3982
J. Phys. Chem. 1988, 92, 3982-3985
the strong benzene-metal interaction and the resulting KekulE distortion detected by LEED. Benzene chemisorbed on the Pt( 111) crystal is less asymmetrically distorted, exhibiting a more uniform expansion of the ring. This structure may suggest a benzene intermediate on the metal surface that can desorb intact at the higher temperatures and pressures of the catalytic r e a ~ t i o n . ~ The benzene on Pd( 11 1) surface was found by LEED to be weakly distorted. On this surface, a higher activation energy of decomposition is apparent: the H, desorption maximum due to benzene decomposition is higher on P d ( l l 1 ) (-555 K) than R h ( l l 1 ) (-490 K42) or P t ( l l 1 ) (-545 K44*45).This seems to correlate with the weaker benzenepalladium interaction observed by LEED. Molecular benzene desorbs from Pd( 111) at the two temperatures of -430 and -530 K.” This indicates that benzene still exists as an intact molecule on the surface at 530 K. (By contrast, Rh( 111) has only a 395 K desorption peak42and Pt( 111) indicating that decomposition is has 375 and 450 K predominant at higher temperatures on these surfaces.) It is known that acetylene can trimerize to form benzene on Pd( 111) crystal surfaces7-14but not on Rh( 111)46or on Pt( 11l).47 Rucker et al. have studiedI3 this reaction at high pressures (200-1200 Torr) in the temperature range of 273-573 K and found that benzene was the only product detected. This might be related to the weak benzene-palladium interaction and the (44) Tsai. M.-C.: Muetterties. E. L. J. Am. Chem. SOC.1982. 104. 2534. (45j Garfunkel, E. L.; Maj, J. J.; Frost, J. C.; Farias, M. H.; Somorjai, G. A. J. Phys. Chem. 1983,87, 3629. (46) Mate, C. M., private communication (47) Kang, D. B.; Anderson, A. B. Surf. Sci. 1985, 15S, 639, and references therein. (48) Gomez-Sal, M. P.; Johnson, B. F. G.; Lewis, T.; Raithby, P. R.; Wright, A. H. J. Chem. SOC.,Chem. Commun. 1985, 1682.
resulting easy molecular benzene desorption at these reaction conditions. The cyclotrimerization occurs on Pd( 11 1) even under UHV conditions, and benzene desorbs at 250 and 490 K after adsorbing acetylene at 130 K and subsequent heating. The Pd( 11 1)-(3X 3)-C6H6+2C0 structure was obtained above room temperature. Structural studies on both acetylene and benzene at low temperatures are necessary to understand such low-temperature benzene formation.
7. Conclusion An ordered (3x3) benzene overlayer was formed on Pd(ll1) by coadsorbing benzene and CO. A dynamical LEED analysis has revealed that both benzene and C O bond over fcc-type hollow sites in a close-packed form with a 2:l C O to C6H6 stoichiometry. Weak distortions from the gas-phase geometry may be present in both molecules. This contrasts with larger benzene distortion on Rh( 1 11) and Pt( 11 1). Clear trends emerge which indicate an increasing metal-benzene bond strength and decreasing C-C bond strength in going from Pd( 11 1) via Rh( 11 1) to Pt( 11 1). These trends are consistent with vibrational spectroscopy results. Acknowledgment. We thank C. M. Mate, D. F. Ogletree, C. Minot, and E. L. Garfunkel for fruitful discussions and assistance. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U S . Department of Energy under Contract No. DE-AC03-76SF00098, We also acknowledge supercomputer time provided by the Office of Energy Research of the U S . Department of Energy and NATO for an US-France exchange grant. H.O. gratefully acknowledges financial support from IBM Japan. Registry No. Pd, 7440-05-3; (20,630-08-0; C6H6, 71-43-2.
