The potent antioxidant alpha-tocopherol is known to trap two hydroxyl radicals leading to the formation of the experimentally observed alpha-tocopherylquinone product. Based on double-hybrid density functional theory calculations, we propose for the first time, a reaction mechanism for the conversion of alpha-tocopherol to alpha-tocopherylquinone. We find that a water-catalysed ring-opening reaction plays a key role in this conversion. The water catalysts act as proton shuttles facilitating the proton transfers and reducing the ring strain in the cyclic transition structures. On the basis of the proposed reaction mechanism, we proceed to design an antioxidant with potentially enhanced antioxidant properties. (C) 2018 Elsevier B.V. All rights reserved.