2112
J. Phys. Chem. 1992,96, 2112-2116
energy conformation. Moreover, in the amorphous region of the polymer film, it is not unreasonable to assume that the boundary conditions placed on the film by the experimental growth conditions will force neighboring polymer strands into conformations which represent local, rather than global, energy minima. This cofacial arrangement of neighboring PET unit cells could represent such a local minimum. Clearly, a complete mapping of the potential energy surface for the interaction of PET dimers is required. A second possibility arises from the demonstrated flexibility of the polymer chains about the saturated ethylenic linkage: namely, that chains of a sufficient length could self-interact, with unit cells at one end of the chain strongly interacting with unit cells at the other end. Preliminary molecular dynamics computations have indicated that chains eight unit cells in length are insufficient to generate a self-interaction. Chains of 16 unit cells can generate significant self-interactions, however, although they are still not sufficient to induce the delocalization of electronic states necessary to generate the observed spectral features (i.e,, the emissions at 368 and 534 nm). Work in this area is ongoing,
and the details will be published fully elsewhere.I*
v.
synopsis
Classical molecular dynamics simulations were performed to characterize the dynamic structure and intermolecular interactions in crystalline poly(ethy1ene terephthalate). Moreover, the spectroscopically parametrized CNDO/S3 molecular model was used to determine the photophysical properties of dimer unit cells at several, dynamically determined, conformations in an effort to understand the nature of the 368- and 534-nm emissions. These computations indicate that these emissions do not arise from dynamic fluctuations of crystalline polymer chains and must originate in the amorphous regions of the polymer films.
Acknowledgment. We are grateful to Dr. C. B. Duke for generously allowing the use of his CNDO/S3 computer programs. Registry NO. PET (SRU), 25038-59-9. (18) LaFemina, J. P.; Gupte, V., in preparation.
Photoreduction of Alkylmethylviologms wlth a-Tocopherol in Dioctadecyldimethylammonium Chlorfde Vesicles Masato Sakaguchi,t Hero Baglioni,t and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: August 13, 1991)
Electron spin resonance (ESR) spectroscopy is used to detect the photoreduction yield of alkylmethylviologens (AV2+) in rapidly frozen dioctadecyldimethylammonium chloride ( D O D A C ) vesicles containing concentrations of a-tocopherol (major component of vitamin E) from 0 to 23 mol %. The observed radicals are alkylmethylviologen cation radicals (AV+) from photoirradiated AV2+ in D O D A C vesicles without a-tocopherol. For 1-3 mol % a-tocopherol, the major radical is A V + and the minor radical is a neutral free radical of a-tocopherol (EO) which is formed by photoinduced conversion from the a-tocopherol cation radical (EH') with D O D A C vesicles acting as proton scavengers. The total ESR intensity increases with an increase of the alkyl chain length of AV2+. The A V + intensity increases slightly with increasing a-tocopherol concentration. For over 9 mol % a-tocopherol, the major radical becomes EO, and at 17 mol % EO alone is observed. This is explained by acceleration of the photoreduction of AV2+ to AV' by electrons released from a-tocopherol and further photoreduction of A V + to AV, which is not detected by ESR spectroscopy. The photoyield for 23 mol % a-tocopherol in D O D A C vesicles without AV2+ is about 2-fold more than that in hexane solution. This enhancement of photoyield suggests that D O D A C may act as a proton scavenger and compartmentalize a-tocopherol to minimize back electron reaction.
Introduction Unilamellar vesicles formed from phospholipids are being used for molecular compartmentalization.'J Veside-compartmentalized, photoionizable molecules have been used as model systems for artificial photosynthesis to achieve net photoinduced charge ~eparation.~Previous work in this laboratory has dealt with enhancement of photoionization by modification of the vesicle interface and interior structure. Such control factors include the phospholipid headgroup type,4 the alkyl chain length of the phospholipid: the interface charge of the phospholipid$@ and the horporation of surface-active additives such as salts: alcoh o l ~and , ~ cholesterol.1*'3 A related approach to control the net photoefficiency is to add variable-length alkyl chains to the photoactive m~lecule.~-'~ This is a control method for the location of the photoactive moiety relative to the vesicle interface. Of analogous interest is the role that electron donors play in the photoreduction of alkylmethylviologens in vesicle solutions. Electron spin resonance (ESR) has been used to monitor the net photoyields in rapidly frozen vesicle solution^.^-'^ 'On leave from Ichimura Gakuen Junior College, Inuyama, Japan. 'Permanent address: Department of Chemistry, University of Florence, 50121 Florence, Italy.
