Red pigment-forming substances from autoxidized linolenate: identification of prostaglandin-like substances

The chemical structures of lipid degradation products capable of reacting with amino acids and forming red pigments were investigated. The red pigment-forming substances (RPS's) derived from autoxidized linolenate in triglyceride of linseed oil were purified successively by gel chromatography on Sephadex LH-20, column chromatography and TLC on Silica gel 60, and HPLC on mu-Porasil. Consequently, three types of RPS's were isolated. IR spectra of RPS's were similar, except for slight differences in the fingerprint region (1300-650 cm-1). These substances included the OH group (3500 cm-1 region), conjugated aldehyde (vC = O 1688 cm-1, vC = C 1635 cm-1) and ketone (vC = O 1740 cm-1) in their molecules. RPS's were analyzed by GLC and GC-MS after derivatization with dimethylhydrazine and/or trimethylsilyl reagents, before and after the reduction with NaBH4 and/or hydrogenation with PtO2. The fragmentation patterns indicated the presence of an ethyl group in addition to the functional groups described above, and the molecular formula was estimated to be C10H14O3. Further elucidation of the structures was obtained by 13C- and 1H-NMR analyses, and evidence was obtained for the presence of a hydroxypentanone ring, a PG-like structure. The sequence of the protons on the ring carbons was unequivocally deduced from the double resonance experiments. All the data taken together suggested that the RPS's were the stereoisomer of 3-(2-ethyl-5-hydroxy-3-oxo) cyclopentanyl-2-propenal.