REACTION AND QUENCHING OF SINGLET MOLECULAR-OXYGEN WITH ESTERS OF POLYUNSATURATED FATTY-ACIDS

The rate constants for the reactive (k(R)) and unreactive (k(Q)) inter action of singlet molecular oxygen with three esters of polyunsaturate d fatty acids (PUFA: cis-methyl oleate, MO; cis-methyl linoleate, MLA and cis-ethyl linolenate, ELN) are determined. The values of the ratio k(Q)/k(R) are 0.51, 0.26 and 0.20 for MO, MLA and ELN, respectively. This variation results principally from that of k(R) because the value s of k(Q) are only slightly different (1.24 x 10(4) M(-1) s(-1) for MO and similar to 1.0 x 10(4) M(-1) s(-1) for MLA and ELN). It is shown that the rate constant k(Q) characterizes mainly an interaction with t he unreactive part of the molecule rather than with the double bonds ( solventlike quenching). Contrary to the already reported case of 1,5-p olyenes for which k(Q) much less than k(R), the present results and th ose obtained from a number of literature data show that for PUFA and t heir esters, neither k(R) + k(Q) nor k(R) are proportional to the tota l number of double bonds or of methylene groups adjacent to the double bonds. Instead, a linear correlation is observed by plotting k(R) vs the number of methylene groups adjacent to two double bonds. It is ded uced that contrary to a common assumption, biallylic hydrogens have a reactivity higher than that of singly allylic hydrogens (reactivity ra tio 1.19). The consequence of this result on the estimation of relativ e contributions of singlet oxygen and radical mechanisms to oxidation processes is discussed. Moreover, the whole of these results allows pr ediction of the values of k(R) and k(Q) for all unsaturated fatty acid s (and their esters) of similar structure.