In the present study the effect of the alkyl chain length of alkylmethylviologens and the addition of a-tocopherol (major component of vitamin E) have been investigated by ESR for the (1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (2) Kalyanasundaram, K. Photochemistry in Microheterogeneous Sysrems; Academic: New York, 1987. (3) See for example: (a) Chamulpathi, V. G.;Tollin, G. Photochem. Photobiol. 1989,49,61. (b) Youn, H. C.; Baral, S.; Fendler, J. H. J . Phys. Chem. 1988, 92, 6320. (c) Patterson, B. C.; Thompson, D. H.; Hurst, J. K. J. Am. Chem. SOC.1988,110,3656. (d) Kevan. L. In Photoinduced Electron Transfer Parr B Fox, M. A., Chanon, M., Eds.;Elsevier: Amsterdam, 1988; pp 329-384. (4) Hiff, T.; Kevan, L. J . Phys. Chem. 1988, 92, 2069. (5) Hiff, T.; Kevan, L. J . Phys. Chem. 1988, 92, 3982. (6) Li, A. S.W.; Kevan, L. J . Am. Chem. Soc. 1983, 105, 5752. (7) Lanot, M. P.; Kevan, L. J . Phys. Ckem. 1989, 93, 998. (8) Sakaguchi, M.; Hu, M. J . Phys. Chem. 1990, 94, 870. (9) Hiff, T.; Kevan, L. J . Phys. Chem. 1988, 93, 3227. (IO) Hironlitsu, I.; Kevan, L. J . Am. Chem. Soc. 1987, 109, 4501. (11) Hiff, T.; Kevan, L. J . Phys. Chem. 1989, 93, 1572. (12) Lanot, M. P.; Kevan, L. J . Phys. Chem. 1989, 93, 5280. (13) Sakaguchi, M.; Kevan, L. J . Phys. Chem. 1991, 95, 5996. (14) Colaneri, M. J.; Kevan, L.; Thompson, D. H. P.; Hurst, J. K. J . Phys. Chem. 1989, 93, 5280. (15) Sakaguchi, M.; Kevan, L. J . Phys. Chem. 1989, 93, 6039.
0022-365419212096-2772$03.00/0 0 1992 American Chemical Society
Photoreduction of Alkylmethylviologens
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2773
photoreduction of alkylmethylviologens and the photoinduced reaction of a-tocopherol in a cationic vesicle system.
