Pulse Radiolysis Studies in Model Lipid Systems. The Influence of Aggregation on Kinetic Behavior of OH Induced Radicals in Aqueous Sodium Linoleate

Abstract

The effect of the solvent composition and of micelle-forming surfactants on the electron-transfer rate between phenothiazine and iron(III), according to the reaction asp + Fe(lI1) e &n + Fe(ll)Solvent composition affects the specific rate constant k l in the order: methanol/water > ethanol/water > n-propanol/water > acetone/water (all 50/50 v.v.).Strong effect is exhibited by micelles in going from a large increase observed for anionic surfactant to a decrease for non-ionic or cationic ones, in respect to the previous cited solvent compositions. The difference in reaction rate observed between an anionic surfactant and a cationic one is about lo4 -10'. The effect of acidity, ionic strength and temperature has been also investigated. E 4097 H H has been investigated by means of the stopped-flow technique.The kinetic and spectral behavior of radicals formed by hydroxyl radical attack on linoleate anions has been studied by pulse radiolysis. Reactivity of .OH toward this surfactant is an order of magnitude greater in monomeric form [k,OH+linolca,c = 8.0. lo9 M -' sec-I] than in micellar form [k.OH+lin(mico,lc) = 1.0' lo9 M-' sec-'1. Abstraction of a hydrogen atom from the doubly allylic position gives rise to an intense absorption in the UV region (Amax = 282-286nm, E -3 . lo4 M-' cm-' ) which may be used as a probe of radical activity at that site. This abstraction may occur, to a small extent, directly via .OH attack. However, greater than 90% of initial attack occurs at other sites. Subsequent secondary abstraction of doubly allylic H atoms appears to occur predominantly by: (1) intramolecular processes in monomers, (2) intermolecular processes in micelles. Disappearance of radicals by secondary processes is slower in the micellar pseudo phase than in monomeric solution.Die kinetischen und spektralen Eigenschaften von Radikalen, die durch die Reaktion von OH-Radikalen mit Linoleat-Anionen entstehen, wurden mit der Pulsradiolyse untersucht. Die Reaktivitat von OH ist gegeniiber dem monomeren Linoleat-Anion etwa eine GroDenordnung groBer (koH+Linolsa, = 8,O' lo9 M -' s -I als gegeniiber der mizellaren Form (koH+Lin(M,rellc) = 1.0. lo9 M -' S -' ). Abspaltung eines Wasserstoffatoms von der zweifachen Allylposition fuhrt zu einer intensiven Absorption im UV-Bereich (Amax = 282-286 nm, E -lo4 M-'), die als Sonde fur die Radikalaktivitat an dieser Stelle dienen kann. Mehr als 90% der OH-Radikale greifen jedoch an anderen Stellen des Molekiils an. Die darauf folgende sekundare Abspaltung von H-Atomen in der zweifachen Allylposition scheint hauptsiichlich durch 1) intramolekulare Prozesse in Monomeren und 2. intermolekulare Prozesse in Mizellen zu erfolgen. Das Verschwinden von Radikalen durch Sekundirprozesse geschieht in der mizellaren Pseudophase langsamer als in der monomeren Losung.cis double-bond moieties incorporated into their mid-zone. Such moieties may consist of one double bondas in the case of oleic acidor multiple non-conjugated bonds as in the cases of linoleic and linolenic acids. *) The research described...

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