Oxidative stability electron spin resonance spectroscopy

KRISTENSEN D, krOger-ohlsen m V and SKIBSTED L H (2002) Radical formation in dairy products Prediction of oxidative stability based on electron spin resonance spectroscopy, in Morello M J, Shahidi F Ho CT Free Radicals in Food, Chemistry, Nutrition and Health Effects, ACS Symposium Series 807, Washington D C, 114-25. 

Fully hindered amines are excellent UV stabilizers, but poor thermal antioxidants, for polyolefins. We have found that partially hindered bicyclic amines, such as 3,3-dialkyldecahydroquinoxalin-2-ones, are excellent UV stabilizers and also excellent thermal antioxidants as well. The oxidation of these bicyclic amines with m-chloroperbenzoic acid was studied by electron spin resonance spectroscopy. They form stable, partially hindered nitroxyl radicals (6-line spectra). However, these primary radicals are easily oxidized to a new, fully hindered nitroxyl radical (3-line spectra) which are also very stable. Field desorption mass spectroscopic studies of the oxidation of these amines show that the primary nitroxyl radicals, which form first, lose two hydrogen atoms and add an oxygen. Thus, these bicyclic partially hindered amines are unique in being both stable and at the same time hydrogen atom donors. 

Thomsen, M.K., Kristensen, D. and Skibsted, L.H. Electron spin resonance spectroscopy for determination of the oxidative stability of food lipids. J. Am. Oil Chem. Soc. 77,725-730 (2000). 

Radical Formation in Dairy Products Prediction of Oxidative Stability Based on Electron Spin Resonance Spectroscopy... 

From the very beginning of the phenothiazine chemistry, its capacity to give rise to different oxidation products, now explained in terms of a suecessive electron loss, was realized. The first systematic study on these topics was by Kehrmann, in the first decade of the present century. He suceeeded in explaining the observed phenomena, and it has been proved by recent work that most of his statements are essentially correct. A second stage was marked by Michaelis s work, which stressed the remarkable stability of the semiquinonoid forms obtained both by oxidation of phenothiazines and by reduction of quinoneiminoid phenothiazine dyestuffs. The development of electron spin resonance (ESR) spectroscopy allowed the direet investigation of the radical species, and marked the beginning of a third era in the study of redox processes in the phenothiazine class. 

The molecular and electronic structures of cyclic disulfide cation radicals of 1,2-dithietane 6 and 1,2-dithiete 7, and radical cations of 1,2-dithiolane 2 (2a-c represent stable conformations determined in terms of the symmetry restriction of Cs, Cz, and Czv), with emphasis on the nature of a two-center three-electron (Zc-ie) sulfur-sulfur bond have been examined by ab initio molecular orbital (MO) calculations <1997JMT(418)171>. Unrestricted Hartree-Fock (UHF)/ MIDI-4(d) computations showed that this bond in organodisulfide radical cation 2 is shorter in comparison to 1,2-dithiolane 2 and possesses partial Jt-bond character (structure A), as previously implied by electron spin resonance (ESR) spectroscopy <1982JA2318>, which correlates best with the form as the most favorable conformation of the cation radical 2. Contrary to the repulsive S-S interaction in the parent 1,2-dithiolane arising from the lone pairs of electrons, the hemi-7t-bond formed by one-electron oxidation should stabilize the five-membered ring of 2, or, for example, a similar cation radical of LA 3 which is involved in diverse biochemical reactions. 

Lipid oxidation is also estimated by instrumental analyses such as by Rancimat test method and used for accelerated stability study [6]. Electron spin resonance can measure radical formation in many types of food matrices [7]. Analysis of volatiles with dynamic headspace/GC-MS is highly sensitive and can provide lot of information with regard to volatile lipid oxidation products and other volatiles with sensory impact in food samples. This type of data can also give information on possible reaction pathways of food deterioration [8]. Front-face fluorescence spectroscopy... 

Based on X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), electron spin resonance (ESR), Mbssbauer, and extended X-ray absorption fine structure spectroscopy (EXAFS) , van Veen and collaborators concluded that the thermal treatment at temperatures where catalytic activity is maximum ( 500-600°C) does not lead to complete destruction of the macrocycles, but rather to a ligand modification which preserves the Me-N4 moiety intact. Furthermore, the stability of this catalytic site is improved because the reactive parts of the ligands are bound to the carbon support and thus are no longer susceptible to an oxidative attack. Thermal treatments at higher temperatures (up to 850°C) led to some decomposition of the Me-N4 moiety, and thus to a decrease of the catalytic activity, and to the reduction of some of the ions to their metallic state. 

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