J. Phys. Chem. 1992,96, 8184-8187
8184
Mechanism of Antloxidant Reaction of Vitamin E. 2. Photoelectron Spectroscopy and ab Initio Calculation Shin-ichi Nagaoka,* Kazuo Mukai,* Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790, Japan
Tomoyuki Itoh, and Shunji Katsumata Department of Fundamental Science, Iwaki Meisei University, Iwaki 970, Japan (Received: April 9, 1992; In Final Form: June I I , 1992)
A photoelectron spectroscopic study and ab initio calculations of the antioxidant action of vitamin E derivatives have been carried out. The vertical ionization energies (I,,'s) of tocopherols (TocH's) were obtained by using photoelectron spectroscopy. The geometries of TocHs were optimized, and the Koopmans' theorem first ionization energies (IK's) for those geometries were calculated with the ab initio method. A plot of I K vs I, is found to be linear. The second-order rate constant for the reaction of TocH with a substituted phenoxyl radical (k,)increases and the activation energy ( E d decreases as Z, (ZK) decreases. A plot of E,,, vs I, ( I K ) is found to be linear. A substantial deuterium kinetic isotope effect on k, is also observed. It is thus considered that both the charge transfer and the proton tunneling play important roles in the antioxidant reaction of TocH.
Introduction It is quite attractive to study the antioxidant action (reactions 1 and 2) of vitamin E (a-,8-, y-, and &tocopherols, Figure 1). LOO' TocH LOOH + Toc' (1)
+ LOO' + Toc'
-
nonradical products
(2)
Here, LOO', TocH, LOOH, and Toc' stand for a peroxyl radical, tocopherol, a peroxide, and tocopheroxyl radical, respectively. In a previous paper,' we measured the second-order rate constants of nondeuterated TocH's and deuterated TocHs (TocDs) with variously substituted phenoxyl radicals (PhO"s), kis, with a stopped-flow spectrophotometer. We also obtained the peak oxidation potentials of TocH's and the half-wave reduction potentials of Ph0"s by using a cyclic voltammetric technique. The geometries of TocH's were optimized with the semiempirical MNDO (modified neglect of diatomic overlap) method. The Koopmans' theorem first ionization energies (IK's)for those geometries were calculated with the ab initio method. The results indicate that k, increases as the total electron-donating capacity of the alkyl substituents at the aromatic ring of TocH or the electron-withdrawing capacity of the substituent of PhO' increases. On the other hand, a substantial deuterium kinetic isotope effect on k, is also observed. It is thus considered that both the charge transfer and the proton tunneling play important roles in the antioxidant reaction of TocH. On the basis of the experimental and calculated results mentioned above, we offered a probable explanation for the mechanism of the antioxidant reaction of TocH.' In the initial stage of the reaction, LOO' and TocH approach each other and their electron clouds begin to overlap. LOO' and TocH are relatively susceptible to accepting and donating an electron, respectively. Thus, the final goal of this process is the transition state which has the - -TocH+). When property of the charge-transfer species (LOO:-LOO' and TocH approach each other to some extent (LOO'&- -TocHd+),the proton tunneling takes place below the transition state. Tunneling allows the proton to cut a corner on the potential energy surface. The tunneling path of TocD is longer than that of TocH. Finally, LOOH and Toc' separate from each other. Our explanation is widely applicable to many protontransfer reactions in addition to the antioxidant and prooxidant reactions of TocH.'q2 There remained, however, a few problems to be investigated further. First, oxidation potential depends on the temperature, the solvent, and so on. In contrast, the ionization energy obtained in the vapor phase by means of the He I photoelectron spectroscopy does not depend on the experimental conditions. It is an inherent
-
0022-365419212096-8184$03.00/0
value in individual molecules and directly reflects the valence electronic structure of the molecule. However, it was difficult to obtain the photoelectron spectra of nonvolatile compounds such as TocH's.l Accordingly, we have constructed a heating apparatus based on irradiation with an infrared ray for the photoelectron spectrometer and have obtained the vertical ionization energies (I;s) of TocH's. Secondly, the geometries of TocH's were optimized with the semiempirical MNDO method owing to computer time constraints in the previous paper.' However, in order to interpret observed features of k i s quantitatively, it is desirable to perform nonempirical molecular orbital calculations at a reliable level of theory. Accordingly, we have carried out ab initio calculations of TocH's and have obtained IK's for the optimized geometries.