Photoionlzatlon of Chlorophyll a in Veslcles: Effects of Variation of the Alkyl Chain Length of the Phosphollpid Thomas Hiff and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: November 23, 1987; I n Final Form: January 26, 1988)
Electron spin resonance has been used to detect the products of photoionized chlorophyll a by red light with or without electron scavengers and to measure their yields in vesicles differing by the alkyl chain length (C,., to CI8)of the phospholipid. Electron spin echo (ESE) spectroscopy has been used to investigate the solubilization sites of photogenerated chlorophyll a cation and tetrabromobenzoquinone anion in these different vesicles. As the alkyl chain length of the phospholipid decreases, the photoionization yield increases. This indicates that the average electron-transfer distance from photoexcited chlorophyll a decreases as the surfactant alkyl chain length decreases. In addition, the ESE of the quinone anion shows an increase in the deuterium modulation depth from D 2 0 at the vesicle surface when the surfactant alkyl chain length decreases.
Introduction Recent work has shown that bacterial photosynthesis is initiated by photoionization of a chlorophyll dimer with eventual electron transfer to a quinone within a protein and lipid environment.’ Organized molecular assemblies like micelles and vesicles are being studied as membrane mimetic systems2 Photoinduced electron-transfer reactions in such media are being widely investigated as model reactions for artificial photosynthetic system^.^-'^ In (,l) Hoff, A. J. In Light Reaction Path of Photosynthesis; Fong, F. K., Ed.; Springer-Verlag: New York, 1982; Chapter 4, p 81. (2) Fendler, J. H . Acc. Chem. Res. 1980, 13, 7. (3) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133. (4) Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 2248.
0022-3654/88/2092-3982$01.50/0
order to achieve net light energy storage with such systems, it is necessary to control the net charge separation efficiency which is mainly determined by the rate of back electron transfer. It has been shown for micelles that these structural factors that affect the photoionization efficiency of an embedded molecule can ( 5 ) Hurley, J. K.; Tollin, G. Sol. Energy 1982, 28, 187.
(6) Narayana, P. A.; Li, A. S.W.; Kevan, L. J. Am. Chem. SOC.1981,103, 3603. (7) Thompson, D. H. P.; Barrette, W. C.; Hurst, J. K. J . Am. Chem. SOC. 1987, 109, 2003. (8) Calvin, M. Photochem. Photobiol. 1983, 37, 349. (9) Fendler, J. H. In Photochemical Conuersion and Storage of Solar Energy; Connolly, J. S . , Ed.; Academic: New York, 1981. (10) Colaneri, M. J.; Kevan, L.; Thompson, D. H. P.; Hurst, J. K. J. Phys. Chem. 1987, 91, 4072.
0 1988 American Chemical Society
Photoionization of Chlorophyll a in Vesicles
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3983
be controllably varied by changing counterions,11,'2headgr~ups,'~,'~ or alkyl chain length of s ~ r f a c t a n t sor ' ~ by adding salts'"'* and water-soluble alcohol^^^^^^ that modify the micelle surface structure. Photoionization of chlorophyll a (Chla) incorporated into micelles or vesicles has been studied mainly by optical spectroscopic m e t h o d ~ ~oriented l - ~ ~ toward determination of the kinetics and formation mechanisms of the chlorophyll cation radical (Chla'). Electron spin resonance (ESR) methods have also been used to study Chla+ formation in vesicle^.^^-^^ In the current investigation ESR is applied to study the effect of the length of the hydrophobic tail of the vesicle-forming phospholipid surfactant on the photoionization yield of Chla+ with and without hydrophilic and hydrophobic electron scavengers. It is shown that a shorter alkyl tail correlates with a higher photoionization yield. Electron spin echo (ESE) techniques29have also been used to study photoinduced Chla+ formation in the presence of an electron acceptor in vesicle systems. Conditions to differentiate echo production from Chla' or the acceptor anion have been developed, and a correlation between the degree of hydration of the vesicle surface and the alkyl chain length of the vesicle-forming surfactant has been established. This provides a model for the alkyl chain length effect on the Chla photoionization yield.
