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J . Phys. Chem. 1986, 90, 2283-2284
pH triggers a long-range electron transfer from the electrode to the outer film through - the inner layer e-(electrode)
+ [b2(py)Ru1110H]2+outer + H+
-
[b~(PY)RU'*OH21~+outer (4) as signalled by the cathodic current spike. The energetics of the process and the sense of the electron flow are depicted in the inset of Figure 2. After letting the electrode stabilize for 30 s, the reverse charge transfer was achieved by jumping the pH back to 5 by the addition of NaOH to the external solution. The anodic current spike arises from long-range electron transfer through the inner film to the electrode via the net reaction shown in eq 5, which is also illus-
The results presented here demonstrate that long-range electron transfer through - a -polymeric film can be induced by pH changes in a spatially separated second film. Conversely, the-pH change induced within the second film by either long-range oxidation (reaction 5 ) or reduction (reaction 4) could provide the basis for a device for coupling simple electron transfer to more complex reactions having a proton demand. This is an essential feature of biological redox membranes whose function has been described by the chemiosmotic hypothesis of M i t ~ h e l l . ~In the bilayers described here, oxidation within a second film in which facile H+ access to an external solution does not exist could lead to a local proton buildup providing a basis for coupling electron transfer to reactions such as hydrolyses where there is a catalytic or stoichiometric proton demand.
~~
trated in Figure 2. The current is considerably smaller in magnitude because of the loss in electroactivity-for [ ( b ~ y ) ~ ( p y ) RuI1OH2l2+in Nafion at pH 5.537
(9) (a) Mitchell, P. Eur. J . Biochem. 1978, 95, 1. (b) Mitchell, P. Science 1979, 206, 1148.
Distance Dependence of Electron Transfer from Liposome-Embedded (AI kanephosphocholine-porp hinato)zinc Eishun Tsuchida,* Masao Kaneko; Hiroyuki Nishide, and Mikio Hoshino' Department of Polymer Chemistry, Waseda University, Tokyo 160, Japan (Received: December 12, 1985; In Final Form: March 20, 1986)
(Alkanephosphocholine-porphinato)zinc forms a geometrically well-defined bilayer liposome with phospholipid. Electron transfer from the liposome-embedded (porphinato)zincs with different alkyl chain lengths to methylviologen present in the outer bulk solution is measured by laser flash photolysis: the intermolecular electron transfer was observed only when the porphyrin plane is located within 12 A from the surface.
Introduction There is currently a great interest in the separation distance between redox centers when electron transfer occurs.14 Evidence has been given that electron transfer in biological molecules takes place over long distances of the order of 10 8, via nonadiabatic pathways. Many of the recent studies on electron-transfer distances were concerned with intramolecular process which can avoid the uncertainties associated with intermolecular electron transfer. Relatively few studies have been devoted to the distance problem in intermolecular electron transfer because it is rather difficult to prepare a geometrically well-established electron-transfer system. A few earlier examples of intermolecular electron transfer include photoinduced electron transfer between a donor and acceptor embedded in monolayer assemblies.5,6 Their studies, however, represent electron transfer by emission quenching, leaving an ambiguity in discussing electron transfer. Photoinduced electron transfer between porphyrins and ferricyanide at a black lipid membrane-water interface' and electron exchange between (4-alkylpyridine)pentaammineruthenium ions separated by vesicle bilayer* have been reported, but the experiments were carried out without changing the distance between the redox centers. One approach to distance-dependent intermolecular electron transfer was a study of electron exchange between hexakis(alky1 isocyanide)manganese c o m p l e ~ e s . ~ Intermolecular electron transfer is receiving more and more attention in conjunction with mimicking highly efficient biological 'Solar Energy Research Group, The Institute of Physical and Chemical Research, Wako 351-01, Japan.
0022-3654/86/2090-2283$01 SO10
reaction systems and with designing artificial energy conversion systems. It is important to obtain information about distance dependence of intermolecular electron transfer between two kinds of redox centers. Recently two of the authors have synthesized, by paying attention to stereostructure and hydrophilic-hydrophobic balance of the porphyrin, a novel and amphiphilic (porphinato)metal derivative having four phosphocholine groups, [5,10,15,20-tetra(cu,cu,cu,cu-o-[2',Y-dirnethyl-20'-((2''-(trimethylammonio)ethyl) phosphonatoxy)alkaneamido]pheny1)porphinatolmetal The (porphinato)metal has high compatibility with phospholipids and forms very stable lipid bilayers.1°J2 The porphyrin plane was embedded in the hydrophobic region of the bilayer, and the distance of the porphyrin plane from the wall surface is a function of the alkyl chain length of the (porphina(1) Nocera, D. G.; Winkler, J. R.; Yokom, K. M.; Bordignon, E.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 5145. (2) Li, T. T.-T.; Weaver, M. J. J . Am. Chem. SOC.1984, 106, 6107. (3) Isied, S . S.: Kuehn. C.; Worosila, G. J . Am. Chem. SOC.1984, 106, 1722. (4) Miller, J. R.; Calcaterra, L. T.;Closs, G. L. J . Am. Chem. SOC.1984, 106, 3047. ( 5 ) Mobius, D. Ber. Bunsenges. Phys. Chem. 1978, 82, 848. (6) Kuhn, J. J . Photochem. 1979, 10, 111. (7) Lee, L. Y . C.; Hurst, J. K. J . Am. Chem. SOC.1984, 106, 7411. (8) Mauzerall, D. Life Sci. Res. Rep. 1979, Z2, 241. (9) Nielson, R. M.; Wherland, S . J . Am. Chem. SOC.1985, 107, 1505. (10) Tsuchida, E.; Nishide, H.; Yuasa, M.; Hasegawa, E.; Matsushita, Y. J . Chem. SOC.,Dalton Trans. 1984, 1147. (1 1) Matsushita, Y.; Hasegawa, E.; Eshima, K.; Tsuchida, E. Chem. Lett. 1983, 1387. (12) Tsuchida, E. Ann. N.Y. Acad. Sci. 1985, 446, 429.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 1I , 1986
to)metal. Thus we have chosen as the reaction centers of electron-transfer system the (porphinato)zinc(II) derivative (ZnP, 1) embedded in phospholipid bilayer and methylviologen (MV2+) present in the outer bulk solution and measured electron transfer from the excited ZnP to the MV2+ by a laser flash photolysis technique to study the distance dependence of the electron transfer.