Experimental Section d-a-, d-p-, d-y-, and d-&tocopherols were kindly supplied from Eisai Co., Ltd., and were used without further purification. Preparation of dl-tocol was reported in a previous paper.' The He I photoelectron spectra of TocH's were measured with a vacuum-ultraviolet photoelectron spectrometer (JASCO Model PE-1A). The basic setup and the experimental procedures were described in detail The heating apparatus for TocHs (Figure 2) consists of a furnace (Shinku-Riko Co., Ltd., RHLE25) and a controller (Shih-Riko Co., Ltd., HPC-5000.2081). A sample was contained in a Pyrex glass tube of 4-mm o.d., which was placed in a Swagelok stainless-steel tube fitting in., Union) connected to a copper pipe of 6-mm 0.d. The tube fitting and copper pipe were placed in a quartz tube which was fitted to the furnace. The quartz tube was evacuated through an ionization chamber. The ionization chamber was connected to the quartz tube but was not in contact with the copper pipe. Accordingly, although the temperature of the copper pipe increased, the ionization chamber was kept at room temperature. The temperature of the copper pipe was maintained at 176,170,160,150, and 173 O C in the experiments of CY-, /3-, y-, and 6-TocH's and tocol, respectively. Calculation Method and hocedure Ab initio self-consistent-field calculations were camed out with the GAUSSIAN 82 programas The basis set used in the present calculations is STO-3G, which reproduces the experimental results fairly Full geometry optimization was performed by the energy gradient method, and IK's for the optimized geometries were calculated. As described previously,' we replaced the two alkyl groups at the 2-position of TocH by two hydrogen atoms to facilitate the calculation. 0 1992 American Chemical Society
Mechanism of Antioxidant Reaction of Vitamin E
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8185
TABLE I: B and IKin Optimized Geometry, I , EamlkSH,lkSD,'and ksH/ksDof a-,8-, 7-, and 6-TocH's and Tocol CY-TWH P-TOCH T-TOCH &TwH tocol
z,/ev 7.22 7.34 7.35 7.47 7.62
6/deg 21.0 21.4 18.9 20.2 20.1
E,,,/kJ mol-' 18.7 21.1 22.2 25.6 27.1
kSH/M-l s-l 5.12 x 103 2.24 x 103 2.42 x 103 1.00 x 103 5.60 X lo2
Z K F
5.75 5.85 5.86 5.98 6.08
kSD/M-' s-l 2.24 X lo2 1.49 X lo2 1.61 X lo2 6.42 X 10' 3.05 X 10'
ksH/k,D 22.9 15.0 15.0 15.6 18.4
.06 I
a-Tocopherol
8-Tocopherol
Hop 6-Tocopherol
tocopherol
"aR y 3
R- ( C H ~ C H ~ C H ~ C H ) ~ C H C J
Toco 1
Figure 1. Structures of TocH's.
7
6
8
ION1 Z A T ION ENERGY
@
PIPE
... INFRARED
10
Figure 3. Photoelectron spectra near the first ionization threshold of P-TocH (-), 6-TocH (---), and two1 (---).
S E C T I O N A L V I E W OF F U R N A C E
...C O P P E R
9 leV I
(a) a - T o o o p h e r o l
Calod
1 1. 400
1.534
LAMP
4
1.406
.
1.301
h
1.087 -088
H 1 1 5 . 5O
f
118.0
1.540
i
100.2 1 IO. 2 108. 8
117.4
.ASS T U B E (b)
H1. 4 0 2
J
QUARTZ TUBE
Expt I
a-Tocopherol
O
1.388
S1. 1.384
o
o
1. 0 4 0
I . 514 1. 300
I. 517 1.393
t
513
1.300
I. 464
I
Figure 2. Heating apparatus based on irradiation with infrared ray and vacuum-ultraviolet photoelectron spectrometer.
Figure 4. (a) Optimized geometry, in A and deg, of a-TocH. The -21 .Oo; dihedral angles obtained are as follows: C4a-C8a-Ol+, c8a-01
