Conformational and Cs⁺complexation properties of norbadione-A: a molecular modeling study

Abstract

We report a quantum mechanical (QM) and classical molecular dynamics (MD) study of the conformational and complexation properties of norbadione-A (NBA), a key pigment involved in the Cs+ complexation by mushrooms. The Z versus E isomers of its pulvinic moieties are compared in their neutral (Pulv0), mono- (Pulv(-1)) and di-deprotonated (Pulv(-2)) states, and the 1H chemical shifts are calculated ab initio. Pulv(-1) is found to be stabilized in the E form by an internal COOH(-)O(enolate) hydrogen-bond. No energy minimum is found for the corresponding COO(-)HO(enol) state, indicating that the conjugated enol function of Pulv0 is more acidic than the COOH function. Further deprotonation leads to the Z and E forms of Pulv(-2) that are close in energy and both account for a marked downfield shift delta of ortho-H8 protons. A similar shift is found upon deprotonation of the enol function of an ester analogue of Pulv0. Therefore, contrary to previous assumptions (ref. 7: P. Kuad, et al., J. Am. Chem. Soc., 2005, 127, 1323), the large shift of delta(H8) around pH 9.5 upon deprotonation of NBA or of pulvinic acid cannot be taken as an indicator of an E-to-Z conformational switch, but merely reflects the pH-induced conformational change of the carboxylate group adjacent to the (H8)-ring. The QM and MD studies on NBA(2-) and NBA(4-) support the view that both species prefer the E/E form with two intramolecular COOH(-)O(enolate) hydrogen-bonds in the gas phase and in solution. Finally, we simulated mono- and di-nuclear complexes of Cs+ with NBA(2-) and NBA(4-) by MD, showing that only the NBA(4-) state populated at high pH values can bind two Cs+ cations, with both E and Z conformations of the pulvinic arms.

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

“…The conformational freedom acquired by breaking the stable 7-membered ring may also participate in enhancing the complexation process. However, it is not possible to infer accurate structural conclusions from the experimental results, since NBA displays a large conformational freedom as well as many coordination sites and p interaction possibilities, which allow a wide variety of structures as shown by a molecular dynamics study in methanol 11 and in the methanol : water mixture (this study). Here, strengthening the previous theoretical calculations, we show that the dicationic complexes can form only when at least NBA's tetradeprotonated state is reached.…”

“…1). The quantum mechanical study 11 showed that NBA's functional groups decrease in basicity in the following order: phenolates 4 naphtholate 4 carboxylates 4 enolates, whereas the experimental study, 15 based on many spectroscopic clues, inverted the order of protonation for carboxylates and enolates. However, the most important feature is that both studies agree in that, after deprotonation of the most acidic function of a pulvinic moiety, a very strong hydrogen bond sets up between the carboxylic and enolic groups, leading to a 7-membered ring with co-planar atoms (see Scheme 2).…”

“…The solutes were immersed either in a water box or in a methanol : water 80 : 20 mixture represented with 3D periodic boundary conditions. The initial norbadione ZZ and EE structures were extracted from the last set of MD simulations in methanol, 11 with coplanar lactone and carboxylate groups. Non-bonded interactions were calculated using a 12 A ˚cutoff with Particle Mesh Ewald correction for long range electrostatics.…”

“…Both studies, however, agree that the complexation mechanism is uncommon. In a more recent publication, 11…”

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

“…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.…”