Experimental Section [ 5,10,15,20-Tetra(a,cu,a,a-o[ 2’,2’-dimethyl-20’-( (2”-(trimethylammonio)ethyl)phosphonatoxy)alkaneamido]pheny1)porphinato]zinc(II)’s (la-d, n = 4, 6, 10, and 18) were synthesized as reported previously.lO,l Liposome-embedded 1 was prepared
I
0 I
I
la b c d
n 4 6 10 18
by using dipalmitoylphosphatidylcholine(DPPC) ([DPPC] / [ZnP] = 50) and pH 7 phosphate buffer by normal thin film methods and subsequent ultrasonication. The concentration of the liposome-embedded ZnP was 0.1 m M and the absorption at the laser excitation wavelength (532 nm) was ca. 0.1. 0.1-2 mM MV2+ was present in the outer bulk solution. Laser flash photolysis was carried out under argon atmosphere by the second harmonic (532 nm) of Nd:YAG laser, Model H Y 500 from JK Lasers Ltd.I3 The flash intensity was ca. 100 mJ cm-2 and the pulse duration was 20 ns. The quenching of the triplet state of ZnP was monitored at 470 nm and the formation of viologen cation radical at 603 nm.
Results and Discussion Incorporation of ZnP in the bilayer of liposome was first confirmed by gel permeation chromatography (Sepharose column) monitored by the absorptions at 300 and 424 nm based on DMPC and ZnP, respectively. The elution curves coincide with each other, which means that ZnP is included in the liposome. The solution was also checked by ultracentrifugation (40000 rpm, 1 h). The (13) Hoshino, M.; Imamura, M.; Watanabe, S.; Hama, Y . J . Phys. Chem. 1984, 88, 45.
Letters TABLE I: Rate Constant ( k ) for Viologen Cation Radical Formation As Monitored by Laser Flash Photolysis at 603 nm (porphinato)zinc n distance,c A 10-9k: M - I s-I la 4 9 2.26 lb 6 12 1.77 IC 10 17 MV’. not detectedb Id 18 21 MV+. not detectedb “ T h e data for MV2’ = 0.5 mM. The bulk MV2+ concentration was varied between 0.1 and 2 mM. * T h e formed MV’. concentration was of those for l a and l b during the time course of 20 times less than as long as those fo l a and l b after the excitation. cEstimated distance from porphyrin plane to bilayer surface.
supernatant did not contain both DPPC and ZnP. This also indicated that ZnP is included in the liposome. From transmitting electron microscopy, the liposome-embedded ZnP looks like a unilamellar or single-walled liposome with diameter ca. 350 A. The fluorescence spectrum of the liposome-embedded ZnP, with emission maxima at 605 and 658 nm using excitation at 413 nm, agreed with those in alcohols. The fluorescence intensity for the liposome-embedded ZnP corresponded to that in butyl alcohol. This result indicates that ZnP is molecularly dispersed in the bilayer and surrounding by a hydrophobic environment even in the aqueous medium. The estimated distances from the porphyrin plane to the polar groups which are located at the bilayer surface are presented in Table I and were calculated from the CPK model of 1. It is assumed that the polar groups are located mixed with each other around the bilayer surface rather than oriented along the alkyl chain perpendicular to the wall. The fluorescence (emission peaks at 605 and 658 nm for 1) was not quenched by MV2+ at all. The T-T absorption spectrum of 1 measured by laser flash showed its maximum at 400 nm. The triplet was quenched by MV2+for the liposome-embedded l a and l b but was not for the liposome-embedded ICand Id. The rate of viologen cation radical formation was measured by laser flash photolysis with 532-nm excitation by monitoring the absorption increases at 603 nm, and the results are shown in Table I. The excitation of l a and l b induced cation radical formation, while ICand Id gave no cation radical of MV2+. The formation of cation radical was also observed at other several wavelengths between 590 and 700 nm. A corresponding decrease rate in the triplet absorption of ZnP at 470 nm was observed for l a and l b but was negligibly small for IC and Id. It is interesting to note that the electron transfer occurs only for the liposome-embedded ZnP whose center is estimated to be located within 12 A from the bilayer surface. These results are consistent with the findings for the intramolecular electron transfer which has been reported to occur at the distance of about 10 A. Since fast formation of cation radical was not observed within the laser pulse duration, the mechanism that MV2+is adsorbed near the ZnP in the bilayer wall and reacts immediately after triplet formation is excluded. The rate constants in Table I, which are of the order of diffusion-controlled value, also show that MV2+ in the bulk solution diffuses to the outer surface of the bilayer and then reacts there. This interpretation is supported by the fact that the rate of cation radical formation for l a was proportional to the MV2+concentration in the bulk. Detailed mechanistic studies are now underway. Acknowledgment. This work was partially supported by a Grant-in-Aid for Special Project Research from the Ministry of Education, Japan. We thank Dr. David Mauzerall, the Rockefeller University, New York, for helpful discussion.