Experimental Section Methylviologen dichloride hydrate (MV’) was purchased from Aldrich and used without further purification. The alkylmedithylviologens (AV2+) N-hexyl-N’-methyl-4,4’-bipyridium chloride (c6v2+), N-dodecyl-N’-methyl-4,4’-bipyridinium dichloride (CI2V2+),and N-hexadecyl-N’-methyl-4,4’-bipyridium dichloride (c]6v2+) were generously provided by D. H. P. Thompson and J. K. Hurst of the Oregon Graduate Center. Deuterium oxide (D20) was obtained from Aldrich (99.8 atom % D). a-Tocopherol was purchased from Aldrich. Dioctadecyldimethylammonium bromide (DODAB) was purchased from Eastman Chemicals and purified by recrystallization from acetone. A methanol/chloroform (70:30 v/v) solution of DODAB was passed twice through an ion exchange resin type AG2X8, 20-50 mesh from Biorad Laboratories. The eluent containing dioctadecyldimethylammonium chloride (DODAC) was evaporated, and the residue was recrystallized two times from acetone/water (953 v/v solution) and evacuated for 2 days at room temperature. Vesicle solutions1e18were prepared as follows. Chloroform and surfactant solutions, containing a-tocopherol if added, were evaporated at 328 K for 1 h under nitrogen gas flow and evacuated at 328 K for 1 h. The resulting films were sonicated in D 2 0 with a Fisher Model 300 sonic dismembrator operating at 30% relative output power with a 4-mm 0.d. microtip under nitrogen atmosphere. Unilamellar DODAC vesicles in D 2 0 were formed by sonication for 15 min at 326 f 2 K.15 After sonication, each AV2+ in D 2 0 solution was added to the DODAC solution in D20, introduced into 2-mm-i.d. by 3-mm-0.d. Suprasil quartz tubes, allowed to stand for 2 h at room temperature, then rapidly frozen by insertion into liquid nitrogen, and stored at 77 K. The respective concentrations of AV2+ and DODAC were 0.3 and 27 mM. Photoirradiation was carried out for 10 min at 77 K with a 300-W Cermax Xenon lamp (LX 300 UV) with a power supply from ILC Technology. The light passed through a 10-cm water filter and a Corning 7-54 glass filter with a transmission range of 240 < X < 400 nm. All ESR spectra were recorded at X-band with a Bruker ESP 300 spectrometer with 100-kHz field modulation at 77 K and 0.2-mW microwave power to avoid power saturation. The magnetic fields were measured with a Varian E-500 nuclear magnetic resonance gaussmeter, and the microwave frequency in the 9-GHz range was directly measured with a Hewlett-Packard 5350 B microwave frequency counter. A simulation program for superposition of two spectra from two kinds of radicals with anisotropic g and hyperfine tensors was developed with a first-order perturbation treatment using Iwasaki’s formalismlg for the spin Hamiltonian. Quadrupole and zero-field terms were ignored. It was assumed that the g and hyperfine tensors are coaxial and that the line shape function is Gaussian. ESR spectral simulations of power patterns for an anisotropic g tensor were carried out using a Melcom-Cosmo 700 I11 computer (Mitsubishi Electric Co.) in the computer center of the Nagoya Institute of Technology. Results ESR spectra from frozen solutions of AV2+in DODAC vesicles with and without a-tocopherol were obtained by photoirradiation at 77 K for 10 min, at which time plateau yields were achieved. The FSR spectra from photoirradiated MV2+in DODAC vesicles with and without a-tocopherol are shown in Figure 1. The spectra for C6V2+,C12V2+,and c16v2+(Figure 2) are similar. The singlet spectra from each irradiated AV2+ in DODAC vesicles without a-tocopherol have g = 2.0037 (Figures l a and 2a). The samples (16) (17) 163. (18) (19)
MV2t/DODAC/a-Tocopherol a -Tocopherol
(01 0 mol %
-j v
q = 2 0050
Figure 1. ESR spectra observed at 77 K from photoirradiated MVZt in DODAC vesicles with a-tocopherol: (a) 0 mol %, (b) 1 mol %; (c) 3 mol %; (d) 9 mol %; (e) 17 mol %; ( f ) 23 mol %. Intensities of the spectra are not normalized. C,,VZt/DODAC/a
-Tocopherol Tocopherol
Q-
(a) 0 mol %
(b) I
(c) 3
(d) 9
q = 2 OO5OJ
Figure 2. ESR spectra observed at 77 K from photoirradiated C,6V2t in DODAC vesicles with a-tocopherol: (a) 0 mol %; (b) 1 mol %; (c) 3 mol %; (d) 9 mol %; (e) 17 mol %; (f) 23 mol %. Intensities of the spectra are not normalized.