Experimental Section The purity of chlorophyll a extracted from spinach by the conventional method30 was determined to be 96% from its extinction coefficient in diethyl ether at 660 nm versus the literature value of 8.63 X 1O4 M-' cm-'. Dimyristoyl-L-a-phosphatidylcholine (DMPC), dipentadecanoyl-L-a-phosphadylcholine (DPePC), dipalmitoyl-DL-a-phosphatidylcholine(DPPC), diheptadecanoyl-L-a-phosphatidylcholine (DHPC), and distearoyl-L-a-phosphatidylcholine (DSPC) from Sigma Chemicals were used as received. These phosphatidylcholines have varying alkyl chain lengths of C14 to CIS,respectively. Buffer solutions were prepared with sodium phosphate from Fisher Inc., sodium pyrophosphate from Aldrich Chemicals, and sodium ethylenediaminetetraacetate (EDTA) from Alfa Products. The electron scavengers (ES) potassium ferricyanide (FC) from MCB, Inc., tetrachloro-p-benzoquinone(TCBQ) from Aldrich, and tetrabromo-p-benzoquinone (TBBQ) from Alfa Products were also used without further purification. Unilamellar liposome solutions of Chla were prepared by the method developed by Huang3' as modified by OettmeierZ6et al. A chloroform solution of phospholipid containing Chla was
(1 1) Szajdzinska-pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman. M. J. J . Am. Chem. SOC.1985. 107. 784. (12) Jones, R. R. M.; Maldonado, R.; Szajdinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126. (13) Li, A. S. W.; Kevan, L. J. Am. Chem. SOC.1983, 105, 5752. (14) Plonka. A,; Kevan, L. J . Phys. Chem. 1985, 89, 2087. (15) Narayana, P. A,; Li, A. S. W.; Kevan, L. J . Am. Chem. SOC.1982, 104, 6502. (16) Maldonado, R.; Kevan, L.; Szajdinska-Pietek, E.; Jones, R. R. M. J . Chem. Phys. 198d, 81, 3958. (17) Plonka, A,; Kevan, L. J . Chem. Phys. 1985,82, 4322. (18) Hiromitsu, I.; Kevan, L. J . Phys. Chem. 1986, 90, 3088. (19) Szajdinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1985. 107. 6467. (20) Szajdinska-Pieiek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Colloid Interface Sci. 1986, I 1 0, 5 14. (21) Ford, W. E.; Tollin, G. Photochem. Photobiol. 1982, 35, 809. (22) Ford, W. E.; Tollin, G. Photochem. Photobiol. 1982, 36, 647. (23) Fang, Y.; Tollin, G. Photochem. Photobiol. 1983, 38, 429. (24) Fang, Y . ;Tollin, G. Photochem. Photobiol. 1984, 39, 685. (25) Tomkiewicz, M.; Corker, G. A. Photochem. Photobiol. 1975,22, 249. (26) Oettmeier, W.; Norris, J. R.; Katz, J. J. K. Z . Naturforsch., C 1976, 31C, 163. (27) Ohta, N.; Kevan, L. J . Phys. Chem. 1985, 89, 3070. (28) Hiromitsu, I.; Kevan, L. J. Am. Chem. SOC.1987, 109, 4501. (29) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley Interscience: New York, 1979; pp 279-341. (30) Strain, H. H.; Svec, W. A. In The Chlorophylls; Vernon, L. P., Seely, G. R., Eds.; Academic: New York, 1966; p 21. (31) Huang, G. H. Biochemistry 1969, 8, 344.
Chlo'
Chla+/TCBQ-
V Figure 1. ESR line shapes of phospholipid vesicle solutions of Chla at 77 K after 2-h red light irradiation (a) with no electron scavenger or with added ferricyanide ions and (b) with TCBQ as an electron scavenger.
