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

Abstract

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.

Survey of the year 2009: applications of isothermal titration calorimetry

“…In 2009 we identified more than 400 articles after searching Web of Science, PubMed, SciDir and OVID databases using ‘isothermal AND titration AND calorimetry’ or ITC or ‘Isothermal Titration Calorimetry’ search terms. These have been classified into the following categories: Pre‐2009 references cited in the text and review articles 1–43 Protein‐protein and protein‐peptide interactions 44–124 Protein/peptide‐small molecule interactions 125–218 Protein/peptide‐metal ion interactions 219–253 Protein/peptide‐nucleic acid interactions 254–273 Protein/peptide‐lipid interactions 274–292 Protein/peptide‐polysaccharide interactions 293–316 Nucleic acid‐small molecule interactions 317–348 Small molecule interactions …”

Atomistic Origin of Selective Cs Accumulation in Mushrooms: DFT Analysis for Alkali Metal Cation Complexation Selectivity of Scissors-like Pigments

“…Since cation complexation of molecules in vivo and vitro conditions are typically performed in water (aqueous solution) 41 as well as in a methanol−water mixture, 14 the solvent effects on the cation binding to the molecules must be subsequently taken into account. To do so, we adopt PCM.…”

“…So far, there have been several investigations on the structural and complexation properties of NBA. Actually, the experimental studies by Garaudée et al, Desage-El Murr et al, and Kuad et al confirmed the unusual complexation selectivity order Cs + > K + > Na + only in a high pH range. Theoretically, Schurhammer et al and Kuad et al performed classical molecular dynamics simulation and found, though qualitatively, a tendency to obey this stability order of Cs + > K + > Na + .…”

“…Actually, the experimental studies by Garaudée et al, Desage-El Murr et al, and Kuad et al confirmed the unusual complexation selectivity order Cs + > K + > Na + only in a high pH range. Theoretically, Schurhammer et al and Kuad et al performed classical molecular dynamics simulation and found, though qualitatively, a tendency to obey this stability order of Cs + > K + > Na + . Thus, the remaining issue has been a more clear quantitative analysis on the Cs + complexation selectivity of NBA.…”

“…by Steffan et al and Aumann et al . Afterward, several investigations were then conducted to find a peculiar atomistic mechanism occurring inside the pileus or fruiting body of the mushrooms. − Consequently, their studies suggested that radioactive Cs accumulation takes place via Cs complexation process of certain metabolite molecules produced inside mushrooms. In particular, one cap pigment produced by fungi, brown-colored norbadione A (NBA, C 35 H 18 O 15 ), was considered to selectively complex cesium cations and therefore to be mainly responsible for the detection of a high-level of 137 Cs and 134 Cs radioactivity in the edible mushrooms.…”