appeared blue, which is a characteristic color of AV+.15y20,21No ESR signal was observed at 77 K from unirradiated samples. The ESR spectra from MV2+and C16v2+in DODAC vesicles with a-tocopherol are shown in Figures 1b-f and 2b-f (spectra are not normalized). The ESR spectra from AV2+ in DODAC vesicles with a-tocopherol (0-3 mol W ) show a major singlet. The samples were blue. In contrast, at higher concentration ranges of a-tocopherol (9-23 mol W ) , the ESR spectra show an apparent quintet with a hyperfine coupling of about 0.5 mT and an apparent g value of 2.0050. The sample was yellow. Small lines with hyperfine couplings of about 2.2 mT are superimposed on each side of the singlet spectrum (Figure lb,c, shown with arrows). The lines are more clearly indicated in the in DODAC vesicles with higher conESR spectra from c]6v2+ centrations of a-tocopherol (Figure 2a-q indicated with arrows). Figure 3 shows the ESR spectrum (Figure 3a) from MV2+(1 mol %) in DODAC vesicles without a-tocopherol and the simu-
Huang, C. Biochemistry 1969, 8, 344. Oettmeirer, W.; Norris, J. R.; Katz, J. J. Z . Naturforsch. 1976, 31c, Lim, Y . Y.; Fendler, J. H. J . Am. Chem. SOC.1979, 101, 3378. Iwasaki, M . J . Magn. Reson. 1974, 16, 417.
(20) Jonson, C. W.; Gutowsky, H. S. J . Chem. Phys. 1963, 39, 58. (21) Evans, A. G.; Evans, J. C.; Baker, M . W. J . Chem. SOC.,Perkin Trans. 2 1977, 1787.
2774 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
Sakaguchi et al. MV'*/DODAC/a-Tocopherol
MV*+/DODAC
A
( 3 mol %)
A' H
lOmT
t
H
g = 2 0036
t
9.20036
Figure 3. (a) ESR spectrum observed at 77 K from photoirradiated MV*+ in DODAC vesicles without a-tocopherol and (b) its simulated spectrum (dashed line).
Figure 5. (a) ESR spectrum observed at 77 K from photoirradiated MV2+ in DODAC vesicles with 3 mol 96 a-tocopherol and (b) its simulated spectrum (dashed line).
DODAC/a-Tocopherol(23 mol X )
1
h I .OmT H
I
t g = 2.0036 Figure 4. (a) ESR spectrum observed at 77 K from photoirradiated 23 mol 9%a-tocopherol in DODAC vesicles without AV2' and (b) its simulated spectrum (dashed line).
lated spectrum (Figure 3b) obtained by spectral simulation of MV+ using the reported hyperfine couplingsZoof (IN = 0.423 mT, uH(CH3) = 0.399 mT, a ~ ( 2 = ) 0.133 mT, a ~ ( 3 = ) 0.157 mT, g, = 1.9993, and g,,= 2.01 18, where the position numbering is as follows. 3
2
MV2+
CgV2+
q2v2+
C,&2+
Alkyl Chain Length Qwe 6. Total photoyields from AV2+ are plotted against the alkyl chain length of alkylmethylviologensin DODAC vesicle with a-tocopherol as follows: (v)0 mol 96; (A)1 mol % ( 0 )3 mol 96; (A) 9 mol 96; ( 0 )17 mol %; (V) 23 mol%. Photoyields from ( 0 )1 mol % and (e) 23 mol 96 a-tocopherol in hexane solution with no AVZt or DODAC vesicles and from (e)23 mol S a-tocopherol in DODAC vesicles without AV2+ arc
also plotted.
