Complexation of Cs+, K+ and Na+ by norbadione A triggered by the release of a strong hydrogen bond: nature and stability of the complexes.

Norbadione A (NBA) is a pigment present in edible mushrooms which is presumed to selectively complex Cs(+) cations. Due to a very uncommon complexation mechanism, we used a combination of several experimental techniques, including (1)H-NMR, (133)Cs-NMR, isothermal calorimetric, potentiometric titrations and molecular dynamics MD simulations to determine the nature of the complexed species, as well as their stability constants for the NBA-M(+) systems (M(+) = Cs(+), K(+), Na(+)) in methanol:water 80:20 solutions at 25.0 degrees C. We show that almost no complexation occurs below pH 7.5, as long as a proton, involved in a strong hydrogen bond, bridges both carboxylic and enolic groups of each pulvinic moiety of NBA. Thus, neutralization of that proton is necessary to both set free potential coordination sites and to trigger a conformational change, two conditions needed to bind successively a first, then a second metallic cation. The stability constants determined in this study are in good agreement with each other, leading to the stability order Cs(+) > K(+) > Na(+) for both mono- and bimetallic complexes, which is the reversed order to the one generally observed for low molecular weight carboxylic ligands in water. According to MD simulations in solution, complexation involves a mixture of Z/E isomers and conformers of NBA with a broad diversity of binding modes. Some pH and environment dependent aggregation phenomena are considered to also contribute to the binding process, and to possibly explain the accumulation of radionuclides in mushrooms.

Molecular puzzle ring: pseudo[1]rotaxane from a flexible cyclodextrin derivative.

A pseudo[1]rotaxane formed by a flexible cyclodextrin (CD) derivative (1-R) with a bulky end group has been investigated on kinetic quantitation. 1-Rs have the cinnamamide moiety as a guest and a bulky end group (R) as a rate-determining moiety of the threading process. The R groups play an important role for the formation of pseudo[1]rotaxane, and kinetics of the self-inclusion process was found to be controlled by the size and shapes of the R groups. 1-Ad and 1-Me derivatives, which have an adamantyl and methyl end group, respectively, formed self-inclusion complexes by threading of the arm moiety with a conformational conversion of altrose from (1)C(4) form to (4)C(1) form. Flexibility of the altro-alpha-CD cavity resulted in an induced fit (from (1)C(4) to (4)C(1)) to the arm moiety, and introducing a bulky end group allowed the stability of this pseudo[1]rotaxane to be enhanced.

scyllo-inositol pentakisphosphate as an analogue of myo-inositol 1,3,4,5,6-pentakisphosphate: chemical synthesis, physicochemistry and biological applications.

myo-Inositol 1,3,4,5,6-pentakisphosphate (Ins(1,3,4,5,6)P(5)), an inositol polyphosphate of emerging significance in cellular signalling, and its C-2 epimer scyllo-inositol pentakisphosphate (scyllo-InsP(5)) were synthesised from the same myo-inositol-based precursor. Potentiometric and NMR titrations show that both pentakisphosphates undergo a conformational ring-flip at higher pH, beginning at pH 8 for scyllo-InsP(5) and pH 9 for Ins(1,3,4,5,6)P(5). Over the physiological pH range, however, the conformation of the inositol rings and the microprotonation patterns of the phosphate groups in Ins(1,3,4,5,6)P(5) and scyllo-InsP(5) are similar. Thus, scyllo-InsP(5) should be a useful tool for identifying biologically relevant actions of Ins(1,3,4,5,6)P(5), mediated by specific binding sites, and distinguishing them from nonspecific electrostatic effects. We also demonstrate that, although scyllo-InsP(5) and Ins(1,3,4,5,6)P(5) are both hydrolysed by multiple inositol polyphosphate phosphatase (MINPP), scyllo-InsP(5) is not dephosphorylated by PTEN or phosphorylated by Ins(1,3,4,5,6)P(5) 2-kinases. This finding both reinforces the value of scyllo-InsP(5) as a biological control and shows that the axial 2-OH group of Ins(1,3,4,5,6)P(5) plays a part in substrate recognition by PTEN and the Ins(1,3,4,5,6)P(5) 2-kinases.

Straightforward detection of the secondary ionisation of the phosphate group and pK determinations by high-resolution solid-state 31P NMR.

The suitability of high-resolution solid-state 31P NMR for a straightforward determination of the protonation state of phosphate groups as well as of their pK2 values extracted from solid state mono : dianionic ratios has been demonstrated.

Inframolecular protonation process of norbadione A: influence of the ionic environment and stereochemical consequences.

The microscopic protonation mechanism, at an inframolecular level, of norbadione A, a pigment extracted from mushrooms and known to complex cesium cations, was determined by using 1H NMR titrations and the cluster expansion method. This study revealed a pH dependent Z to E isomer switch that occurs in both pulvinic moieties. As a consequence, norbadione A can exist in solution in four stereomeric forms (E-E, E-Z, Z-E, and Z-Z), which can be of interest in the development of molecular-level devices. In the presence of 0.15 M NaCl, the calculated microconstants showed an unusual apparent cooperativity between the enol groups, which results from the release of the sodium cations upon protonation of norbadione A.