Energy and electron transfer reactions of polyenic acids with variable chain lengths Asma Zaidi a, y , He Li a, y , Hans-Richard Sliwka a, * , y , Vassilia Partali a, y , Hansgeorg Ernst b , Thor B. Melø c, * a Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway b BASF SE, Fine Chemical Research, 67056 Ludwigshafen, Germany c Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway article info Article history: Received 12 June 2012 Received in revised form 25 September 2012 Accepted 15 October 2012 Available online 22 October 2012 Keywords: Antioxidant Carotenoids Electron transfer Energy transfer Carboxylic acids abstract Highly unsaturated conjugated fatty acids (carotenoic acids) with 5e15 conjugated double bonds were irradiated by flash pulses in the presence of the sensitizer riboflavin. The resulting triplet and radical cation transients were analyzed and the underlying energy and electron transfer reactions determined experimentally and by calculation. The two key antioxidant reactions of carotenoids summarize in a r , the ratio of the reaction constants for electron and energy transfer. The optimal antioxidant ratio a r cumu- lated in the polyenic acid with 11 double bonds. The photophysical formation of radical cations was chemically ascertained by reaction of the acids with FeCl 3 . Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The electron-rich carotenoids are eminent electron donators as well as distinguished triplet energy acceptors. The combination of these properties established the reputation of carotenoids as potent antioxidants. Antioxidation can be characterized (a) by electron transfer, i.e., the carotenoid delivers an electron to a molecule de- preciated of electrons, or (b) by energy transfer, i.e., the carotenoid combats oxygen in its highly reactive singlet electron configuration taking over oxygen’s excessive energy. Other possible antioxidant mechanisms operate to a minor degree with carotenoids (radical addition, H-transfer, chemical oxygen quenching) or occur with specific radicals ðO 2 $ Þ. 1e3 The antioxidant effect of carotenoids has been determined (a) by varying the peripherals (rings, substituents) keeping the polyene chain constant or (b) by keeping the peripherals constant varying the chain lengths. 4,5 The first method ranked astaxanthin and dihydroxyisorenieratene as superior antioxidants; 6,7 the result of the second method is not uniquely identified. However, it is commonly assumed the longer a polyene chain the better the antioxidant properties. On the other hand, systematic measurements with different carotenoids have suggested an optimal chain length for diverse functions, e.g., 11 double bonds for singlet oxygen quenching, 7e11 or 9 and 11 double bonds for photobleaching pro- tection, 10 or 9e11 double bonds for light harvesting function, and 7 double bonds for photon-to-current conversion (solar cell). 8e14 Since an all-purpose method for determining the total antioxi- dant activity (TAA), 15 total antioxidant capacity (TAC), 16 or total antioxidant potential (TAP) 17 is lacking the results depend above all on the test system. 18 The common antioxidant acronyms largely ignore carotenoids as a singlet oxygen quencher (energy transfer reaction). 19 Our own antioxidant measurements showed to either energy or electron transfer but did not encompass both actions. 20e25 A theoretical study with 28 miscellaneous carotenoids established the 1 O 2 -quenching constant, the ionization energy and the HOMOeLUMO energy difference as distinct descriptors for the an- tioxidant property. 26 Another work concentrated only on electron transfer processes and related the antioxidant properties to the re- dox potentials. 27 We concentrated in this study on both the energy and electron transfer reaction of a series of carotenoic acids (Car) specified as Cn (n¼number of carbon atoms) with N C]C bonds Cn:N, i.e., C20:5eC45:15. The compounds are interrelated; the only variable is the number of double bonds (Scheme 1). Generally, Car do not easily react with other organic compounds. Therefore, Car are not affected by riboflavin (Rib), tocopherol (Toc), chlorophyll (Chl), nitronaphthalene (NN), or other sensitizers (Sen). 20,28e30 However, when these sensitizers are irradiated, they * Corresponding authors. E-mail addresses: hrs@nvg.ntnu.no (H.-R. Sliwka), thor.melo@ntnu.no (T.B. Melø). y Fax: þ47 73596155. Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.10.041 Tetrahedron 69 (2013) 219e227