The ESR spectra from MV2+ (1 mol 5%) in DODAC with a-tocopherol (1-23 mol 5%) were simulated by superimposing simulation spectra of MV+ and EO using the hyperfine couplings and g values in Figures 3b and 4b, except for their relative in-
tensities. Each intensity is obtained by double integration of the simulation spectrum normalized to the total intensity of CI6V2+ in DODAC vesicles without a-tocopherol. Therefore the photoyields can be compared directly. An example of these simulations is shown in Figure 5 for 3 mol 5% a-tocopherol for an MV+/EO intensity ratio of 0.65:0.15. The corresponding ratio for 1 mol % is 0.690.07, for 9 mol % 0.31:1.24, and for 17 and 23 mole % 0.O:l.O or only the EO radical. The ESR spectrum from 0.3 mM (corresponding to 1 mol %) a-tocopherol in hexane with no AV2+ or DODAC vesicles was a singlet simulated best by the parameters for the parent cation EH+ Of UH(CH~)= 0.24 mT,23and UH(CH~)= 0.12 mT,23 g = 2.0010, and g,,= 2.0096 than by those for the parent anion. h It is clear that this singlet is different from the quintet of the EO radical (Figure 4). Total photoyields from AV2+ in DODAC vesicles with and without a-tocopherol are determined by double integration of the ESR spacua, normalized to the yield of C16V+in DODAC vesicles without a-tocopherol. These yields are plotted against the alkyl chain length (Figure 6) and against the concentration of a-tocopherol (Figure 7). The error bars in the figures were determined as the average deviations in duplicate or triplicate experiments. If no error bars are shown, the deviations were within the symbol size. The total yields increase with alkyl chain length in the a-tocopherol concentration range 0-9 mol % (Figure 7). For over
(22) Tsuchiya, J.; Niki, E.; Kamiya, Y. Bull. Chem. SOC.Jpn. 1983, 56, 229.
(23) Depew, M. C.; Craw, M. T.; MacCormik, K.; Wan, J. K. S. J . Photochem. Phoiobiol., B 1987, I , 229.
U
U
The ESR spectrum (Figure 4a) from a-tocopherol (23 mol %) in DODAC vesicles without AV2+and the simulation (Figure 4b) of the neutral chromanoxy free radical2*of a-tocopherol (EO) are obtained by using-the reported hyperfine couplings22of UH(CH3, 9) = 0.602 mT, aH(CH3, 10) = 0.458 mT, aH(CHp, 11) = 0.094 mT, and aH(CH2,4) = 0.148 mT and anisotropic g values g, = 2.0065 and g,,= 2.0035 where the numbering system is as follows. 11
io
9
The Journal of Physical Chemistry, Vol. 96, No. 6 , 1992 2775
Photoreduction of Alkylmethylviologens
*O . -l
0
IO
20
a-Tocopherol (mol %) Figure 7. Total photoyields from AV2+ ( ( 0 )MV'; (0) C6V2+;(A) CIZV2+;(0)&V2+) in DODAC vesicles are plotted against the a-tocopherol concentration. Photoyields from ( 0 ) 1 mol % and (a) 23 mol % a-tocopherol in hexane solution with no AV2+ or DODAC vesicles and from (e) 23 mol % a-tocopherol in DODAC vesicles without AV2+are also plotted.
9 mol 9% a-tocopherol, the total yields remain constant at about 1.5 arbitrary units and are independent of the alkyl chain length of AV2+ (Figure 6). The yield of 23 mol % a-tocopherol in DODAC vesicles with no AV2+is about 1.55 units (Figures 6 and 7), which is identical with the yields of AV2+ (1 mol %) in DODAC vesicles with a-tocopherol (9-23 mol %). In contrast, the yield of a-tocopherol (23 mol %) alone in hexane solution is about 0.75 units (Figures 6 and 7). The yield of a-tocopherol (1 mol %, identical concentration as AVZ+)is about 0.55 units (Figures 6 and 7), which is lower than that of MV2+ in DODAC vesicles without a-tocopherol, which is the minimum yield.
