Vitamin
E
is localized in membranes and functions as an efficient inhibitor of lipid peroxidation in biological systems. In this study, we measured the second-order rate constants (
ks) for the reaction of tocotrienol homologues (α-, β-, γ-, and δ-Toc-
3
Hs) with the aroxyl radical (ArO•) used as a model for lipid peroxyl radicals (LOO•) in the membranes of egg yolk phosphatidylcholine (EYPC) vesicles by stopped-flow spectrophotometry, and compared them to those of tocopherol homologues (α-, β-, γ-, and δ-TocHs). The relative rate constants of Toc-
3
H homologues to α-Toc-
3
H in membranes (α/β/γ/δ=100/
83
.
7
/63.2/20.2) were not much different to those of TocH homologues to α-TocH (α/β/γ/δ=100/
88
.4/
83
.
8
/17.
3
). Each
ks value of Toc-
3
H homologues in membranes was 60-80% of that of the corresponding TocH homologues except for the almost identical
ks values of δ-homologues, but there was no difference in EtOH solution between each
ks value of the corresponding homologues of Toc-
3
H and TocH. These results indicate that the difference of the alkyl-side chain structure of vitamin
E
causes a change in the mobility of vitamin
E
molecules and/or the location of their antioxidant OH-groups in membranes, resulting in lowered radical-trapping rates of Toc-
3
Hs. By use of the ratio of the
kinh value of α-TocH with LOO• (
3
.20×10
6 M−1s
−1) to the
ks value of α-TocH with ArO• (
8
.05×10
4 M−1s
−1) in chlorobenzene (that is, 39.
8
), the
kinh value for the reaction of α-TocH with LOO• in membrane was estimated to be 1.03×10
5 M−1s
−1.
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