- Thermodynamics of Reaction in Heterogeneous Systems (Water-Organic Phases) between the Ionophore Monensin and Alkali-Metal Cations
-
Gibbs functions, enthalpies, entropies, and volumes of reaction for the reaction of bacterial ionophore monensin with alkali-metal cations in various water-organic biphasic solvent systems are obtained by using various experimental methods.In addition, weak but perceptible solubilization of the complexes formed is shown to occur in the aqueous phase.Their dissociation in water is studied.All the data are discussed in terms of structure and solvation of the variuos species.The selectivity sequence is not solvent dependent though the enthalpic and entropic contributions for different cations vary with the solvent system involved.On the basis of these data, both biological functional aspects and analytical aspects relevant to the testing of ionophores are discussed.
- Hebrant, Marc,Pointud, Yvon,Juillard, Jean
-
-
Read Online
- Rates and Equilibria of Alkali Metal and Silver Ion Complex Formation with Monensin in Ethanol
-
Measurements are reported on the stability constants and the rates of formation and dissociation of alkali metal and silver complexes of monensin in ethanol.Among the alkali metal complexes the order of stability is Li++>K+>Rb+>Cs+ (with Cs+ ca.Li+).Compared with the neutral antibiotic ionophores, the stability constants of monensin complexes are in general higher and show a much sharper peak selectivity.Dissociation rate constants are very sensitive to cation size and reflect a similar (inverse) variation with cation size of the stability constants; the formation rate constants increase monotonically with increasing cation size from Li+ (9.0E7 M-1s-1) to Cs+(2.5E9 M-1S-1.The silver complex, AgMon, has a stability constant slightly higher than that of the most stable alkali metal complex, NaMon, and a remarkably high formation rate constant, kf=3.5E10 M-1s-1, approaching that of a diffusion-controlled reaction, despite the high solvation energy of Ag+.The kinetic and thermodynamic properties of monensin complexes are compared with those of other naturally occuring and synthetic macrocyclic ionophores.
- Cox, B. G.,Truong, Ng van,Rzeszotarska, J.,Schneider, H.
-
-
Read Online
- BIOSYNTHESIS OF THE POLYETHER ANTIBIOTIC MONENSIN-A. RESULTS FROM THE INCORPORATIONS OF LABELLED ACETATE AND PROPIONATE AS A PROBE OF THE CARBON CHAIN ASSEMBLY PROCESSES
-
The incorporation of sodium - and (S)--propionate into the polyether antibiotic monensin-A in cultures of Streptomyces cinnamonensis occurs with retention of label only at C-4 and C-6, whereas during the incorporation of sodium (R)-propionate the deuterium label is lost to the medium.These results are consistent with the formation of (S)-methylmalonyl-CoA from the labelled propionate by carboxylation of propionyl-CoA with loss of the 2-pro-R hydrogen.The (S)-methylmalonyl-CoA is subsequently incorporated into the antibiotic by a decarboxylative condensation occuring with overall inversion.The incorporations of sodium -and -acetates into monensin-A provide evidence for a pathway of metabolism leading to methylmalonyl-CoA that does not proceed via succinyl-CoA.Instead, acetyl-CoA may be processed via butyryl-CoA and isobutyryl-CoA, to afford (S)-methylmalonyl-CoA.
- Sood, Gulshan R.,Ashworth, Doreen M.,Ajaz, Abid A.,Robinson, John A.
-
p. 3183 - 3194
(2007/10/02)
-
- BUTYRATE METABOLISM IN STREPTOMYCETES. CHARACTERIZATION OF AN INTRAMOLECULAR VICINAL INTERCHANGE REARRANGEMENT LINKING ISOBUTYRATE AND BUTYRATE IN STREPTOMYCES CINNAMONENSIS
-
The incorporations of varicus carbon-13 and deuterium labelled forms of isobutyrate into the polyether antibiotic monensin-A have provided evidence for the existence of a novel rearrangement in whole cells of Streptomyces cinnamonensis, which leads to the conversion of isobutyrate into butyrate.This rearrangement is shown to proceed in an intramolecular fashion by migration of the carboxy carbon of isobutyrate to the 2-pro-S methyl, with a concomitant back migration of a hydrogen atom from this methyl group predominantly into the 3-pro-R position in butyrate.Formally, therefore, the carboxy carbon is replaced with overall retention of configuration, in a vicinal interchange rearrangement.The significance of these observations with regard to the coenzyme requirements of the rearrangement, and its relationship to polyether and macrolide antibiotic production in Streptomycetes is discussed.
- Reynolds, Kevin A.,O'Hagan, David,Gani, David,Robinson, John A.
-
p. 3195 - 3208
(2007/10/02)
-
- ETUDE THERMODYNAMIQUE DE LA COMPLEXATION DES IONS SODIUM, POTASSIUM ET RUBIDIUM PAR LES IONOPHORES BACTERIENS: MONENSINE, GRISORIXINE ET NIGERICINE
-
Enthalpies in anhydrous methanol for both dissociation of carboxylic group and complexation of alkali metal cations by these natural ionophores (as 1-1 neutral association between the cation and the carboxylate form of the ionophore) are obtained from calorimetric measurements.Using equilibrium constants measured independently, it is possible to reach the corresponding entropies.The results now obtained on the thermodynamic of these two reactions are discussed in terms of "opened" or "closed" structure of the ionophores and of interactions with the solvent medium: - both acid and "empty" anion form can be considered as open in methanol; - nigericin and grisorixin have, as far as G, H, S, are concerned a very similar behaviour.The standard quantities are only shifted from grisorixin to nigericin, owing to the solvation of the supplementary OH group of the second ionophore; - monensin have a very distinct behaviour from the two other ionophores.Such thermodynamic studies are expected to give some informations on the mechanism of action of these bacterial ionophores.
- Pointud, Yvon,Juillard, Jean,Jeminet, Georges,David, Lucien
-
-
