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

• Published in: Journal of the American Chemical Society 2005-02, Vol.127 (4), p.1323-1333
• Main Authors: Kuad, Paul, Borkovec, Michal, Desage-El Murr, Marine, Le Gall, Thierry, Mioskowski, Charles, Spiess, Bernard
• Format: Article
• Language: English
• Subjects: 4-Butyrolactone - analogs & derivatives; 4-Butyrolactone - chemistry; Biological and medical sciences; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; Kinetics; Mechanisms. Catalysis. Electron transfer. Models; Molecular biophysics; Nuclear Magnetic Resonance, Biomolecular; Phenylacetates - chemistry; Physical chemistry in biology; Potentiometry; Protons; Spectrophotometry, Ultraviolet; Stereoisomerism
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0483185

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.