evaporated for 3 h at 63 OC under nitrogen atmosphere. The resulting film was sonicated in an aqueous buffer solution with a Fisher Model 300 sonic dismembrator operated at 30 W with a 4-mm-0.d. microtip for 1 h at 5 5 "C (above the liquid crystal gel-phase transition temperature). Protiated and deuteriated buffer solutions contained 0.1 M sodium phosphate, 0.1 M sodium pyrophosphate, and 1 mM EDTA respectively in triply distilled water ( H 2 0 ) or D 2 0 from Aldrich. The pH was adjusted to 7.0 with sulfuric acid. If added, FC was introduced after sonication. TCBQ or TBBQ was introduced before evaporation to ensure penetration into the vesicle. The added amounts of phospholipid, Chla, and ES were kept at 45, 1.5, and 15 mM, respectively, for photoionization yield measurements, but the concentration of TBBQ was lowered to 0.15 mM for electron spin echo experiments. The entire sample preparation was carried out under minimal light in a nitrogen atmosphere. The vesicle solutions were sealed into 2-mm-i.d. X h n " m . d . Suprasil quartz tubes and into 7 5 - ~ L(1-mm-0.d.) Pyrex micropipets for photoirradiation at 77 K and at room temperature, respectively. Photoirradiation at 77 K was done with a 300-W Cermax xenon lamp with a power supply from ILC Technology. The light was passed through a IO-cm water filter and a Corning 2412 glass filter for red light irradiation (A > 590 nm). Photoirradiation at room temperature was performed in the ESR cavity. The light intensity was 3000 W m-2 at the sample position for ESR measurements and was 4500 W m-2 at the sample position for ESE measurements. ESR spectra were recorded on a Varian E-4 spectrometer at 9 GHz with 100-kHz field modulation at 77 K and at room temperature at 0.2-mW microwave power. Optical absorption spectra were measured in a 1-mm path length quartz cell with a Perkin-Elmer Model 330 spectrophotometer at room temperature. With this spectrophotometer the signal intensity of a reference is automatically subtracted from the signal intensity of the sample. The vesicle solutions were diluted six times with distilled water for the optical absorption measurements since the absorption intensities of Chla in the undiluted vesicle solutions were too strong to be measured. Lambert-Beer's law was satisfied in the diluted liposome solutions. Microwave power saturation experiments were done using a Varian E-9 ESR at 4.2 K with 270-Hz field modulation. Electron spin echo spectra were recorded with a home-built s p e c t r ~ m e t e r ~ ~ at 4.2 K at about 3300 G. Fourier-transformed ESE spectra were obtained with the fast Fourier transform routine of the Nicolet 1280 computer used for acquisition of the ESE data. Results A . Electron Spin Resonance Studies. At room temperature, no ESR signal is detected before or after photoirradiation with or without scavengers. At 77 K, Chla-containing vesicles prepared (32) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979,83, 3378.
3984 The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Hiff and Kevan TWO-PULSE ECHO Chlo+/TBBQ-/DPPC
\
-
\
I
0 30
I
€
L
0
a
NO ES
C14 C15 Cl6 C17 C18 ALKYL CHAIN LENGTH
1
IO0 200 300 400 REPETITION TIME, ms
I
500
Figure 3. ESE intensity at 4.2 K versus the repetition time of a two-pulse echo sequence a t 7 = 0.56 ps for (0)photoionized Chla/TBBQ (1O:l ratio) in DPPC vesicles and for ( 0 )chemically generated TBBQ- in lo--’ M in ethanol with M NaOH. The vertical scales are arbitrary and coincide for a repetition time of 100 ms.
Figure 2. Corrected photoionization yields of irradiated phospholipid vesicle solutions of Chla a t 77 K, with no electron scavenger (ES), with added ferricyanide (FC), and with added TCBQ, against the length of the alkyl moiety of the phospholipid.
without an electron scavenger and with added ferricyanide produced a symmetric single ESR line as shown in Figure la. The g value is 2.0026, and the derivative peak-to-peak line width is 10 G. According to previous results27and to the optical spectra that show a characteristic peak at 672 nm, this ESR line can be attributed to Chla+. Ferricyanide is ESR silent because of a short spin-lattice relaxation time at 77 K. Vesicles containing Chla with TCBQ as an electron scavenger produced an asymmetric single ESR line at g = 2.0050 as shown in Figure 1b. It has been reported that the TCBQ- anion radical has g = 2.0057.33 Thus, this asymmetric ESR line is assigned to a superposition of Chla+ and TCBQ- by analogy to differential microwave power saturation measurementsz7and 35-GHz meas u r e m e n t ~on~ Chla’ ~ and TBBQ-. The photoionization yields of Chla+ were determined from the doubly integrated ESR spectra at 0.2-mW microwave power at which no power saturation occurred and after 2-h photoirradiation at which plateau yields were reached. With no added electron scavenger and with added ferricyanide the ESR signal arises entirely from Chla+ and is symmetric. Corrections were made for a very small dark signal possibly arising from stray light, and all intensities were normalized to constant Chla concentration as measured by optical absorption of the solutions before photoirradiation. With added TCBQ the dark signal was somewhat larger, which might indicate some contribution from redox impurities. Also, with added TCBQ the corrected doubly integrated ESR intensity was assumed to be. due equally to Chla+ and TCBQand accordingly was divided by two to obtain an intensity proportional to the Chla’ concentration. This was a good assumption since at [TCBQ]/[Chla] = 10, as used, the yield of Chla+ versus TCBQ concentration had plateaued. These corrected yields of Chla’ are plotted in Figure 2 versus the alkyl chain length of the surfactant. It is seen that the photoionization yield is 1.6 times greater with ferricyanide as compared to TCBQ as an electron scavenger. Most importantly, it is seen that the photoionization yield decreases linearly with increasing alkyl chain length of the vesicle-forming surfactant in the presence of added electron scavengers. However, with no added electron scavenger the photoionization yield, although small, seems independent of the alkyl chain length. B. Electron Spin Echo Studies. N o electron spin echo signal was seen after photoirradiation of the vesicles in the absence of added electron scavengers, probably because of the weakness of (33) Blois, Jr., M. S.; Brown, H.W.; Moling, J. E. In Free Radicals in Biological Systems; Blois, Jr., M. S., Brown, H. W., Lenmon, R. M., Lindblom, R. O., Weissbluth, M., Eds.; Academic: New York, 1961; p 117. (34) Hiff, T.; Kevan, L. Phorochem. Phorobiol., submitted for publication.
b 400 ms
0
05
10
15
20
25
7.tL5
Figure 4. Two-pulse ESE spectra of photoirradiated Chla with TBBQ (1O:l ratio) in DPPC vesicles prepared in D,O (a) at 30-ms repetition time and (b) at 400-ms repetition time.
the ESR signal. Likewise, no echo was observed with added ferricyanide. This, however, is probably due to a shortening of the echo decay below our detection limits due to the added paramagnetic ferricyanide. With added TCBQ at 15 mM an echo was observed, but no deuterium modulation was seen when the vesicle solutions were made in D20. However, when the quinone concentration was greatly reduced, clear deuterium modulation was detected. This was also found to be the case for tetrabromep-benzoquinone (TBBQ) for which ESE modulation in such vesicle systems has previously been reported.35 Based on the above considerations a series of ESE experiments were carried out with [Chla] = 1.5 mM and [TBBQ] = 0.15 mM in DPPC vesicles. Since Chla+ and TBBQ- have quite different relaxation times,27 the effect of pulse sequence repetition time versus echo intensity was investigated as shown in Figure 3. At short repetition times echo contributions from species with relaxation times longer than the repetition time should be suppressed due to partial saturation. Figure 3 shows that the echo intensity of TBBQ- alone, produced by chemical reduction by NaBH,,36 shows little variation with pulse sequence repetition time from 50 to 500 ms. In contrast, the echo intensity of the photoionized Chla/TBBQ system in DPPC vesicles shows a significant increase with increasing repetition time. This is attributed to superimposed echo contributions from TBBQ- and Chla+, with Chla’ being more abundant due to the low [TBBQ] used. The variation with repetition time can be interpreted as partial saturation at low repetition times. With this interpretation TBBQ- makes the major contribution to the echo intensity at 30-ms repetition time and (35) Ohta, N.; Kevan, L. J . Chem. Phys. 1985, 83, 4382. (36) Hales, B J.; Case, E. E. Biochim. Biophys. Acta 1981, 637, 291.
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3985
Photoionization of Chlorophyll a in Vesicles 04 -
2 0 0
-
I o w N 0.3 -
a
r 5-
-
i 1
II
03
b T
-
t
1 I
I
I
I
I
C14 C15 C16 C17 C18 C14 C15 C16 C17 C16 ALKYL CHAIN LENGTH
Figure 5. Normalized ESE modulation depth against the alkyl chain length in the phospholipid of vesicles containing photoionized Chla/ TBBQ (1O:l) in D20at 30-msrepetition time (a) from two-pulse ESE spectra and (b) from three-pulse ESE spectra.
