hydrocarbon-like environment. The polarity boundary is rather diffuse. In order to properly evaluate the factors affecting carboxylic ionophore mediated transport, it is necessary to determine the effects of each of the microenvironments encountered within the membrane on the conformation of the ionophore and the stability of the ionophore-ion inclusion complex. 

The molecular basis of action of the ionophore antibiotics involves their ability to form lipid-soluble complexes with metal ions. Mueller and Rudin [182] suggested that the macrocyclic ionophores form coordination complexes by hydrogen bonding and dipole interactions between the cation and the carboxyl oxygen atoms of the antibiotics which project towards the centre of the ring. The carboxyl oxygen atoms of the antibiotic replace the water molecules in the hydration shell of the cation. Complexes of this type involving K ions... 

Macrocyclic ionophores, e.g. (11), catalyse the butylaminolysis of 4-nitrophenyl acetate in chlorobenzene. There is a large dependence of the rate enhancement... 

The potential of the polar cavity of macrocyclic ionophores such as (78) to stabilize polar transition states by electrostatic interaction has been suggested as an explanation for the catalytic effect of (78) on the aminolysis of p- nitrophenyl acetate by butylamine. 

---