Submitted to: Tetrahedron
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2004
Publication Date: 2/6/2006
Citation: Heider, E.M., Harper, J.K., Grant, D.M., Hoffman, A., Dugan, F.M., Tomer, D.P., O'Neill, K.L. 2006. Exploring unusual antioxidant activity in a benzoic acid derivative: a proposed mechanism for citrinin.. Tetrahedron. 62(6):1199-1208. Interpretive Summary: A recent survey of endophytic fungi present in Ginko biloba leaves in the Northwest (USA) yielded a bioactive fungus identified as Penicillium citrinum. Cultivation of this organism provided an active fraction that gave a single pure compound, citrinin. Previous work has demonstrated that citrinin is carcinogenic and able to inhibit transcription. Citrinin is also reported to interact with DNA and has anti-oxidant activity. Strong antifungal activity was observed against Pythium and Scleroderma. Samples of Rhizoctonia and Geotrichum exhibited slowed growth relative to a control at concentrations of 2 ' 10 'g/mL. Antibacterial activity was found against Staphylococcus aureus. Citrinin consumption was found to kill tobacco hornworms in one day at the 20 'g level using citrinin coated tobacco leaves. Actual citrinin consumption by hornworms was significantly less than 20 'g as only a small fraction of the leaf was consumed. Stereochemistry (alternative forms of the citrinin molecule) has a great impact on the antioxidant activity of citrinin.
Technical Abstract: A mechanism is proposed for the unusual antioxidant activity in citrinin based on computed O ' H bond dissociation enthalpies (BDE). These data clearly show that citrinin itself is not the active species, but rather a pair of hydrated diastereomeric Micheal addition products consisting of substituted 2,6-dihydroxy benzoic acids. These diastereomers act as radical scavengers via O ' H bond dissociation with computed BDE's ranging from 79.5 ' 82.3 kcal/mol for the two hydroxy groups present. These data represent the first facile O ' H bond dissociation in a phenol containing a strongly electron withdrawing group. This unusual reactivity arises from an intramolecular network of radical stabilizing hydrogen bonds that decrease O ' H BDE's by as much as 29.0 kcal/mol. The additional influence of stereochemistry on BDE is computed to be ~ 3 kcal/mole. Data presented are for gas phase molecules, but solvents are unlikely to significantly modify these results since most polar groups are involved in intramolecular hydrogen bonds in the conformations considered. Additional bioactivities are reported here and reemphasize that citrinin and the Michael addition products are likely too toxic for use as antioxidants in organisms. This study, however, clearly identifies specific reaction sites in the active form, thus strongly guiding rational design of synthetic derivatives with more favourable biocompatibility.