Visible light induced photo-oxidation of highly branched isoprenoid (HBI) alkenes: Significant dependence on the number and nature of double bonds

Visible light induced, type II (i.e. involving singlet oxygen, 1O2) photosensitized oxidation of mono-, di-, tri- and tetraunsaturated C25 highly branched (HBI) alkenes in solution was investigated. The photodegradation rate was shown to be dependent on the number and nature of the double bonds, especially the number of trisubstituted ones. HBI alkenes with at least one trisubstituted double bond were photodegraded at similar or higher rates than some other lipids (e.g. vitamin E) known to be very reactive towards 1O2. The 1O2 mediated photo-oxidation of HBI alkenes likely involves direct reaction of 1O2 with the trisubstituted double bond via a concerted [`]ene' addition, leading to formation of a hydroperoxide at each carbon of the original double bond. The enhanced photoreactivity of HBI alkenes with at least one trisubstituted double bond was confirmed in dead cells of the diatom Haslea ostrearia. In addition, since HBI alkenes with such double bonds were photodegraded at similar or higher rates compared to polyunsaturated fatty acids, vitamin E and chlorophyll a, they should be significantly photochemically altered within the euphotic zone of the ocean. In contrast, HBIs with only di- and monosubstituted double bonds seem to be relatively inert with respect to type II photoprocesses. The results support the apparent good preservation of the monoene IP25 in sediments, while the interpretation of sedimentary occurrences of HBI trienes may need further consideration.