Antioxidant activity, NMR, X-ray, ECD and UV/vis spectra of (+)-terrein: Experimental and theoretical approaches Zuhra Bashir Khalifa Al Trabolsy a , El Hassane Anouar a,⇑ , Nur Shahidatul Shida Zakaria a , Manar Zulkeflee b , Mizaton Hazizul Hasan b , Maisarah Mohd Zin b , Rohaya Ahmad a,c , Sadia Sultan a,d , Jean-Frédéric F. Weber a,d a Atta-ur-Rahman Institute for Natural Product Discovery, Universiti Teknologi MARA, Puncak Alam Campus, 42300 Bandar Puncak Alam, Selangor D. E., Malaysia b Pharmacology Toxicology Research Laboratory, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam Campus, 42300 Bandar Puncak Alam, Selangor D. E, Malaysia c Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor D. E., Malaysia d Department of Pharmacology and Chemistry, Faculty of Pharmacy, Universiti Teknologi MARA, Puncak Alam Campus, 42300 Bandar Puncak Alam, Selangor D. E., Malaysia highlights  Stereochemistry of isolated terrein was confirmed by using DFT and TD- DFT calculations.  Antioxidant activity of terrein is relatively influence by the type of isomers (cis or trans).  Antioxidant activity of terrein is independent to the stereochemistry of stereogenic centers. graphical abstract article info Article history: Received 16 July 2013 Received in revised form 22 November 2013 Accepted 16 December 2013 Available online 22 December 2013 Keywords: Terrein Aspergillus terreus DFT ECD spectra Antioxidant activity abstract Fungal metabolite terrein isolated from Aspergillus terreus is endowed with diverse biological and antiox- idant activities. To determine the stereochemistry of the isolated terrein, we combined spectroscopic methods (CD and NMR spectra) and theoretical calculations (DFT and TD-DFT methods). Stereochemistry effects on the antioxidant activity of isolated terrein were evaluated by calculating bond dissociation enthalpies (BDEs), ionization potentials (IPs) and spin density delocalization of terrein and isoterrein ster- eoisomers with B3P86/6-31+G (d, p) method in gas and polarizable continuum model. The results showed a good agreement between experimental data and theoretical calculations which confirmed the (+)-ter- rein stereochemistry of isolated metabolite. Theoretical calculations showed that the antioxidant activity is relatively influenced by isomeric geometry of the terrein (a variation of 2 kcal/mol between BDEs of terrein and isoterrein isomers), while chirality has no influence on the antioxidant activity [0.2 kcal/ mol difference between BDEs of (+)- and (–)-terrein]. The low antioxidant activity of (+)-terrein with respect to trolox and ascorbic acid was explained by the positive free Gibbs energy of the hydrogen atom transfer (HAT) mechanism and high BDE values of the 2-OH active site. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Terrein (alternative names: trans-2,3-dihydroxy-4-propenylcy- clopent-4-enone; trans-4,5-dihydroxy-3-propenyl-2-cylopenten- 1-one) is a bioactive fungal metabolite isolated from Aspergillus 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.12.034 ⇑ Corresponding author. Tel.: +60 3 3258 4771; fax: +60 3 3258 4770. E-mail address: anouarelhassane@yahoo.fr (E.H. Anouar). Journal of Molecular Structure 1060 (2014) 102–110 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc