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Inframolecular Protonation Process of Norbadione A:  Influence of the Ionic Environment and Stereochemical Consequences

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journal contribution
posted on 02.02.2005, 00:00 by Paul Kuad, Michal Borkovec, Marine Desage-El Murr, Thierry Le Gall, Charles Mioskowski, Bernard Spiess
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.