PHOTO-INITIATED PEROXIDATION OF LIPIDS IN MICELLES BY AZAAROMATICS

The monoazaaromatics, pyridine (1), hexyl nicotinate (2), and quinolin e (3) and diazaaromatics, pyrimidine (4) and purine (5), readily act a s photo-initiators for the peroxidation of methyl linoleate in 0.50 M SDS at 37 degrees C giving free radical chain oxidations of linoleate. Quantitative kinetic runs on the order in substrate, RH, and in the r ate of chain initiation, R(i), showed that the classical rare law for autoxidation, -d[O-2]/dt = (k(p)/(2 k(t)(1/2)))[RH] x R(i)(1/2), is ap plicable to these photo-initiated oxidations. The oxidizability of met hyl linoleate under these conditions is 2.92 x 10(-2) M(-1/2) s(-1/2). These peroxidations were inhibited by chromanol phenolic antioxidants of the vitamin E class, such as lipid-soluble 2,2,5,7,8-pentamethyl-6 -hydroxychroman (PMHC) and water-soluble 2-carboxy-2,5,7,8-tetramethyl -6-hydroxychroman (Trolox) and derived rare constants for inhibition o f peroxidation were k(inh) (PMHC) = 4.35 x 10(4) M(-1) S-1 and k(inh) (Trolox) = 2.81 x 10(4) M(-1) s(-1) during inhibited oxidation of meth yl linoleate photo-initiated by 4. The products from photo-initiated p eroxidation of methyl linoleate by 1 through 5 were determined by redu ction and high-performance liquid chromatography analyses to be the 9- and 13-positional hydroperoxides of the four geometrical isomers: cis -9, trans-11 (6), trans-10, cis-12 (7), trans-9, trans-11 (8), and tra ns-10, trans-12 (9)-octadecadienoates typical of the free radical chai n mechanism of lipid peroxidation. Products from dye-sensitized oxidat ion by Methylene Blue or Rose Bengal of methyl linoleate gave a produc t distribution of six hydroperoxides typical of oxidation by singlet o xygen. Thermal or photo-initiated peroxidation of methyl linoleate in SDS gave some selectivity of oxidation at the 13-position of the linol eate chain. The ratio of 13- to 9-oxidation varied in the range 1.23 t o 1.14 as the cis/trans to trans/trans ratio of geometrical isomers va ried from 0.44 to 1.25 during photooxidation of increased amounts of l inoleate in SDS. This selectivity is attributed to loss of the pseudo symmetry around the pentadienyl system in the lipid chain in the SDS s ystem during the peroxidation.