No abstract

The nature and mechanism of formation of the thiobarbituric acid (TBA)-reaction material produced in the autoxidation of polyunsaturated fatty acids (PUFA) or their esters has been studied. On the basis of chemical studies and spectroscopic evidence, it is concluded that the TBA test detects malonaldehyde which arises at least in part from the acid-catalyzed or thermal decomposition or endoperoxides (2,3-dioxanorbornane compounds). These endoperosides have structures related to those of the endoperoxides produced in the biosynthetic sequence leading to prostaglandins. A mechanism is proposed in which these endoperoxides are formed in a free radical cyclization process operating in competition with hydroperoxide formation during the autoxidation of PUFA or their esters containing three or more double bonds. When 20:3 or 20:4 PUFA undergo autoxidation, some of the natural, physiologically active prostaglandins would be produced, although in very low yield, along with many other stereo- and positional isomers. Thus, it is possible that some of the complex symptoms of lipid peroxidation in vivo could be due to nonenzymatically produced prostaglandins or their steroisomers.

The relative reactivities of a series of hydrogen donors toward the methyl radical were measured in solution at 110°. Methyl radicals were generated by the thermal decomposition of ferf-butyl peracetate in mixtures of the hydrogen donor RH and the standard substrate tert-butyl mercaptan-S-t/, and the ratio [CHJ [CH3D] was determined by mass spectrometry: CH3• + RH CH4 + R• (¿HH) and CH3• + tm-BuSD -CH3D + tert-BuS• (¿sd)• A wide range of substrates was studied, including compounds containing alkyl, allylic, benzylic, and cycloalkyl hydrogens, and the data are combined to give a scale of rate constants for reaction of methyl radicals with any given type of hydrogen. Our scale of reactivities for hydrogen-abstraction reactions and Szwarc's scale3 for addition reactions can be put on a common basis using the reactivity of isooctane as the bridge: CH3• + (olefinic or aromatic compound) -» CH3-C-C • (¿add) and CH3• + C8H18 -CH4 -\-C8H17• (¿¡so). This permits us to tabulate the relative reactivity toward methyl radicals of virtually every type of hydrocarbon-saturated, olefinic, or aromatic; from these data the relative reactivity of any hydrocarbon of interest can be predicted. The selectivity of the methyl radical toward various types of hydrogens is compared with that of a number of other radicals.The methyl radical is found to be quite similar to the phenyl radical in selectivity. Calculations show the relative reactivity of a series of donors toward the methyl radical is the same in the gas phase and in solution.(1) Reactions of Radicals. Part 41. This work was supported in part by NIH Grant GM-11908. (2) (a)

Neat samples of polyunsaturated fatty acids were exposed to ozone in air in a flow system, and the formation of peroxides, conjugated dienes and thiobarbituric acid (TBA)-reactive material was followed as a function of time. The effect of ozone is to shorten the induction period normally observed in autoxidation studies, but the ozone, at the concentrations used here (0-1.5 ppm), appears to have no effect on the rates of product formation after the induction period. During the induction period, increasing ozone concentrations give rise to substantially increased rates of peroxide (or materials which titrate like peroxide) formation, a slightly increased rate of conjugated diene formation, and no significant increase in the rate of production of TBA-reactive material. Vitamin E lengthens the induction period but appears to have no other effect. Some of these data are in conflict with earlier reports of Menzel et al.

Isomer distribution data are reported for the addition of tritium atoms, generated by photolysis of thiophenol-/, to monosubstituted benzenes at 40°. These data are combined with literature values of the total rate constant for the addition of H atoms to substituted benzenes, and relative rate constants are calculated for the rate of addition to the ortho, meta, and para positions (so called partial rate factors). A Hammett correlation of the meta and para partial rate factors gives a p of -0.24 ± 0.09; a correlation of meta data only gives a p of -0.28 ± 0.12. These p values are compared with values derived from studies of H atoms generated by aqueous radiolysis. A compilation is given of Hammett p values for the abstraction of hydrogen from the benzylic position of toluenes by various free radicals and the addition of radicals.to monosubstituted benzenes. Unexpectedly, the p values for the two reactions are quite similar for the five of the six radicals for which both p values are known; it is suggested that this similarity may be general. The use of the Hammett equation to correlate radical reactions is discussed.The hydrogen atom adds to aromatic compounds to form a cyclohexadienyl radical.3-7 Kinetic data for this reaction (eq 1) have been published from a X ortho, meta, and para isomers number of laboratories using aqueous solutions of the aromatic compounds and high energy radiolysis of water to generate the H atoms. The techniques used in these studies give the total rate constant for eq 1 and do not distinguish the partial rate constants k0, km, and kp for addition to the carbons ortho, meta, or para to the X substituent. Despite the fact that the fraction of the hydrogen atoms which add to each of the three isomeric positions in the aromatic compounds was not determined, the data for eq 1 have been correlated using the Hammett equation. Since it was not possible to decide whether am or values should be used with the data available, Anbar, Meyerstein, and Neta8 and Neta and Schuler9 correlated the values of ktotal with both meta and para values. Using this method, they obtain Hammett correlations with p values of -0.7 and -0.4,(1) This research was supported in part by National Institutes of Health Grant 11908 and National Science Foundation Grant GP 3820.(2) This publication is part VI in the series, "Reactions of the Hydrogen Atom in Solution." Previous publications in this series: (a) part I, W.