Chemical and quantum mechanical studies of the free radical C-C bond formation in the lipoxygenase-catalyzed dimerisation of octadeca-9,12-diynoic acid

Abstract

Triple bond analogues of poly-unsaturated fatty acids are well-known inactivators of lipoxygenases. In an earlier study we proposed that, since 11-oxo-octadeca-9,12-diynoic acid (11-oxo-ODYA) is the only oxygenated product formed during the irreversible inactivation of soybean lipoxygenase-1, the inactivation should proceed via a C11 centered octadeca-9,12-diynoic acid radical (ODYA radical).[1] In the present study we investigated the lipoxygenase-catalysed formation of the ODYA radical. In the reaction of lipoxygenase with ODYA in the absence of dioxygen and in the presence of 13(S)-hydroperoxy-octadeca-9Z, 11E-dienoic acid (13-HPOD), free ODYA radicals were formed which resulted in the formation of three dimeric ODYA products in which one ODYA moiety is linked via its C9 (12%), C11 (72%) or C13 (16%) to the C11 methylene of the other ODYA moiety. With the ab initio Hartree-Fock method, using the 2,5-heptadiynyl radical as a model compound, the electron spin in the ODYA radical was calculated to be located for 12.0, 75.0 and 12.0% on carbon atoms C9, C11 and C13 of the ODYA radical, respectively. The ODYA-ODYA dimer formation could thus be explained on the basis of the electron spin distribution in the ODYA radical. The dimer formation, i. e. reaction of an ODYA radical with an ODYA molecule was compared with the reaction of the ODYA radical with dioxygen. On the basis of this comparison it is concluded that a) the ODYA dimer formation occurs at the carbon atom with the highest electron spin population; b) ODYA dimer formation is predominantly a kinetically determined process; c) the electron spin distribution in the ODYA radical can be used to predict the composition of the dimer mixture; and d) the regiospecific oxygen addition in the formation of 11-oxo-ODYA is enzymatically controlled.