Chain-breaking reactions against lipid peroxidation performed by carotenes, including beta-carotene (beta-CAR) and lycopene (LYC), have been studied using density functional theory. We chose linoleic acid (LAH) as the lipid model and examined two mechanisms: hydrogen abstraction and addition. Our computed reaction diagrams reveal that the addition mechanism is able to offer a larger extent of chain-breaking protection than hydrogen abstraction. In the case of hydrogen abstraction, the resulting carotene radical CAR(-H)(*) has a smaller O(2) affinity than the linoleic acid radical (LA(*)). Formation of the addition adduct radical ROO-CAR(*) is energetically favorable, and it has an even smaller tendency to react with O(2) than CAR(-H)(*). Comparatively, ROO-beta-CAR(*) is less likely to react with O(2) than ROO-LYC(*). Both the hydrogen abstraction and addition radicals (CAR(-H)(*) and ROO-CAR(*)) react readily with a second ROO(*) radical via either hydrogen abstraction or addition.

The reaction mechanism of carotenes (CARs) in chain termination against lipid peroxidation was studied using density functional theory. In the presence of peroxide (ROO(·)), the reaction barrier for its addition to CAR to form ROO-CAR(·) is smaller than those for its hydrogen abstractions from CAR and linoleic acid (LAH), respectively. In contrast, the reaction barriers for the O(2) additions of the carbon-centered radicals are ordered as ROO-CAR(·) > CAR(-H)(·) > LA(·). Thus, the chain-termination function of CAR is best demonstrated by trapping the addition radical and suppressing O(2) addition. For either the ROO-CAR(·) or CAR(-H)(·) radicals, β-carotene has noticeably higher O(2) addition barriers than those of their lycopene counterparts. The reaction barrier for the rearrangement of ROO-CAR(·) into RO(·) + epoxide is much smaller than that for the formation of cyclic ether and is comparable to that of O(2) addition. Since RO(·) has a stronger tendency toward hydrogen abstraction than ROO(·), the rearrangement of the addition adduct has to be recognized as an important factor while evaluating the chain-termination potency of a molecule. The overall reaction energy profile reveals that the protective function of CAR is mostly exhibited via formation of the addition adduct. Comparatively, β-carotene is a more potent antiradical agent than lycopene against peroxide-initiated lipid peroxidation.