Discussion Effect of a-Tocopherol Addition on the Process of AVZ+to AV Photoreduction in LWDAC Vesicles. The apparent g value (2.0037) from photoirradiated AV2+ in DODAC vesicles with a-tocopherol (0-3 mol %) coincides with that of AV+ reported el~ewhere.'~J~ The simulated spectrum of MV+ (Figure 4b), given by using the hyperfine couplings reported,20coincides with the observed spectrum in Figure 4a. Previous ESR studies have shown that substitutions onto the viologen moiety have relatively little effect on the unpaired spin distribution in the viologen cation radical.20s21No ESR signal was observed from unirradiated AV2+ with and without a-tocopherol in DODAC vesicles. The singlet spectra (Figures l a and 2a) are thus assigned to the alkylmethylviologen cation radical, AV+. The simulated spectrum of EO (Figure 4b) with anisotropic g values coincides with the spectrum (Figure 4a) observed from a-tocopherol (23 mol %) in DODAC vesicles without AV2+,which shows an apparent quintet with about 0.5-mT hyperfine coupling and an anisotropic g value. The simulated spectrum of the EO radical with an anisotropic g value seems reasonable, because the alkoxy-type radical, RCH20, produced in X-irradiated single crystals of nucleosides and nucleotides has large anisotropic g values. The ranges of principal g values are g, = 2.054-2.093, gint= 2.005-2.009, and gmin= 1.95-2.000.24 The observed spectrum, therefore, is assigned to the neutral free radical of a-tocopherol, EO. The spectra observed from MV2+ (1 mol %) in DODAC vesicles with a-tocopherol (17 and 23 mol %) coincide with the simulated spectrum of EO (Figure 4b), except for the intensities. The good
a-Tocopherol (mol %) Figure 8. Photoyields of MV' and EO, obtained from spectral simulations, versus a-tocopherol concentration: (0)MV'; ( 0 )EO; (A)MV+ and EO.
agreement between the observed and simulated spectra indicates that the observed spectrum is composed of EO alone even in the presence of MV2+. The lack of MV+ in the spectrum suggests that secondary photoreduction of MV+ to MV, which is unobservable by ESR, occurs in DODAC vesicles under photoirradiation. Positions of hyperfine couplings marked by arrows in Figures lb,c and 2a-c coincide with an octet spectrum that has been assigned to a secondary alkyl surfactant radical denoted DAC,*3l3 which is produced by radical conversion from AV+ to DODAC under photoirradiation. The small lines are assigned to the DAC radical produced by radical conversion from AV+ under photoirradiation.8J3 The DAC radical intensity increases with increasing a-tocopherol concentration up to 3 mol % in the DODAC vesicles (Figures lb,c and 2a-c). This increase of the DAC radical is explained by an increase of the AV+ due to electrons released from a-tocopherol under photoirradiation. For over 9 mol % a-tocopherol, the DAC radical is unobservable. This disappearance can be explained by a decrease of the AV+ concentration due to secondary photoreduction of AV+ to AV, which is accelerated by electrons released from a-tocopherol under photoirradiation. The simulated superimposed spectra of MV+ and EO are in good agreement with the observed spectra except for the wings which show contributions from the DAC radial, which is not included in the simulation. The MV+ yield slightly increases with an increase of a-tocopherol and then decreases with a further increase of a-tocopherol until it is not observable (Figure 8). MV+ cannot migrate at 77 K. Therefore, the loss of MV+ is suggested to be due to secondary photoreduction of MV+ to MV by electrons from a-tocopherol to produce EH+. EO is not observed from a-tocopherol in the absence of DODAC vesicles. However, in the presence of DODAC vesicles, EO is observed instead of EH+ (Figure 4). It has been reported that EO is obtained by deprotonation of EH+ in the presence of a proton scavengerZ5or in an acid aqueous solution.26 The DODAC vesicular solutions may be slightly acidic due to hydrolysis so that EH+ is deprotonated to produce EO. Also the compartmentalization of the vesicle solutions may inhibit back proton transfer to help stabilize EO. We thus suggest that the photoreduction of AV2+in DODAC vesicles with and without a-tocopherol occurs as follows. (25) Eloranta, J.; Mamalainen, E.; Salo, E.; Makela, R.; Kekalainen, U.
(24) Bernhard, W. A.; Close, D. M.; Hiitterman, J.; Zehner, H. J . Chem. Phys. 1977, 67, 1211.
Acta Chem. Scand. 1983, A37, 383.
(26) Svanholm, U.; Bechgaard, K.; Parker, V. D. J . Am. Chem. SOC.1974,
96, 2409.
2776 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
+
AV2+ e(C1) AV+
+ DODAC-H
hv
AV+
AV+-H hu
+
AV2+ e(E)
EH+
hv
vts~clesolution
AV+
( i ) a-Tocopherol (0 mol
e
+ DAC
+ e(E)
-+ + -
a-tocopherol F=r EH+
(1)
AV+
EO
AV
(2) (3) (4)
H+
(5)
(6)
5%).