Chla+ makes the major contribution at 500-ms repetition time. This interpretation is supported by the different modulation patterns observed at the low and high repetition rates as shown in Figure 4. At 30-ms repetition time deuterium modulation (0.55-ps period) from DzO is observed as well as protium modulation (0.08-ps period). This implies that TBBQ- interacts with DzOat the vesicle interface. In contrast, a t 400-ms repetition time 14N modulation (0.35-ps period) is observed from Chla+ interacting with its own porphyrin ring; if deuterium modulation is present, it is masked by the stronger nitrogen modulation. Experiments in vesicles prepared in H 2 0 showed the same modulation pattern at 400-ms repetition time. These frequency assignments were supported by Fourier transformation (FT) of the time domain data although the FT spectra are quite noisy. The detection of deuterium modulation associated with TBBQallows us to investigate a surfactant alkyl chain length effect on this deuterium modulation. This is done in Figure 5 for both two-pulse and three-pulse ESE data. The normalized modulation depth3’ is measured at the minimum of the first period from an extrapolated unmodulated echo decay curve divided by the depth to the base line. Figure 4 shows a clear trend of decreasing normalized deuterium modulation depth with increasing alkyl chain length of the surfactant. The two-pulse data have better signal-to-noise, but overall the two-pulse and three-pulse data show comparable trends.
Discussion The photoionization efficiency of Chla in dialkylphosphatidylcholine vesicles does depend on the alkyl chain length in the presence of added electron scavengers as shown in Figure 2. This can most simply be interpreted as a change in the average distance between Chla and the electron scavenger within the vesicle structure. As the alkyl chain length is increased, the overall (37) Szajdinska-Pietek,E.; Maldonado, R.; Kevan, L.; Berr, S. S.; Jones, R. R. M. J . Phys. Chem. 1985, 89, 1547.
dimension of the vesicle is increased and the average locus of positions for Chla relative to the vesicle interface increases. A water-soluble electron scavenger like ferricyanide is localized at the vesicle interface. Thus, it is understandable that the average Chla-ferricyanide distance increases with increasing alkyl chain length. For a lipid-soluble electron acceptor like TCBQ it is less clear why the average Chla-TCBQ distance should increase with alkyl chain length. However, the slope of the alkyl chain length dependence is similar for ferricyanide and TCBQ as electron scavengers. Thus, the greater overall lipid volume with increasing alkyl chain length seems to independently increase the separation between Chla and TCBQ within the vesicle structure. If the positions of both Chla and TCBQ are determined independently, then their average interaction distance will increase with total vesicle volume. In the case of no added electron scavenger there appears to be no alkyl chain length effect, although the photoionization yield is small and experimental variations may obscure any trend. In previous work it has been suggested that bulk water at the vesicle interface can act as an electron acceptor for photoexcited Chla.z8 In this case one would expect an alkyl chain length effect similar to that found for ferricyanide, which is localized at the vesicle interface. This apparent discrepancy is not understood. However, other recent work3ssuggests that the degree of water organization at the vesicle interface affects the photoionization yield in the absence of molecular electron acceptors. Similar effects may play a role in the present system. It is interesting to note that an apparently similar alkyl chain length effect has been previously observed for the photoionization of tetramethylbenzidine in micelle^.'^ In the micellar systems there was no added electron acceptor but the photoionization yield was efficient. The ESEM data of TBBQ- versus surfactant alkyl chain length in Figure 5 show a striking parallel to the photoionization yield data. The ESEM data show that the average interaction of TBBQ- with water at the vesicle interface decreases with increasing alkyl chain length. This dipolar interaction depends on the interaction distance as well as the average number of interacting water molecules. Since the vesicle interface structure and hence the number of interacting waters should not be affected by changing the surfactant alkyl chain length, we conclude that the trend in the ESEM data can best be interpreted as an average increase in the interaction distance of TBBQ- from the vesicle interface with increasing alkyl chain length. This is the same type of alkyl chain length effect as deduced from the photoionization yield, and similar interpretive reasoning applies.
Acknowledgment. This research was supported by a grant from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy. Registry No. DMPC, 18194-24-6; DPePC, 3355-27-9; DPPC, 279768-4; DHPC, 70897-27-7; DSPC, 8 16-94-4; FC, 13746-66-2; TCBQ, 118-75-2; TBBQ, 488-48-2; chlorophyll a, 479-61-8. (38) Baglioni, P.; Kevan, L. J . Phys. Chem. 1987, 91, 2106.
