Photo‐oxidation of lipids impregnated on the surface of dried seaweed (Porphyra yezoensis Ueda). Hydroperoxide distribution

Abstract

AbstractThe distribution of hydroperoxide isomers generated by photo‐oxidation of natural lipids impregnated on the surface of dried seaweed previously exposed to visible light and without added photosensitizer were studied. The surface of dried seaweed was impregnated with linoleic acid methyl ester, and the sample was divided into two parts. One part was exposed to light from a 100‐W tungsten bulb (4500 lux) in a low‐temperature room (5°C). The other part was kept in the dark as a control. Positional isomers of the hydroperoxides generated from the impregnated linoleic acid methyl ester were separated individually by HPLC and further identified by MS. The dried seaweed kept in the dark contained four hydroperoxide isomers, namely, 13‐hydroperoxy‐cis‐9, trans‐11‐octadecadienoate, 13‐hydroperoxy‐trans‐9, trans‐11‐octadecadienoate, 9‐hydroperoxy‐trans‐10,cis‐12‐octadecadienoate, and 9‐hydroperoxy‐trans‐10, trans‐12‐octadecadienoate. For the dried seaweed exposed to light, the oxidized lipids contained not only the same four isomers, but also 12‐hydroperoxy‐cis‐9, trans‐13‐octadecadienoate and 10‐hydroperoxy‐trans‐8,cis‐12‐octadecadienoate. When fresh seaweed was dried in the sunlight, the formation of 12‐cis,trans‐ and 10‐cis,trans‐hydroperoxides of naturally occurring methyl linoleate was verified. Dried seaweed was then impregnated with eicosapentaenoic acid ethyl ester and exposed to light. Light exposure also generated certain hydroperoxide isomers attributable to singlet oxygen oxidation, namely, 6‐hydroperoxy‐trans‐4,cis‐8, cis‐11,cis‐14,cis‐17‐ethyl and 17‐hydroperoxy‐cis‐5,cis‐8,cis‐11, cis‐14,trans‐18‐ethyl eicosapentaenoate. When dried sea‐weed without any impregnated lipids was exposed to the light for 24 h in a cold room (5°C), characteristic isomers, including both the 20‐carbon FA isomers 6‐OOH and 17‐OOH as well as the 18‐carbon FA isomers 10‐OOH and 12‐OOH, were detected in the light‐exposed sample but were not found in the control. These results clearly show that singlet oxygen oxidation of lipids occurred in the seaweed exposed to light. We concluded that this lipid oxidation was catalyzed by chlorophyll as a photosensitizer in seaweed.