AVZ+reacts with an electron, e, which probably comes from the chloride counterion to produce A V (eq 1). The radical conversion from AV+ to the DAC radical occurs in the presence of DODAC vesicles under photoirradiation (eq 2). (ii)a-Tocopherol(1-3 mol %), Photoreduction of AV2+ to AV+ is accelerated by electrons released from a-tocopherol (eqs 3 and 4). The AV+ concentration increases with an increase of a-tocopherol concentration, and the radical conversion from AV+ to the DAC radical is accelerated by the high concentration of AV+ (eq 2). EH+is produced by electrons released from a-tocopherol (eq 3) followed by deprotonation of EH+ in the vesicle solution under photoirradiation to result in EO (eq 5). (iii) a-Tocopherol (9-23 mol %). AV+ produced by primary photoreduction of AV2+(ap 1 and 4) reacts with electrons releastd from a-tocopherol at high concentrations (eq 3) to give AV (eq 6), which is not detected by ESR. The radical is depressed by the low concentration of AV+. Finally, EH+converts to EO by H+transfer to the vesicle solution under photoirradiation (q5).
E f W of a-Tocoplerol Addition 011 the Photoreduction Yield ia DODAC Vesidea Yields of MV+, EO, and the sum of MV+ and EO (total yield) from MV2+in DODAC vesicles with a-tocopherol (+3 mol %) are obtained by double integration of the simulated spectra and normalized to the yield of CI6V2+in DODAC vesicles without a-tocopherol (Figure 8). These yields can be compared. The yield of MV+ in DODAC
of
Additions and Corrections vesicles increases slightly with increasing a-tocopherol to about 3 mol % a-tocopherol and then decreases with further increase of a-tocopherol. The initial increase of the MV+ yield is due to an additional photoreduction process by electrons released from a-tocopherol (eq 3). Above 9 mol % a-tocopherol, the decrease of the MV+ yield is due to its conversion from MV+ to MV. The yield of the EO in DODAC vesicles increases with a-tocopherol until about 17 mol 9%. This increase of EO yield suggests that the deprotonation of EH+ is promoted by DODAC molecules under photoirradiation. The total yields from AV2+in DODAC vesicles with a-tocopherol increase with increasing alkyl chain length of AVZ+at a-tocopherol concentrations up to 3 mol 96 (Figure 6). This increase is greater for AV2+ with longer alkyl chains due to a location in less hydrated r e g i ~ n s , which ~ ~ . ~ promotes ~ an increase of the DAC radical. For 9 mol 46 and greater a-tocopherol, the total yields remain almost constant versus alkyl chaii length. The constant yield may be due to a balance of electron back reaction via eq 3 and the scavenging effects of DODAC molecules. The total yields versus a-tocopherol concentration (Figure 8) show an increase to 9 mol % and then an approximate plateau. The yield fmm a4cqherol(23 mol 96) only in hexane is lower than that from a-tocopherol (23 mol 96) in DODAC vesicles (Figures 6 and 7). This suggests that DODAC vesicle solutions act to stabilize the EO radical. These results do demonstrate that one can "tune" the magnitude of the alkyl chain length effect of AV2+on the total photoreduction yield by the addition of a-tocopherol in DODAC vesicles. However, the tuning range is rather narrow. This limited tuning range is related to the secondary photoreduction of AV+ to AV and the photoinduced conversion of a-tocopherol.
Acknowledgment. We thank Professor H.Kashiwabara for use of the Melcom Cosmo 700 I11 computer at the Nagoya Institute ofTechndogy. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US.Department of Energy. P.B. thanks CNR for partial financial support.
ADDITIONS AND CORRECTIONS 1991, Volume 95
Kevin AsBley,* Frederick Weinert, M.beab C.Samant, H.W, and M.R.PMlpott*: Infrared Spectroelectrochemicl Study of Cyanide Adsorption on Palladium Surfaces. Pages 7411 and 7412. Figures 4 and 6 were inadvertently switched. The figure captions are correct, but the figures themselves should be exchanged.
